--- Created: 2026/10/23 14:01 share_link: https://notesx.azraftaohid.me/bc03ithl share_updated: 2026-10-23T15:19:48+06:00 --- 1. [Section A: River Engineering (`EWCE 461/SA`)](#Section%20A:%20River%20Engineering%20(%60EWCE%20461/SA%60)) 1. [SA Lecture 1: Introduction to River Systems, Longitudinal Profile & Channel Classification](#SA%20Lecture%201:%20Introduction%20to%20River%20Systems,%20Longitudinal%20Profile%20&%20Channel%20Classification) 1. [Q1.1: Explain with a sketch the different zones along a river longitudinal profile and how sediment size, slope, discharge, width, depth, and velocity change from upstream to downstream.](#Q1.1:%20Explain%20with%20a%20sketch%20the%20different%20zones%20along%20a%20river%20longitudinal%20profile%20and%20how%20sediment%20size,%20slope,%20discharge,%20width,%20depth,%20and%20velocity%20change%20from%20upstream%20to%20downstream.) 2. [Q1.2: Define floodplain, natural levee/terrace, thalweg, tributary, and distributary.](#Q1.2:%20Define%20floodplain,%20natural%20levee/terrace,%20thalweg,%20tributary,%20and%20distributary.) 3. [Q1.3: What is an alluvial river? How are alluvial rivers classified based on sinuosity and multichanneledness? Explain with a sketch.](#Q1.3:%20What%20is%20an%20alluvial%20river?%20How%20are%20alluvial%20rivers%20classified%20based%20on%20sinuosity%20and%20multichanneledness?%20Explain%20with%20a%20sketch.) 4. [Q1.4: Classify rivers based on sediment load and compare their width-depth ratio, slope, and bank stability.](#Q1.4:%20Classify%20rivers%20based%20on%20sediment%20load%20and%20compare%20their%20width-depth%20ratio,%20slope,%20and%20bank%20stability.) 5. [Q1.5: What is Planform Index ($PFI$)? Write its equation, threshold values, and the $PFI$ status of the Jamuna River.](#Q1.5:%20What%20is%20Planform%20Index%20($PFI$)?%20Write%20its%20equation,%20threshold%20values,%20and%20the%20$PFI$%20status%20of%20the%20Jamuna%20River.) 2. [SA Lecture 2: Fluvial Geomorphology, Channel Patterns & Hydraulic Geometry](#SA%20Lecture%202:%20Fluvial%20Geomorphology,%20Channel%20Patterns%20&%20Hydraulic%20Geometry) 1. [Q2.1: Define channel pattern. Explain the four variables that influence channel pattern (slope, sediment load/size, sinuosity, and width-depth ratio / bank cohesiveness) with sketches.](#Q2.1:%20Define%20channel%20pattern.%20Explain%20the%20four%20variables%20that%20influence%20channel%20pattern%20(slope,%20sediment%20load/size,%20sinuosity,%20and%20width-depth%20ratio%20/%20bank%20cohesiveness)%20with%20sketches.) 2. [Q2.2: What is a flashy river and where are flashy rivers found in Bangladesh?](#Q2.2:%20What%20is%20a%20flashy%20river%20and%20where%20are%20flashy%20rivers%20found%20in%20Bangladesh?) 3. [Q2.3: Explain the concept of hydraulic geometry of alluvial channels and its two types.](#Q2.3:%20Explain%20the%20concept%20of%20hydraulic%20geometry%20of%20alluvial%20channels%20and%20its%20two%20types.) 4. [Q2.4: [Math] Establish the hydraulic geometry relationship between channel width ($W$) and discharge ($Q$) ($W = aQ^b$) from observed field data.](#Q2.4:%20%5BMath%5D%20Establish%20the%20hydraulic%20geometry%20relationship%20between%20channel%20width%20($W$)%20and%20discharge%20($Q$)%20($W%20=%20aQ%5Eb$)%20from%20observed%20field%20data.) 5. [Q2.5: Define bankfull discharge. Explain how the width-depth ($W/D$) ratio varies with water depth and how to find bankfull depth from cross-section data.](#Q2.5:%20Define%20bankfull%20discharge.%20Explain%20how%20the%20width-depth%20($W/D$)%20ratio%20varies%20with%20water%20depth%20and%20how%20to%20find%20bankfull%20depth%20from%20cross-section%20data.) 6. [Q2.6: Explain why the longitudinal bed slope of a river decreases downstream (concave-upward profile).](#Q2.6:%20Explain%20why%20the%20longitudinal%20bed%20slope%20of%20a%20river%20decreases%20downstream%20(concave-upward%20profile).) 3. [SA Lecture 3: Meander Geometry, Mechanics & Formation Theories](#SA%20Lecture%203:%20Meander%20Geometry,%20Mechanics%20&%20Formation%20Theories) 1. [Q3.1: Why does mean flow velocity increase slightly from upstream to downstream even though river slope decreases?](#Q3.1:%20Why%20does%20mean%20flow%20velocity%20increase%20slightly%20from%20upstream%20to%20downstream%20even%20though%20river%20slope%20decreases?) 2. [Q3.2: Briefly discuss the five theories of river meandering.](#Q3.2:%20Briefly%20discuss%20the%20five%20theories%20of%20river%20meandering.) 3. [Q3.3: [Math] Establish the relationship between Slope and Discharge ($S = pQ^{-z}$) or Meander Wavelength and Channel Width ($SP = aW^b, M_L = 2 \cdot SP$).](#Q3.3:%20%5BMath%5D%20Establish%20the%20relationship%20between%20Slope%20and%20Discharge%20($S%20=%20pQ%5E%7B-z%7D$)%20or%20Meander%20Wavelength%20and%20Channel%20Width%20($SP%20=%20aW%5Eb,%20M_L%20=%202%20%5Ccdot%20SP$).) 4. [Q3.4: Define pool, riffle, and helical current. Explain with a sketch how helical flow causes outer-bank erosion and inner-bank point bar deposition at a river bend.](#Q3.4:%20Define%20pool,%20riffle,%20and%20helical%20current.%20Explain%20with%20a%20sketch%20how%20helical%20flow%20causes%20outer-bank%20erosion%20and%20inner-bank%20point%20bar%20deposition%20at%20a%20river%20bend.) 4. [SA Lecture 4: Channel Stability & Sediment Transport Modes](#SA%20Lecture%204:%20Channel%20Stability%20&%20Sediment%20Transport%20Modes) 1. [Q4.1: On what parameters does channel stability depend? Why is a meandering river considered more stable than a braided river? Explain using Planform Stability Ratios ($PS_1$ and $PS_2$).](#Q4.1:%20On%20what%20parameters%20does%20channel%20stability%20depend?%20Why%20is%20a%20meandering%20river%20considered%20more%20stable%20than%20a%20braided%20river?%20Explain%20using%20Planform%20Stability%20Ratios%20($PS_1$%20and%20$PS_2$).) 2. [Q4.2: Classify sediment load in detail and distinguish between bed load, suspended load, wash load, and bed-material load with a figure.](#Q4.2:%20Classify%20sediment%20load%20in%20detail%20and%20distinguish%20between%20bed%20load,%20suspended%20load,%20wash%20load,%20and%20bed-material%20load%20with%20a%20figure.) 3. [Q4.3: Explain with a figure why bed load is "capacity-dependent" while suspended/wash load is "supply-dependent," and why wash load never settles in rivers.](#Q4.3:%20Explain%20with%20a%20figure%20why%20bed%20load%20is%20%22capacity-dependent%22%20while%20suspended/wash%20load%20is%20%22supply-dependent,%22%20and%20why%20wash%20load%20never%20settles%20in%20rivers.) 4. [Q4.4: Why is there no sharp demarcation between bed load and suspended load (why are they interchangeable)?](#Q4.4:%20Why%20is%20there%20no%20sharp%20demarcation%20between%20bed%20load%20and%20suspended%20load%20(why%20are%20they%20interchangeable)?) 5. [Q4.5: What is dissolved load and why is it not considered part of sediment load?](#Q4.5:%20What%20is%20dissolved%20load%20and%20why%20is%20it%20not%20considered%20part%20of%20sediment%20load?) 6. [Q4.6: Explain with sketches the vertical distribution of suspended sediment size ($d_{10}, d_{50}, d_{90}$) and concentration over the flow depth.](#Q4.6:%20Explain%20with%20sketches%20the%20vertical%20distribution%20of%20suspended%20sediment%20size%20($d_%7B10%7D,%20d_%7B50%7D,%20d_%7B90%7D$)%20and%20concentration%20over%20the%20flow%20depth.) 5. [SA Lecture 5: Stream Power & Incipient Motion (Shields & Hjulström)](#SA%20Lecture%205:%20Stream%20Power%20&%20Incipient%20Motion%20(Shields%20&%20Hjulstr%C3%B6m)) 1. [Q5.1: Define stream power. Prove that bed shear stress is proportional to velocity squared ($\tau_0 \propto V^2$) and stream power is proportional to velocity cubed ($\Omega \propto V^3$).](#Q5.1:%20Define%20stream%20power.%20Prove%20that%20bed%20shear%20stress%20is%20proportional%20to%20velocity%20squared%20($%5Ctau_0%20%5Cpropto%20V%5E2$)%20and%20stream%20power%20is%20proportional%20to%20velocity%20cubed%20($%5COmega%20%5Cpropto%20V%5E3$).) 2. [Q5.2: Distinguish between stream competence and stream capacity.](#Q5.2:%20Distinguish%20between%20stream%20competence%20and%20stream%20capacity.) 3. [Q5.3: Prove that the critical shear stress ($\tau_c$) required to erode coarse-grained sediment is a function of only sediment diameter ($d$).](#Q5.3:%20Prove%20that%20the%20critical%20shear%20stress%20($%5Ctau_c$)%20required%20to%20erode%20coarse-grained%20sediment%20is%20a%20function%20of%20only%20sediment%20diameter%20($d$).) 4. [Q5.4: Explain with sketches the Shields Diagram and Hjulström Curve, highlighting why sand erodes most easily, why clay needs high velocity to erode, and why erosion velocity exceeds suspension velocity.](#Q5.4:%20Explain%20with%20sketches%20the%20Shields%20Diagram%20and%20Hjulstr%C3%B6m%20Curve,%20highlighting%20why%20sand%20erodes%20most%20easily,%20why%20clay%20needs%20high%20velocity%20to%20erode,%20and%20why%20erosion%20velocity%20exceeds%20suspension%20velocity.) 5. [Q5.5: [Math] Check riverbed erodibility, find minimum non-erodible grain size ($d$), or find maximum stable discharge ($Q_{\max}$) using the Shields parameter ($\tau^*$).](#Q5.5:%20%5BMath%5D%20Check%20riverbed%20erodibility,%20find%20minimum%20non-erodible%20grain%20size%20($d$),%20or%20find%20maximum%20stable%20discharge%20($Q_%7B%5Cmax%7D$)%20using%20the%20Shields%20parameter%20($%5Ctau%5E*$).) 6. [SA Lecture 6: Bedforms, Flow Regimes, Roughness & Fall Velocity](#SA%20Lecture%206:%20Bedforms,%20Flow%20Regimes,%20Roughness%20&%20Fall%20Velocity) 1. [Q6.1: What is flow regime? Explain with sketches the sequence of bedforms in lower and upper flow regimes, why upper-regime bedforms are rare in nature, and how roughness changes across regimes.](#Q6.1:%20What%20is%20flow%20regime?%20Explain%20with%20sketches%20the%20sequence%20of%20bedforms%20in%20lower%20and%20upper%20flow%20regimes,%20why%20upper-regime%20bedforms%20are%20rare%20in%20nature,%20and%20how%20roughness%20changes%20across%20regimes.) 2. [Q6.2: Define sphericity and explain how particle shape affects drag coefficient ($C_D$) and fall velocity ($\omega$).](#Q6.2:%20Define%20sphericity%20and%20explain%20how%20particle%20shape%20affects%20drag%20coefficient%20($C_D$)%20and%20fall%20velocity%20($%5Comega$).) 7. [SA Lecture 7: Suspended Load (Rouse Equation) & Bed Load Formulas](#SA%20Lecture%207:%20Suspended%20Load%20(Rouse%20Equation)%20&%20Bed%20Load%20Formulas) 1. [Q7.1: Why is determining suspended sediment load in a river complex and uncertain?](#Q7.1:%20Why%20is%20determining%20suspended%20sediment%20load%20in%20a%20river%20complex%20and%20uncertain?) 2. [Q7.2: Define shear velocity ($u_*$) and fall velocity ($\omega$). What is the Rouse Number ($R_n$) and how does it control the vertical distribution and transport mode of sediment?](#Q7.2:%20Define%20shear%20velocity%20($u_*$)%20and%20fall%20velocity%20($%5Comega$).%20What%20is%20the%20Rouse%20Number%20($R_n$)%20and%20how%20does%20it%20control%20the%20vertical%20distribution%20and%20transport%20mode%20of%20sediment?) 3. [Q7.3: [Math] Calculate suspended sediment concentration ($C_y$) at elevation $y$ using the Rouse Equation.](#Q7.3:%20%5BMath%5D%20Calculate%20suspended%20sediment%20concentration%20($C_y$)%20at%20elevation%20$y$%20using%20the%20Rouse%20Equation.) 4. [Q7.4: [Math] Estimate bed load transport rate from dune dimensions ($L, \Delta H$) or by Meyer-Peter & Müller (MPM) gravel-bed equation.](#Q7.4:%20%5BMath%5D%20Estimate%20bed%20load%20transport%20rate%20from%20dune%20dimensions%20($L,%20%5CDelta%20H$)%20or%20by%20Meyer-Peter%20&%20M%C3%BCller%20(MPM)%20gravel-bed%20equation.) 8. [SA Lecture 8: Dynamic Adjustment of Rivers — Lane's Relationship, Aggradation & Degradation](#SA%20Lecture%208:%20Dynamic%20Adjustment%20of%20Rivers%20%E2%80%94%20Lane's%20Relationship,%20Aggradation%20&%20Degradation) 1. [Q8.1: State Lane's relationship ($Q_s d \propto Q_w S$) and mention its limitations.](#Q8.1:%20State%20Lane's%20relationship%20($Q_s%20d%20%5Cpropto%20Q_w%20S$)%20and%20mention%20its%20limitations.) 2. [Q8.2: Explain with sketches the expected morphological changes (aggradation or degradation) in a river due to: (i) Upstream & downstream of a dam / clear-water release, (ii) Water supply diversion, (iii) Increase in flood discharge, (iv) Tributary confluence, and (v) Distributary off-take.](#Q8.2:%20Explain%20with%20sketches%20the%20expected%20morphological%20changes%20(aggradation%20or%20degradation)%20in%20a%20river%20due%20to:%20(i)%20Upstream%20&%20downstream%20of%20a%20dam%20/%20clear-water%20release,%20(ii)%20Water%20supply%20diversion,%20(iii)%20Increase%20in%20flood%20discharge,%20(iv)%20Tributary%20confluence,%20and%20(v)%20Distributary%20off-take.) 3. [Q8.3: [Math] Predict downstream slope change and aggradation/degradation using Quantitative Modified Lane's Relationship.](#Q8.3:%20%5BMath%5D%20Predict%20downstream%20slope%20change%20and%20aggradation/degradation%20using%20Quantitative%20Modified%20Lane's%20Relationship.) 9. [SA Lecture 9 & Lecture 10: River Scour, Complex Piers, Lacey–Inglis Method & Countermeasures](#SA%20Lecture%209%20&%20Lecture%2010:%20River%20Scour,%20Complex%20Piers,%20Lacey%E2%80%93Inglis%20Method%20&%20Countermeasures) 1. [Q9.1: What is local scour? List the types of scour and distinguish between scour and long-term degradation.](#Q9.1:%20What%20is%20local%20scour?%20List%20the%20types%20of%20scour%20and%20distinguish%20between%20scour%20and%20long-term%20degradation.) 2. [Q9.2: What is contraction scour and why is it generally not significant (or when is it neglected) at a bridge site?](#Q9.2:%20What%20is%20contraction%20scour%20and%20why%20is%20it%20generally%20not%20significant%20(or%20when%20is%20it%20neglected)%20at%20a%20bridge%20site?) 3. [Q9.3: Explain the scour process around a bridge pier with a figure. Distinguish between horseshoe vortex and wake vortex, and define critical scour velocity ($V_c$) and equilibrium scour depth ($d_s$).](#Q9.3:%20Explain%20the%20scour%20process%20around%20a%20bridge%20pier%20with%20a%20figure.%20Distinguish%20between%20horseshoe%20vortex%20and%20wake%20vortex,%20and%20define%20critical%20scour%20velocity%20($V_c$)%20and%20equilibrium%20scour%20depth%20($d_s$).) 4. [Q9.4: Explain how flow depth ($y$), pier width ($b_p$) (narrow vs. wide piers), sediment size ($d_{50}$), and angle of attack ($\theta$) affect local pier scour.](#Q9.4:%20Explain%20how%20flow%20depth%20($y$),%20pier%20width%20($b_p$)%20(narrow%20vs.%20wide%20piers),%20sediment%20size%20($d_%7B50%7D$),%20and%20angle%20of%20attack%20($%5Ctheta$)%20affect%20local%20pier%20scour.) 5. [Q9.5: Why is pier scour depth generally higher than abutment scour depth?](#Q9.5:%20Why%20is%20pier%20scour%20depth%20generally%20higher%20than%20abutment%20scour%20depth?) 6. [Q9.6: Distinguish between clear-water scour and live-bed scour with a graph. Which gives a higher scour depth and why?](#Q9.6:%20Distinguish%20between%20clear-water%20scour%20and%20live-bed%20scour%20with%20a%20graph.%20Which%20gives%20a%20higher%20scour%20depth%20and%20why?) 7. [Q10.1: What is a complex pier? Sketch Cases I–IV when a normal pier must be treated as a complex pier, and explain why complex pier scour is greater than normal pier scour.](#Q10.1:%20What%20is%20a%20complex%20pier?%20Sketch%20Cases%20I%E2%80%93IV%20when%20a%20normal%20pier%20must%20be%20treated%20as%20a%20complex%20pier,%20and%20explain%20why%20complex%20pier%20scour%20is%20greater%20than%20normal%20pier%20scour.) 8. [Q10.2: [Math] Calculate the effective/equivalent pier diameter ($b_e$) for a complex bridge pier with its pile cap at riverbed level.](#Q10.2:%20%5BMath%5D%20Calculate%20the%20effective/equivalent%20pier%20diameter%20($b_e$)%20for%20a%20complex%20bridge%20pier%20with%20its%20pile%20cap%20at%20riverbed%20level.) 9. [Q10.3: [Math & Theory] Calculate maximum scour depth around a bridge pier using the Lacey–Inglis method and state its limitations.](#Q10.3:%20%5BMath%20&%20Theory%5D%20Calculate%20maximum%20scour%20depth%20around%20a%20bridge%20pier%20using%20the%20Lacey%E2%80%93Inglis%20method%20and%20state%20its%20limitations.) 10. [Q10.4: Explain sacrificial piles as a flow-altering countermeasure against bridge pier scour.](#Q10.4:%20Explain%20sacrificial%20piles%20as%20a%20flow-altering%20countermeasure%20against%20bridge%20pier%20scour.) 10. [SA Lecture 11: River Training & Bank Protection Works](#SA%20Lecture%2011:%20River%20Training%20&%20Bank%20Protection%20Works) 1. [Q11.1: Explain with examples the differences between active (transverse) and passive (longitudinal) river training structures.](#Q11.1:%20Explain%20with%20examples%20the%20differences%20between%20active%20(transverse)%20and%20passive%20(longitudinal)%20river%20training%20structures.) 2. [Q11.2: Discuss the advantages and disadvantages of the soft approach / Nature-Based Solutions (NBS) / bioengineering in riverbank protection.](#Q11.2:%20Discuss%20the%20advantages%20and%20disadvantages%20of%20the%20soft%20approach%20/%20Nature-Based%20Solutions%20(NBS)%20/%20bioengineering%20in%20riverbank%20protection.) 3. [Q11.3: What is a spur/groyne and what is a slack flow zone? Explain with sketches how a series of spurs develops a new bankline, permeable vs. impermeable groynes, and attracting, deflecting, and repelling groynes.](#Q11.3:%20What%20is%20a%20spur/groyne%20and%20what%20is%20a%20slack%20flow%20zone?%20Explain%20with%20sketches%20how%20a%20series%20of%20spurs%20develops%20a%20new%20bankline,%20permeable%20vs.%20impermeable%20groynes,%20and%20attracting,%20deflecting,%20and%20repelling%20groynes.) 4. [Q11.4: State the design rules for groynes (length, spacing, number) and their role in flood/erosion management in Bangladesh.](#Q11.4:%20State%20the%20design%20rules%20for%20groynes%20(length,%20spacing,%20number)%20and%20their%20role%20in%20flood/erosion%20management%20in%20Bangladesh.) 5. [Q11.5: Differentiate between open-joint (flexible) and close-joint (rigid) revetments and state where each is suitable.](#Q11.5:%20Differentiate%20between%20open-joint%20(flexible)%20and%20close-joint%20(rigid)%20revetments%20and%20state%20where%20each%20is%20suitable.) 6. [Q11.6: Define and differentiate between Riprap and Gabion revetments.](#Q11.6:%20Define%20and%20differentiate%20between%20Riprap%20and%20Gabion%20revetments.) 7. [Q11.7: What are Bandals and how do they protect riverbanks and improve dry-season navigation channels?](#Q11.7:%20What%20are%20Bandals%20and%20how%20do%20they%20protect%20riverbanks%20and%20improve%20dry-season%20navigation%20channels?) 11. [SA Lecture 12: Inland Navigation, Meander Cut-Offs & River Dredging](#SA%20Lecture%2012:%20Inland%20Navigation,%20Meander%20Cut-Offs%20&%20River%20Dredging) 1. [Q12.1: Why is inland water transport the cheapest mode of transport? State its advantages over road and rail.](#Q12.1:%20Why%20is%20inland%20water%20transport%20the%20cheapest%20mode%20of%20transport?%20State%20its%20advantages%20over%20road%20and%20rail.) 2. [Q12.2: Explain the morphological and flow factors that affect the navigability of a river.](#Q12.2:%20Explain%20the%20morphological%20and%20flow%20factors%20that%20affect%20the%20navigability%20of%20a%20river.) 3. [Q12.3: Explain the three methods of achieving or improving the navigability of a river.](#Q12.3:%20Explain%20the%20three%20methods%20of%20achieving%20or%20improving%20the%20navigability%20of%20a%20river.) 4. [Q12.4: Explain with a figure the morphological consequences of a meander cut-off.](#Q12.4:%20Explain%20with%20a%20figure%20the%20morphological%20consequences%20of%20a%20meander%20cut-off.) 5. [Q12.5: Explain the four phases of dredging, compare hydraulic vs. mechanical dredgers (and cutter suction dredger), and distinguish between capital and maintenance dredging.](#Q12.5:%20Explain%20the%20four%20phases%20of%20dredging,%20compare%20hydraulic%20vs.%20mechanical%20dredgers%20(and%20cutter%20suction%20dredger),%20and%20distinguish%20between%20capital%20and%20maintenance%20dredging.) 6. [Q12.6: Explain with longitudinal profile figures why capital/navigation dredging is generally not sustainable in alluvial rivers.](#Q12.6:%20Explain%20with%20longitudinal%20profile%20figures%20why%20capital/navigation%20dredging%20is%20generally%20not%20sustainable%20in%20alluvial%20rivers.) 2. [Section B: Flood Management (`EWCE 461/SB`)](#Section%20B:%20Flood%20Management%20(%60EWCE%20461/SB%60)) 1. [SB Lecture 1: Overview of Floods, Types & Causes of Floods in Bangladesh](#SB%20Lecture%201:%20Overview%20of%20Floods,%20Types%20&%20Causes%20of%20Floods%20in%20Bangladesh) 1. [Q1.1: Why are normal floods considered a blessing ("Borsha") rather than a disaster ("Bonna")? What are the positive outcomes of normal and flash floods?](#Q1.1:%20Why%20are%20normal%20floods%20considered%20a%20blessing%20(%22Borsha%22)%20rather%20than%20a%20disaster%20(%22Bonna%22)?%20What%20are%20the%20positive%20outcomes%20of%20normal%20and%20flash%20floods?) 2. [Q1.2: Explain the four major types of floods in Bangladesh with their locations, timing, and impacts.](#Q1.2:%20Explain%20the%20four%20major%20types%20of%20floods%20in%20Bangladesh%20with%20their%20locations,%20timing,%20and%20impacts.) 3. [Q1.3: Explain the geographical, physiographic, and hydro-meteorological factors responsible for recurring floods in Bangladesh.](#Q1.3:%20Explain%20the%20geographical,%20physiographic,%20and%20hydro-meteorological%20factors%20responsible%20for%20recurring%20floods%20in%20Bangladesh.) 4. [Q1.4: Briefly describe the river system network of Bangladesh and the dominance of the Brahmaputra–Jamuna system.](#Q1.4:%20Briefly%20describe%20the%20river%20system%20network%20of%20Bangladesh%20and%20the%20dominance%20of%20the%20Brahmaputra%E2%80%93Jamuna%20system.) 2. [SB Lecture 2: Flood Process in Urban and Rural Areas](#SB%20Lecture%202:%20Flood%20Process%20in%20Urban%20and%20Rural%20Areas) 1. [Q2.1: Define urban flooding. How does urban flooding differ from rural flooding? Explain the major impacts of urban flooding.](#Q2.1:%20Define%20urban%20flooding.%20How%20does%20urban%20flooding%20differ%20from%20rural%20flooding?%20Explain%20the%20major%20impacts%20of%20urban%20flooding.) 3. [SB Lecture 3: Flood Management Measures, Dams & Hydraulic Structures](#SB%20Lecture%203:%20Flood%20Management%20Measures,%20Dams%20&%20Hydraulic%20Structures) 1. [Q3.1: Outline the four phases of planning for flood management.](#Q3.1:%20Outline%20the%20four%20phases%20of%20planning%20for%20flood%20management.) 2. [Q3.2: Differentiate between "Flood Control" and "Flood Management." Illustrate short-term and long-term measures for flood management.](#Q3.2:%20Differentiate%20between%20%22Flood%20Control%22%20and%20%22Flood%20Management.%22%20Illustrate%20short-term%20and%20long-term%20measures%20for%20flood%20management.) 3. [Q3.3: On what factors do damages from a flood event depend?](#Q3.3:%20On%20what%20factors%20do%20damages%20from%20a%20flood%20event%20depend?) 4. [Q3.4: Explain structural and non-structural measures for flood management and how they are applied in Bangladesh.](#Q3.4:%20Explain%20structural%20and%20non-structural%20measures%20for%20flood%20management%20and%20how%20they%20are%20applied%20in%20Bangladesh.) 5. [Q3.5: Draw a typical dam/reservoir layout showing storage zones, classify dams according to use, differentiate between Weir, Barrage, and Dam, and list the causes of dam failure.](#Q3.5:%20Draw%20a%20typical%20dam/reservoir%20layout%20showing%20storage%20zones,%20classify%20dams%20according%20to%20use,%20differentiate%20between%20Weir,%20Barrage,%20and%20Dam,%20and%20list%20the%20causes%20of%20dam%20failure.) 6. [Q3.6: Write short notes on: (i) Floodwall and Levee, (ii) Gravity Dam, and (iii) Diversion Dam.](#Q3.6:%20Write%20short%20notes%20on:%20(i)%20Floodwall%20and%20Levee,%20(ii)%20Gravity%20Dam,%20and%20(iii)%20Diversion%20Dam.) 4. [SB Lecture 4: Diversion Channels, Floodways & Floodwalls](#SB%20Lecture%204:%20Diversion%20Channels,%20Floodways%20&%20Floodwalls) 1. [Q4.1: Define flood diversion channel (floodway). Explain its importance, advantages, and disadvantages for flood management, and distinguish between SPF and PMF.](#Q4.1:%20Define%20flood%20diversion%20channel%20(floodway).%20Explain%20its%20importance,%20advantages,%20and%20disadvantages%20for%20flood%20management,%20and%20distinguish%20between%20SPF%20and%20PMF.) 5. [SB Lecture 5: Improvement of Channel Capacity by Dredging](#SB%20Lecture%205:%20Improvement%20of%20Channel%20Capacity%20by%20Dredging) 1. [Q5.1: Explain the purposes of river dredging and the morphological, hydraulic, and environmental impacts of dredging.](#Q5.1:%20Explain%20the%20purposes%20of%20river%20dredging%20and%20the%20morphological,%20hydraulic,%20and%20environmental%20impacts%20of%20dredging.) 6. [SB Lecture 6: Land & Watershed Management for Flood Mitigation](#SB%20Lecture%206:%20Land%20&%20Watershed%20Management%20for%20Flood%20Mitigation) 1. [Q6.1: What is watershed management and what are its main components? Why does land/watershed management play a key role compared to structural flood measures?](#Q6.1:%20What%20is%20watershed%20management%20and%20what%20are%20its%20main%20components?%20Why%20does%20land/watershed%20management%20play%20a%20key%20role%20compared%20to%20structural%20flood%20measures?) 2. [Q6.2: Explain the effect of land-use change processes (urbanization, floodplain roads/embankments, and agricultural practices) on flood hazards, and outline the land-use & flood management planning process.](#Q6.2:%20Explain%20the%20effect%20of%20land-use%20change%20processes%20(urbanization,%20floodplain%20roads/embankments,%20and%20agricultural%20practices)%20on%20flood%20hazards,%20and%20outline%20the%20land-use%20&%20flood%20management%20planning%20process.) 7. [SB Lecture 7: Flood Proofing Strategies](#SB%20Lecture%207:%20Flood%20Proofing%20Strategies) 1. [Q7.1: Define flood proofing and state its advantages and disadvantages.](#Q7.1:%20Define%20flood%20proofing%20and%20state%20its%20advantages%20and%20disadvantages.) 2. [Q7.2: Briefly explain the seven considerations for developing a flood proofing strategy.](#Q7.2:%20Briefly%20explain%20the%20seven%20considerations%20for%20developing%20a%20flood%20proofing%20strategy.) 3. [Q7.3: Explain the six flood proofing strategies outlined by FEMA and how flood proofing is practiced in vulnerable char/coastal areas of Bangladesh.](#Q7.3:%20Explain%20the%20six%20flood%20proofing%20strategies%20outlined%20by%20FEMA%20and%20how%20flood%20proofing%20is%20practiced%20in%20vulnerable%20char/coastal%20areas%20of%20Bangladesh.) 8. [SB Lecture 8: Floodplain Zoning & Flood Mapping](#SB%20Lecture%208:%20Floodplain%20Zoning%20&%20Flood%20Mapping) 1. [Q8.1: Define floodplain, flood map (FIRM, FBFM, FHBM), and floodplain zoning. Explain the advantages and disadvantages of floodplain zoning ("Floodplain zoning reduces the risk and cost of damages").](#Q8.1:%20Define%20floodplain,%20flood%20map%20(FIRM,%20FBFM,%20FHBM),%20and%20floodplain%20zoning.%20Explain%20the%20advantages%20and%20disadvantages%20of%20floodplain%20zoning%20(%22Floodplain%20zoning%20reduces%20the%20risk%20and%20cost%20of%20damages%22).) 2. [Q8.2: Describe the different flood zones classified based on annual flood occurrence probability (Zones 1, 2, 3a, 3b & FEMA zones), their roles in flood management, and the approaches and data required for flood zoning.](#Q8.2:%20Describe%20the%20different%20flood%20zones%20classified%20based%20on%20annual%20flood%20occurrence%20probability%20(Zones%201,%202,%203a,%203b%20&%20FEMA%20zones),%20their%20roles%20in%20flood%20management,%20and%20the%20approaches%20and%20data%20required%20for%20flood%20zoning.) 9. [SB Lecture 9: Flood Hazard Mapping, Risk Analysis & Frequency Analysis](#SB%20Lecture%209:%20Flood%20Hazard%20Mapping,%20Risk%20Analysis%20&%20Frequency%20Analysis) 1. [Q9.1: Define and relate Flood Hazard ($H$), Exposure ($E$), Susceptibility, Vulnerability ($V$), Flood Damage ($D$), and Flood Risk ($R$). Explain how flood damage is assessed using stage-damage curves.](#Q9.1:%20Define%20and%20relate%20Flood%20Hazard%20($H$),%20Exposure%20($E$),%20Susceptibility,%20Vulnerability%20($V$),%20Flood%20Damage%20($D$),%20and%20Flood%20Risk%20($R$).%20Explain%20how%20flood%20damage%20is%20assessed%20using%20stage-damage%20curves.) 2. [Q9.2: [Math] Calculate annual flood risk ($R$) for island/floodplain communities and determine the economic risk-reduction benefit of building an embankment.](#Q9.2:%20%5BMath%5D%20Calculate%20annual%20flood%20risk%20($R$)%20for%20island/floodplain%20communities%20and%20determine%20the%20economic%20risk-reduction%20benefit%20of%20building%20an%20embankment.) 3. [Q9.3: [Math] Perform flood or rainfall frequency analysis using Normal and Log-Normal probability distributions.](#Q9.3:%20%5BMath%5D%20Perform%20flood%20or%20rainfall%20frequency%20analysis%20using%20Normal%20and%20Log-Normal%20probability%20distributions.) 10. [SB Lecture 10: Flood Forecasting and Warning System (FFWS)](#SB%20Lecture%2010:%20Flood%20Forecasting%20and%20Warning%20System%20(FFWS)) 1. [Q10.1: Define flood forecasting and flood warning. Explain the role of flood forecasting in flood management, and describe the data requirements, models, and limitations of flood forecasting in Bangladesh.](#Q10.1:%20Define%20flood%20forecasting%20and%20flood%20warning.%20Explain%20the%20role%20of%20flood%20forecasting%20in%20flood%20management,%20and%20describe%20the%20data%20requirements,%20models,%20and%20limitations%20of%20flood%20forecasting%20in%20Bangladesh.) 11. [SB Lecture 11: Flood Damage Assessment & Economic Evaluation of Flood Mitigation Projects](#SB%20Lecture%2011:%20Flood%20Damage%20Assessment%20&%20Economic%20Evaluation%20of%20Flood%20Mitigation%20Projects) 1. [Q11.1: Differentiate between tangible (direct and indirect) and intangible flood damages with examples for urban and rural areas.](#Q11.1:%20Differentiate%20between%20tangible%20(direct%20and%20indirect)%20and%20intangible%20flood%20damages%20with%20examples%20for%20urban%20and%20rural%20areas.) 2. [Q11.2: Briefly explain the five approaches for the economic evaluation of flood mitigation projects.](#Q11.2:%20Briefly%20explain%20the%20five%20approaches%20for%20the%20economic%20evaluation%20of%20flood%20mitigation%20projects.) 3. [Q11.3: [Math] Recommend the optimal design river stage (height) for a flood levee from flood stage, damage, return period, and construction cost data.](#Q11.3:%20%5BMath%5D%20Recommend%20the%20optimal%20design%20river%20stage%20(height)%20for%20a%20flood%20levee%20from%20flood%20stage,%20damage,%20return%20period,%20and%20construction%20cost%20data.) --- # EWCE 461: River Engineering & Flood Management — Past Exam Question & Answer Bank (2019–2025) > **How to Use This Guide (1 Mark = 1 Minute Rule):** > > - Every answer is calibrated for **handwritten exam speed** at **1 minute per mark** (including reading the question and writing/sketching by hand). > - **2 marks (2 mins):** 2 short sentences (~25–35 words). > - **4–5 marks (4–5 mins):** 4–5 short bullet points (~50–75 words) + a simple 30-second sketch if asked. > - **6–8 marks (6–8 mins):** 6–8 short bullet points or a compact table (~80–120 words). > - **10–12 marks (10–12 mins):** 8–10 structured bullet points (~130–180 words). > - **Numerical Problems:** Only the **step-by-step methodology and formulas** are given. --- ## Section A: River Engineering (`EWCE 461/SA`) --- ### SA Lecture 1: Introduction to River Systems, Longitudinal Profile & Channel Classification - **Source:** [SA/EWCE 461-Lec1.pdf](SA/EWCE%20461-Lec1.pdf) --- #### Q1.1: Explain with a sketch the different zones along a river longitudinal profile and how sediment size, slope, discharge, width, depth, and velocity change from upstream to downstream. - **Appeared in:** 2022 (Q1b, 6m), 2024 (Q1a, 5m), 2025 (Q2b, 6m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 1](SA/EWCE%20461-Lec1.pdf) p. 3–7 **Sketch to Draw (45 sec):** ```text Elevation ^ | \ 1. Erosional Zone (Steep, Boulders/Cobbles, Many Tributaries) | \ | \___ 2. Transport Zone (Moderate Slope, Gravel/Sand, Single Channel) | \_______ 3. Depositional Zone (Flat, Silt/Clay, Delta Branches) +-------------------------------------------------------------> Distance Downstream ``` **Three Longitudinal Zones:** 1. **Erosional (Source) Zone:** Steep mountain headwaters where many tributaries join; bed erodes downward (degradation) and holds boulders and cobbles. 2. **Transport (Transfer) Zone:** Middle valley where sediment inflow equals outflow (stable bed); flows in a single meandering or braided channel with gravel and sand. 3. **Depositional (Sink) Zone:** Flat coastal/delta plain where the river splits into distributaries and deposits fine sand, silt, and clay (aggradation). **Changes from Upstream to Downstream:** - **Decrease downstream:** Channel slope ($S$) and bed sediment grain size ($D_{50}$). - **Increase downstream:** Water discharge ($Q$), channel width ($W$), flow depth ($D$), and mean flow velocity ($V$, which rises slightly because deeper flow reduces friction more than flatter slope slows it down). --- #### Q1.2: Define floodplain, natural levee/terrace, thalweg, tributary, and distributary. - **Appeared in:** 2024 (Q6d, 2m) - **Time & Length Budget:** 2–3 Minutes | [SA: Lec 1](SA/EWCE%20461-Lec1.pdf) p. 8–13 - **Floodplain:** Low, flat land beside a river channel that gets covered by water when floods overtop the banks (typically every $1.5\text{–}2\text{ years}$). - **Natural Levee:** Raised ridge of coarse sand/silt deposited right along the riverbanks during floods. - **Terrace:** An old, abandoned floodplain left at a higher level after the river cuts deeper into its valley. - **Thalweg:** A continuous line connecting the deepest points along a river bed. - **Tributary vs. Distributary:** A **tributary** is a smaller stream that flows *into* a main river (upstream), whereas a **distributary** branches *off and away* from the main river (in a delta). --- #### Q1.3: What is an alluvial river? How are alluvial rivers classified based on sinuosity and multichanneledness? Explain with a sketch. - **Appeared in:** 2019 (Q1a, 6m), 2025 (Q1a, 5m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 1](SA/EWCE%20461-Lec1.pdf) p. 17–18, 25–26 - **Alluvial River:** A river whose bed and banks are made of loose sediment (sand, silt, gravel) deposited by the river itself. Because the river can erode and rebuild its own banks, it is self-forming and changes shape easily. **Classification by Sinuosity & Multichanneledness:** 1. **Sinuosity Index ($SI$):** Ratio of curved channel length to straight valley length ($SI = L_{\text{channel}} / L_{\text{straight}}$, ranging from $1$ to $\sim 4$). 2. **Multichanneledness:** Whether the river flows in a **single channel** or splits around islands/bars into **multiple channels**. **Sketch & Four River Patterns:** ```text SINGLE CHANNEL MULTIPLE CHANNELS +-----------------------------+-----------------------------+ LOW SINUOSITY | 1. STRAIGHT (SI < 1.05) | 3. BRAIDED (Low SI, >1 ch.) | (Nearly straight) | ====== (Alternate bars) | ===(bar)===(bar)=== | +-----------------------------+-----------------------------+ HIGH SINUOSITY | 2. MEANDERING (SI > 1.5) | 4. ANASTOMOSING (High SI) | (Winding bends) | ~~\__/~~\__/~~ (Point bar)| ~~(island)~~(island)~~ | +-----------------------------+-----------------------------+ ``` - *Note:* **Meandering** and **Braided** are common in nature; **Straight** and **Anastomosing** are rare. --- #### Q1.4: Classify rivers based on sediment load and compare their width-depth ratio, slope, and bank stability. - **Appeared in:** 2022 (Q2b, 5m), 2023 (Q2a, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 1](SA/EWCE%20461-Lec1.pdf) p. 22–24 | River Class | Dominant Sediment | Bed Load (% of Total) | Width/Depth ($W/D$) | Slope & Bank Stability | Typical Pattern | | :--- | :--- | :--- | :--- | :--- | :--- | | **1. Suspended-Load** | Silt & clay (cohesive) | $< 3\%$ | Narrow & deep ($W/D < 10$) | Gentle slope; sticky, strong banks resist erosion | Single meandering ($SI > 2.0$) | | **2. Mixed-Load** | Silt, sand & fine gravel | $3\%\text{–}11\%$ | Moderate ($10 < W/D < 40$) | Moderate slope; moderately stable banks | Sinuous / meandering ($SI \approx 1.3\text{–}2.0$) | | **3. Bed-Load** | Coarse sand & gravel (non-cohesive) | $> 11\%$ | Wide & shallow ($W/D > 40$) | Steep slope; loose banks erode easily | Braided multi-channel ($SI < 1.3$) | --- #### Q1.5: What is Planform Index ($PFI$)? Write its equation, threshold values, and the $PFI$ status of the Jamuna River. - **Appeared in:** 2024 (Q8c, 8m — part) - **Time & Length Budget:** 4 Minutes | [SA: Lec 1](SA/EWCE%20461-Lec1.pdf) p. 29–30 - **Definition:** Planform Index ($PFI$) measures the degree of braiding by giving the average percentage of total river width occupied by each active flow branch. - **Formula:** $$PFI = \frac{\sum T_i}{B} \times \frac{100}{N}$$ where $T_i =$ water surface top width of branch $i$, $B =$ total overall river corridor width, and $N =$ number of active braided branches across the section. - **Classification Thresholds (a lower $PFI$ means stronger braiding):** - **Highly Braided:** $PFI < 3$ - **Moderately Braided:** $3 \le PFI \le 20$ - **Low Braided:** $PFI > 20$ - **Jamuna River Status:** $PFI$ ranges from **$4.3$ to $18.2$**, classifying the Jamuna as a **moderately to highly braided river** whose braiding intensity has increased over recent decades. --- ### SA Lecture 2: Fluvial Geomorphology, Channel Patterns & Hydraulic Geometry - **Source:** [SA/EWCE-461-Lec2.pdf](SA/EWCE-461-Lec2.pdf) --- #### Q2.1: Define channel pattern. Explain the four variables that influence channel pattern (slope, sediment load/size, sinuosity, and width-depth ratio / bank cohesiveness) with sketches. - **Appeared in:** 2019 (Q2a, 10m), 2021 (Q1a, 5m), 2022 (Q1a, 8m), 2023 (Q1a, 12m), 2024 (Q1c, 5m), 2025 (Q1b, 5m; Q2a, 5m) - **Time & Length Budget:** 5–10 Minutes | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 2–13 - **Channel Pattern:** The top-view (planform) shape of a river reach as seen from above—classified as **straight**, **meandering**, or **braided**. - *(Note: Water discharge $Q$ controls channel size—width $W$, depth $D$, meander wavelength $M_L$—but does not by itself set the basic pattern).* **Four Controlling Variables:** 1. **Channel Slope ($S$):** - For a constant discharge $Q$, a very flat slope gives a **straight** channel, a moderate slope produces a **meandering** channel, and a steep slope above a critical threshold ($S_{cr} = 0.012 Q^{-0.44}$) causes a **braided** channel. 2. **Sediment Load & Grain Size ($Q_s, d_{50}$):** - Fine suspended sediment (silt/clay) forms cohesive banks and a single **meandering** channel. - Heavy coarse bed load (sand/gravel) exceeds the river's carrying power, dumping mid-channel bars that split the flow into a **braided** pattern. 3. **Sinuosity ($SI$):** - As valley slope increases from zero, sinuosity rises (straight $\to$ highly meandering) up to a maximum stability peak; beyond the threshold slope, banks collapse, bends cut off, and sinuosity drops into a straighter **braided** belt. 4. **Width-Depth Ratio ($W/D$) & Bank Cohesiveness (Silt-Clay %):** - High silt-clay percentage in banks makes soil sticky and erosion-resistant $\to$ deep, narrow channel ($W/D < 10$) $\to$ **meandering**. - Sandy, non-cohesive banks erode easily $\to$ wide, shallow channel ($W/D > 40$) $\to$ **braided**. **Sketches to Draw (1 min):** ```text (a) Slope vs Discharge (Leopold & Wolman) (b) Sinuosity vs Slope (Schumm) Slope S ^ Sinuosity ^ Peak Meandering | BRAIDED | / \ | ---------------- S = 0.012 Q^(-0.44) | / \ BRAIDED | MEANDERING | / \ (Bars) | ................ | / \ | STRAIGHT 1.0 +-----/---------\------> +----------------------> Discharge Q 0 Flat Steep Slope S ``` --- #### Q2.2: What is a flashy river and where are flashy rivers found in Bangladesh? - **Appeared in:** 2025 (Q2b, 6m — part) - **Time & Length Budget:** 2 Minutes | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 16 - **Flashy River:** A steep mountain-fed river where heavy rain causes water level and discharge to rise very fast, reach a sharp high peak, and fall back down within hours or a few days. - **Locations in Bangladesh:** Found along the **north-eastern (Sylhet/Sunamganj haor border, e.g., Khowai, Manu)**, **northern**, and **south-eastern hilly regions (Chattogram Hill Tracts, e.g., Halda, Sangu, Matamuhuri)**. --- #### Q2.3: Explain the concept of hydraulic geometry of alluvial channels and its two types. - **Appeared in:** 2023 (Q1b, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 19–25 - **Concept:** Introduced by Leopold & Maddock (1953), hydraulic geometry expresses water surface width ($w$), mean depth ($d$), and mean velocity ($u$) as simple power functions of water discharge ($Q$): $$w = a Q^b, \qquad d = c Q^f, \qquad u = k Q^m$$ - **Continuity Conditions:** Since $Q = w \cdot d \cdot u = (a c k) Q^{b+f+m}$, the constants must satisfy: $$a \times c \times k = 1 \qquad \text{and} \qquad b + f + m = 1$$ - **Two Types of Hydraulic Geometry:** 1. **At-a-station:** Shows how $w, d, u$ change **over time** at a single cross-section as flow rises and falls (typical exponents: $b=0.26, f=0.40, m=0.34$). 2. **Downstream:** Shows how $w, d, u$ change **along the river's length** at a reference frequency (such as bankfull or mean annual discharge) as tributaries add water ($b=0.50, f=0.40, m=0.10$). --- #### Q2.4: [Math] Establish the hydraulic geometry relationship between channel width ($W$) and discharge ($Q$) ($W = aQ^b$) from observed field data. - **Appeared in:** 2019 (Q1c, 14m), 2022 (Q1c, 16m) - **Time & Length Budget:** Methodology Only | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 21, 25 **Step-by-Step Methodology:** 1. **Write the power-law equation:** $$W = a Q^b$$ 2. **Linearize by taking $\log_{10}$ on both sides:** $$\log_{10} W = \log_{10} a + b \log_{10} Q \quad \Longleftrightarrow \quad Y = A + b X$$ where $X = \log_{10} Q$, $Y = \log_{10} W$, and $A = \log_{10} a$. 3. **Tabulate values:** For all $n$ given data pairs $(Q_i, W_i)$, compute $X_i = \log_{10} Q_i$, $Y_i = \log_{10} W_i$, $X_i^2$, and $X_i Y_i$. 4. **Find exponent $b$ and coefficient $a$ (by linear regression or log-log graph):** $$b = \frac{n\sum X_i Y_i - (\sum X_i)(\sum Y_i)}{n\sum X_i^2 - (\sum X_i)^2}, \qquad A = \bar{Y} - b\bar{X}, \qquad a = 10^A$$ 5. **Write final equation:** Substitute $a$ and $b$ into $W = a Q^b$ and comment on whether width grows slowly ($b < 0.3$, cohesive banks) or rapidly ($b \ge 0.5$, erodible/braided banks) with discharge. --- #### Q2.5: Define bankfull discharge. Explain how the width-depth ($W/D$) ratio varies with water depth and how to find bankfull depth from cross-section data. - **Appeared in:** 2021 (Q1d, 12m — Math & Theory), 2024 (Q1b, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 26–29 - **Bankfull Discharge:** The flow rate that just fills the main river channel to the top of its banks before spilling out onto the flat floodplain. It occurs roughly once every **$1.5\text{ to }2\text{ years}$** and is the main channel-forming (dominant) discharge because it does the most long-term sediment transport work. **Variation of $W/D$ Ratio with Depth (with Sketch):** ```text Floodplain ________| |________ <-- Above bankfull: W jumps, W/D increases sharply | | Bankfull | | <-- Bankfull level: Minimum W/D ratio \ | Channel | / <-- Below bankfull: D rises faster than W, W/D falls \______|________________|______/ ``` - **Below bankfull:** As water rises inside the steep banks, depth $D$ increases faster than top width $W$, so the **$W/D$ ratio decreases**. - **At bankfull stage:** The **$W/D$ ratio reaches its minimum value**. - **Above bankfull:** Water spills onto the wide, flat floodplain; top width $W$ jumps suddenly while depth $D$ increases very little, causing the **$W/D$ ratio to shoot up sharply**. **Methodology for Numerical Problem (2021 Q1d):** 1. For each row of Depth ($D$) and Width ($W$) in the given table, calculate the ratio $W/D$. 2. Identify the depth where **$W/D$ is minimum** (right before width $W$ jumps abruptly)—that depth is the **bankfull depth**. --- #### Q2.6: Explain why the longitudinal bed slope of a river decreases downstream (concave-upward profile). - **Appeared in:** 2022 (Q1b, 6m), 2024 (Q1a, 5m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 2](SA/EWCE-461-Lec2.pdf) p. 30–32 Rivers develop a concave-upward profile where slope drops exponentially downstream ($S = S_0 e^{-\alpha x}$) for **three reasons**: 1. **Abrasion of sediment ($d = d_0 e^{-cx}$):** Stones and sand grains collide and grind against each other as they travel, becoming smaller and rounder downstream; finer grains need a much flatter slope to be carried. 2. **Upstream sediment source & valley deposition:** Most sediment erodes from steep mountain headwaters, while part of the load settles along the valley floor, lowering the coarse sediment concentration downstream. 3. **Tributary water inflow:** Tributaries add more water discharge ($Q$) downstream relative to coarse sediment load ($Q_s$), so the larger flow can transport the sediment on a gentler slope. --- ### SA Lecture 3: Meander Geometry, Mechanics & Formation Theories - **Source:** [SA/EWCE-461-Lec3.pdf](SA/EWCE-461-Lec3.pdf) --- #### Q3.1: Why does mean flow velocity increase slightly from upstream to downstream even though river slope decreases? - **Appeared in:** 2025 (Q2b, 6m — part) - **Time & Length Budget:** 3 Minutes | [SA: Lec 3](SA/EWCE-461-Lec3.pdf) p. 1 - By Manning's equation, $V = \frac{1}{n} d^{2/3} S^{1/2}$. - Going downstream, slope $S$ decreases, which tends to reduce velocity. - However, flow depth $d$ increases strongly as tributaries join, and bed roughness $n$ decreases because boulders/gravel turn into fine sand/silt. - Because the increase in depth ($d^{2/3}$) and drop in roughness ($1/n$) outweigh the decrease in slope ($S^{1/2}$), mean velocity **increases slightly downstream**. --- #### Q3.2: Briefly discuss the five theories of river meandering. - **Appeared in:** 2019 (Q3a, 6m), 2022 (Q2a, 6m), 2025 (Q1c, 5m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 3](SA/EWCE-461-Lec3.pdf) p. 5–7 1. **Earth's Rotation (Coriolis Force) Theory:** Earth's spin deflects flowing water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, pressing current against one bank and initiating bends. 2. **Disturbance Theory:** During floods, excess sediment deposits as a bar on the riverbed, locally steepening the slope and deflecting the main current into one bank; the deflected flow then rebounds to the opposite bank downstream. 3. **Excess Energy Theory:** A river with too much energy lengthens its path by curving into meanders, which flattens its slope until it reaches a stable minimum-energy state. 4. **Secondary Flow Theory:** Transverse cross-currents (helical spiral flow) develop inside the channel, creating an uneven velocity profile that erodes one bank and deposits on the other. 5. **Wave Motion (Flag Fluttering) Theory:** Like a flag waving in a steady wind, turbulent shear stress along the river boundaries creates natural wave-like oscillations that grow into regular meander bends. --- #### Q3.3: [Math] Establish the relationship between Slope and Discharge ($S = pQ^{-z}$) or Meander Wavelength and Channel Width ($SP = aW^b, M_L = 2 \cdot SP$). - **Appeared in:** 2019 (Q2d, 10m), 2023 (Q1c, 13m), 2024 (Q2d, 15m) - **Time & Length Budget:** Methodology Only | [SA: Lec 3](SA/EWCE-461-Lec3.pdf) p. 3, 13–14 **Step-by-Step Methodology:** 1. **Write the power-law relation:** - For Slope–Discharge: $S = p Q^{-z}$ - For Pool-to-Pool Spacing vs. Width: $SP = a W^b$ 2. **Linearize with $\log_{10}$:** - $\log_{10} S = \log_{10} p - z \log_{10} Q$ - $\log_{10} (SP) = \log_{10} a + b \log_{10} W$ 3. **Perform linear regression (or plot $\log Y$ vs. $\log X$):** Calculate slope ($-z$ or $b$) and intercept ($\log_{10} p$ or $\log_{10} a$) from the given table. 4. **Convert Pool-to-Pool Spacing ($SP$) to Meander Wavelength ($M_L$) (for 2023 Q1c & 2024 Q2d):** - Since one complete S-shaped meander wavelength contains **two bends (two pools)**, multiply the spacing equation by $2$: $$M_L = 2 \times SP = 2 a W^b \quad (\text{typically } M_L \approx 10 W\text{ to }11 W)$$ --- #### Q3.4: Define pool, riffle, and helical current. Explain with a sketch how helical flow causes outer-bank erosion and inner-bank point bar deposition at a river bend. - **Appeared in:** 2024 (Q2c, 5m), 2025 (Q4c, 2m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 3](SA/EWCE-461-Lec3.pdf) p. 11–12, 16–20 - **Pool:** Deep scour zone located along the outer (concave) bank of a meander bend (asymmetric V-shaped cross-section). - **Riffle:** Shallow, straight crossing zone between two bends where coarse sediment accumulates (symmetric cross-section). - **Helical Current:** A 3D corkscrew (spiral) flow pattern that develops as water rounds a curve. **Mechanism & Sketch at a River Bend:** ```text INNER BANK (Convex) OUTER BANK (Concave) [Point Bar Deposition] Surface Water ---> [Undercutting & Erosion] \___.___._____ (Fast, Centrifugal) | | \____ Downflow| | Deep Pool \_____ <--- Bottom Water |__| \_______(Slow, Pressure Gradient) ``` 1. **At the water surface:** Fast-moving water experiences a strong outward **centrifugal force** that exceeds the inward pressure gradient, pushing surface water toward the **outer bank**. 2. **At the outer bank:** Water piles up (super-elevation) and dives downward, scouring the toe of the outer bank to form a deep **pool**. 3. **Along the riverbed:** Slow bottom water has weak centrifugal force, so the inward **pressure gradient** pushes it toward the **inner bank**, carrying eroded bed sediment to build a **point bar**. --- ### SA Lecture 4: Channel Stability & Sediment Transport Modes - **Source:** [SA/EWCE-461-Lec4.pdf](SA/EWCE-461-Lec4.pdf) --- #### Q4.1: On what parameters does channel stability depend? Why is a meandering river considered more stable than a braided river? Explain using Planform Stability Ratios ($PS_1$ and $PS_2$). - **Appeared in:** 2022 (Q2b, 5m), 2023 (Q2a, 5m), 2024 (Q2a, 6m), 2025 (Q2c, 6m) - **Time & Length Budget:** 6 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 1–7 **Parameters Controlling Channel Stability:** 1. **Sediment load type** (ratio of coarse bed-material load to fine wash load). 2. **Bank & bed cohesiveness** (percentage of sticky silt and clay). 3. **Width-depth ratio ($W/D$)** and **channel slope ($S$)**. **Why Meandering is More Stable than Braided:** 1. **Strong cohesive banks:** Meandering rivers have silt-clay banks that resist erosion and keep the channel narrow and deep ($W/D < 10$), whereas braided rivers have loose sandy banks ($W/D > 40$) that collapse easily. 2. **Predictable vs. unpredictable changes:** In a meandering river, erosion (outer pool) and deposition (inner point bar) happen at fixed, predictable places at balanced rates. In a braided river, mid-channel bars and deep channels shift unpredictably during every flood. **Planform Stability Ratios ($PS_1$ & $PS_2$):** $$PS_1 = \frac{\text{Stream Eroding Power}}{\text{Bank Material Strength}}, \qquad PS_2 = \frac{\text{Bed Load Supply}}{\text{Bed Load Transport Capacity}}$$ - **Low $PS_1$ and Low $PS_2$:** Banks are strong enough to resist flow and the stream easily carries its bed load $\to$ **stable single-channel meandering river**. - **High $PS_1$ or High $PS_2$:** Flow overpowers weak banks ($PS_1 \uparrow$) or excess bed load dumps on the bed ($PS_2 \uparrow$) $\to$ **unstable multi-channel braided river**. --- #### Q4.2: Classify sediment load in detail and distinguish between bed load, suspended load, wash load, and bed-material load with a figure. - **Appeared in:** 2019 (Q2b, 6m), 2021 (Q1c, 8m), 2022 (Q2c, 8m), 2023 (Q2b, 5m), 2024 (Q3a, 5m), 2025 (Q1d, 5m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 8–12 **Classification Diagram (1 min):** ```text +--- Dissolved Load (Chemical ions in solution - not true sediment) | TOTAL STREAM LOAD + +--- Bed Load (Rolls, slides, saltates near bed) | By Transport Mode ->| +--- Particulate +--- Suspended Load (Held up by turbulent eddies) Sediment Load | = Suspended Bed-Material + Wash Load By Presence in Bed->+--- Bed-Material Load (Sizes found in riverbed) +--- Wash Load (Fine clay/silt <0.06 mm not in bed) ``` **Key Distinctions:** 1. **Bed Load:** Coarse sand and gravel that rolls, slides, or bounces (**saltates**) within a thin layer ($\sim 2$ grain diameters) right above the riverbed. 2. **Suspended Load:** Finer sand, silt, and clay kept up in the water column by upward turbulent eddies. 3. **Bed-Material Load:** Sediment sizes found in appreciable amounts in the riverbed (includes **all bed load + the coarser part of suspended load**). 4. **Wash Load:** Very fine clay and silt ($< 0.0625\text{ mm}$) washed in from overland erosion that travels in continuous suspension without ever settling in the riverbed. --- #### Q4.3: Explain with a figure why bed load is "capacity-dependent" while suspended/wash load is "supply-dependent," and why wash load never settles in rivers. - **Appeared in:** 2021 (Q1c, 8m), 2022 (Q2c, 8m), 2023 (Q3c, 5m) - **Time & Length Budget:** 6–8 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 9–12, 20; [SA: Lec 7](SA/EWCE-461-Lec7.pdf) p. 13 **Sketch to Draw (45 sec):** ```text Transport Rate ^ | / Transport Capacity Curve (Increases with Flow / Stream Power) | / | / * Actual Bed Load = Capacity-Limited (Follows curve; plenty in bed) | / | / . . . . * Actual Wash/Fine Suspended Load = Supply-Limited | / . (Far below capacity; depends only on watershed erosion) +-----------------------------------------> Grain Size (Fine ---> Coarse) |<- Wash Load ->|<- Bed-Material Load ->| (< 0.0625 mm) (> 0.0625 mm) ``` 1. **Why Bed Load is Capacity-Dependent:** - The riverbed itself holds an unlimited supply of coarse sand and gravel. - How much bed load moves depends **only on the river's hydraulic carrying capacity** (velocity, bed shear stress $\tau_0$, and stream power). 2. **Why Suspended (Wash) Load is Supply-Dependent:** - Flowing water can carry vastly more fine silt and clay than is ever present in the river. - Therefore, its transport rate is **not limited by flow capacity**, but depends strictly on how much fine soil is washed into the river by rainfall and watershed erosion (**upstream supply**). 3. **Why Wash Load Never Settles:** - Clay and fine silt particles ($< 0.06\text{ mm}$) have an almost zero settling velocity (e.g., a $0.001\text{ mm}$ clay particle takes $180\text{ days}$ to fall $10\text{ m}$ in still water). - **Thermal molecular bombardment (Brownian motion)** and even the weakest upward turbulent eddies are stronger than gravity on these tiny grains, keeping them suspended indefinitely. --- #### Q4.4: Why is there no sharp demarcation between bed load and suspended load (why are they interchangeable)? - **Appeared in:** 2023 (Q2c, 5m), 2024 (Q3b, 5m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 9–12 - **Turbulence fluctuates constantly:** Upward eddy velocity ($v'_y$) in a river changes from moment to moment and from place to place. - **Continuous exchange:** A medium sand grain rolling or bouncing as **bed load** is lifted into **suspended load** whenever a strong turbulent eddy exceeds its settling speed ($v'_y > \omega$). - **Settling back:** A few seconds later, when the eddy weakens or the flow slows down, the same grain drops back to the bed and moves again as **bed load**. - Because the same particles constantly switch between rolling/bouncing and suspension as flow energy changes, there is **no fixed boundary** between the two modes. --- #### Q4.5: What is dissolved load and why is it not considered part of sediment load? - **Appeared in:** 2023 (Q2d, 5m; Q3d, 5m) - **Time & Length Budget:** 3 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 8, 17–18 - **Dissolved Load:** Invisible chemical ions carried in true solution in river water from rock weathering and groundwater ($\text{HCO}_3^-, \text{Ca}^{2+}, \text{SO}_4^{2-}, \text{Cl}^-, \text{Na}^+, \text{Mg}^{2+}, \text{K}^+$). - **Why excluded from sediment load:** 1. Dissolved ions become chemically part of the water fluid itself and have **zero settling velocity**. 2. They require **no flow energy or turbulence** to stay up and **never deposit on the bed or erode banks**, so they have no effect on river bedforms or channel morphology. --- #### Q4.6: Explain with sketches the vertical distribution of suspended sediment size ($d_{10}, d_{50}, d_{90}$) and concentration over the flow depth. - **Appeared in:** 2024 (Q1d, 5m), 2025 (Q2d, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 13–14 **Sketches to Draw (45 sec):** ```text (a) Concentration by Grain Size (b) Log-Height Linear Profile Depth y ^ d10 (Silt) log(y) ^ \ | | \ d50 (Fine Sand) | \ Linear decrease | | \ \ d90 (Coarse Sand) | \ on log scale | | \ \___ | \ Bed +-----+------\--------\---> Conc. C +--------------\----> Conc. C (or Size d) ``` 1. **Logarithmic Decrease Upward:** Both sediment concentration ($C$) and average particle size ($d$) are highest right near the bed and decrease **logarithmically** toward the water surface (plotting as a straight line against $\log y$). 2. **Fine Particles ($d_{10}$, silt/clay):** Have tiny settling velocities, so turbulence mixes them almost **uniformly from bed to surface** (nearly vertical line). 3. **Medium Particles ($d_{50}$, fine sand):** Concentration increases noticeably from surface to bed. 4. **Coarse Particles ($d_{90}$, coarse sand):** Heavy grains resist upward eddies and stay **heavily concentrated near the bottom** of the channel. --- ### SA Lecture 5: Stream Power & Incipient Motion (Shields & Hjulström) - **Source:** [SA/EWCE-461-Lec5.pdf](SA/EWCE-461-Lec5.pdf) --- #### Q5.1: Define stream power. Prove that bed shear stress is proportional to velocity squared ($\tau_0 \propto V^2$) and stream power is proportional to velocity cubed ($\Omega \propto V^3$). - **Appeared in:** 2019 (Q2c, 4m), 2022 (Q6c, 4m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 5](SA/EWCE-461-Lec5.pdf) p. 1–6 - **Stream Power ($\Omega$):** The rate at which flowing water expends energy against the riverbed and banks per unit channel length: $\Omega = \rho g Q S$ ($\text{W/m}$), or per unit bed area: $\omega = \frac{\Omega}{w} = \tau_0 V$ ($\text{W/m}^2$). **Proof:** 1. **Bed shear stress:** $\tau_0 = \rho g h S \implies hS = \frac{\tau_0}{\rho g}$. 2. **Substitute into Chezy's velocity formula ($V = C\sqrt{hS}$):** $$V = C \sqrt{\frac{\tau_0}{\rho g}} \implies \tau_0 = \left(\frac{\rho g}{C^2}\right) V^2 \implies \mathbf{\tau_0 \propto V^2}$$ 3. **Substitute $\tau_0 \propto V^2$ into Stream Power ($\Omega = w \tau_0 V$):** $$\Omega = w \left(\frac{\rho g}{C^2} V^2\right) V = \left(\frac{w \rho g}{C^2}\right) V^3 \implies \mathbf{\Omega \propto V^3}$$ --- #### Q5.2: Distinguish between stream competence and stream capacity. - **Appeared in:** 2023 (Q2d, 5m — part) - **Time & Length Budget:** 2 Minutes | [SA: Lec 5](SA/EWCE-461-Lec5.pdf) p. 7 - **Stream Competence (Calibre):** The **maximum particle size (diameter $d$)** that a river can move at a given flow velocity or bed shear stress. - **Stream Capacity:** The **maximum total weight or volume of sediment** of a given size that a river can transport past a cross-section per unit time (depends on discharge $Q$ and stream power $\Omega$). --- #### Q5.3: Prove that the critical shear stress ($\tau_c$) required to erode coarse-grained sediment is a function of only sediment diameter ($d$). - **Appeared in:** 2021 (Q3b, 5m), 2024 (Q2b, 4m) - **Time & Length Budget:** 4–5 Minutes | [SA: Lec 5](SA/EWCE-461-Lec5.pdf) p. 12–13, 19–20 **Proof from Incipient Motion Moment Balance:** 1. **Forces on a resting bed grain of diameter $d$:** - **Submerged gravity (resisting) force:** $F_g = c_1 d^3 g (\rho_s - \rho)$ with moment arm $a_1 \sin\alpha$. - **Hydrodynamic drag (driving) force:** $F_D = c_2 d^2 \tau_c$ with moment arm $a_2 \cos\alpha$. 2. **Equate overturning and resisting moments at the threshold of motion ($\tau_0 = \tau_c$):** $$a_2 c_2 d^2 \tau_c \cos\alpha = a_1 c_1 d^3 g (\rho_s - \rho) \sin\alpha$$ 3. **Solve for critical shear stress $\tau_c$:** $$\tau_c = \underbrace{\left(\frac{a_1 c_1}{a_2 c_2}\tan\alpha\right)}_{\tau^*\text{ (Shields Parameter)}} g (\rho_s - \rho) d = \tau^* g (\rho_s - \rho) d$$ 4. **For coarse sediment ($d > 2\text{ mm}$, rough turbulent flow):** - Shields parameter is constant: $\tau^* = 0.06$. - Using $\rho_s = 2650\text{ kg/m}^3$, $\rho = 1000\text{ kg/m}^3$, and $g = 9.81\text{ m/s}^2$: $$\tau_c = 0.06 \times 9.81 \times (2650 - 1000) \times d \cong \mathbf{970 d} \quad (\text{N/m}^2)$$ - Hence, $\tau_c$ is a function of **sediment diameter $d$ only**. --- #### Q5.4: Explain with sketches the Shields Diagram and Hjulström Curve, highlighting why sand erodes most easily, why clay needs high velocity to erode, and why erosion velocity exceeds suspension velocity. - **Appeared in:** 2019 (Q3b, 6m), 2021 (Q2c, 5m), 2025 (Q3a, 6m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 5](SA/EWCE-461-Lec5.pdf) p. 17–24 **Sketches to Draw (1 min):** ```text (a) Shields Diagram (tau* vs Re* or d) (b) Hjulstrom Curve (Velocity V vs Grain Size d) tau* ^ \ Cohesive Velocity ^ \ Cohesive | EROSION / | \ Clay/Silt Coarse Gravel V | \ Clay | (Entrainment) / 0.06 | \ _________________ | \__________|________________/ | \ Sand / | TRANSPORT | / Small gap 0.03 | \__(Min)___/ | (Suspension) | / for Gravel +-----------------------------------> | Huge gap | / DEPOSITION 0.06 mm 2 mm Grain Size d +---------------+-----------------> Size d ``` **1. Shields Diagram Insights ($\tau^*$ vs. Grain Size / $Re_*$):** - **Fine–Medium Sand ($0.06\text{–}2\text{ mm}$) is easiest to erode ($\tau^*_{\min} \approx 0.03$):** Grains are large enough to stick up out of the smooth laminar sublayer into turbulent flow, yet light and non-cohesive. - **Fine Silt & Clay ($< 0.06\text{ mm}$) need high $\tau^*$:** Strong electrochemical **cohesion** binds particles together, and tiny grains lie hidden inside the smooth viscous sublayer. - **Coarse Gravel ($> 2\text{ mm}$) has constant $\tau^* = 0.06$:** Actual shear stress needed to move them ($\tau_c = 970 d$) rises linearly with particle weight/diameter. **2. Hjulström Curve Insights (Velocity vs. Grain Size):** - **Why Erosion Velocity $>$ Suspension Velocity:** Lifting a resting grain from the bed requires extra force to overcome inter-particle friction, interlocking, or cohesion; once lifted into the water column, much weaker turbulence can keep it floating. - **Gap between Erosion and Deposition Curves:** - **Huge gap for Clay/Silt:** High velocity is needed to break cohesive bonds, but once eroded, clay stays suspended down to almost zero velocity. - **Tiny gap for Coarse Gravel:** A small drop in velocity below the erosion threshold causes gravel to settle immediately. --- #### Q5.5: [Math] Check riverbed erodibility, find minimum non-erodible grain size ($d$), or find maximum stable discharge ($Q_{\max}$) using the Shields parameter ($\tau^*$). - **Appeared in:** 2022 (Q2d, 11m), 2023 (Q2e, 10m), 2024 (Q3d, 10m), 2025 (Q2e, 8m) - **Time & Length Budget:** Methodology Only | [SA: Lec 5](SA/EWCE-461-Lec5.pdf) p. 11, 20; [SA: Lec 4](SA/EWCE-461-Lec4.pdf) p. 19 **Step-by-Step Methodology:** 1. **Find Flow Depth ($h$) and Mean Velocity ($V$) (if $Q$, width $W$, Chezy $C$, and slope $S$ are given):** $$Q = W h C \sqrt{h S} = W C S^{1/2} h^{3/2} \implies h = \left(\frac{Q}{W C \sqrt{S}}\right)^{2/3}, \qquad V = \frac{Q}{W h}$$ 2. **Calculate Actual Bed Shear Stress ($\tau_0$):** $$\tau_0 = \rho g h S \qquad \text{or (if slope } S \text{ is not given):} \qquad \tau_0 = \rho g \left(\frac{V}{C}\right)^2$$ where $\rho = 1000\text{ kg/m}^3$ and $g = 9.81\text{ m/s}^2$. 3. **Calculate Critical Shear Stress ($\tau_c$) from Shields Equation:** $$\tau_c = \tau^* g (\rho_s - \rho) d_{50}$$ where $\rho_s = 2650\text{ kg/m}^3$ and $\tau^*$ is given (typically $0.056$ or $0.06$). 4. **Answer the Specific Question Prompt:** - **Case A — Erodibility Check:** If $\tau_0 > \tau_c$, the bed **will erode**; if $\tau_0 \le \tau_c$, the bed is **stable**. - **Case B — Minimum Non-Erodible Sediment Size ($d_{\min}$) & Soil Type (2022 Q2d, 2024 Q3d):** Set $\tau_c = \tau_0$ and solve for $d$: $$d_{\min} = \frac{\tau_0}{\tau^* g (\rho_s - \rho)}$$ Classify $d_{\min}$ (in $\text{mm}$): Clay ($<0.002$), Silt ($0.002\text{–}0.0625$), Sand ($0.0625\text{–}2.0$), Gravel ($2\text{–}64$), Cobble ($64\text{–}256$), Boulder ($>256$). - **Case C — Maximum Stable Depth & Discharge without Erosion (2023 Q2e):** Set $\tau_0 = \tau_c$ to find maximum permissible depth $h_{\max}$, then compute $Q_{\max}$: $$h_{\max} = \frac{\tau_c}{\rho g S}, \qquad Q_{\max} = W h_{\max} C \sqrt{h_{\max} S}$$ --- ### SA Lecture 6: Bedforms, Flow Regimes, Roughness & Fall Velocity - **Source:** [SA/EWCE-461-Lec6.pdf](SA/EWCE-461-Lec6.pdf) --- #### Q6.1: What is flow regime? Explain with sketches the sequence of bedforms in lower and upper flow regimes, why upper-regime bedforms are rare in nature, and how roughness changes across regimes. - **Appeared in:** 2019 (Q1b, 10m), 2022 (Q3a, 5m), 2023 (Q3a, 7m; Q3b, 3m), 2024 (Q3c, 5m), 2025 (Q1e, 5m) - **Time & Length Budget:** 5–10 Minutes | [SA: Lec 6](SA/EWCE-461-Lec6.pdf) p. 1–18 - **Flow Regime:** The combined characteristic state of riverbed geometry (bedforms), water surface profile, and flow resistance as stream power and Froude number ($Fr = V/\sqrt{gd}$) change. **Sketches of Lower vs. Upper Flow Regime Bedforms (1 min):** ```text LOWER FLOW REGIME (Subcritical, Fr < 1: Water surface OUT OF PHASE with bed) Water: ~~~~\____/~~~~\____/~~~~ (Boils over troughs, dips over crests) Bed: ____/~~~~\____/~~~~\____ 1. Plane Bed -> 2. Ripples -> 3. Dunes (High roughness) UPPER FLOW REGIME (Supercritical, Fr > 1: Water surface IN PHASE with bed) Water: ~~~~/~~~~\____/~~~~\~~~~ (Standing waves / breaking antidunes) Bed: ____/~~~~\____/~~~~\____ 4. Transition -> 5. Upper Plane Bed -> 6. Antidunes ``` **Comparison of Lower vs. Upper Flow Regime:** | Feature | Lower Flow Regime ($Fr < 1$, Subcritical) | Upper Flow Regime ($Fr > 1$, Supercritical) | | :--- | :--- | :--- | | **Bedform Sequence** | Plane bed (no motion) $\to$ **Ripples** ($d<0.7\text{ mm}$) $\to$ **Dunes** | Washed-out dunes $\to$ **Plane bed (with motion)** $\to$ **Standing waves** $\to$ **Antidunes** | | **Water Surface vs. Bed** | **Out of phase** (water dips over dune crests) | **In phase** (water waves match bed waves) | | **Roughness ($n$)** | **High** ($n = 0.014\text{–}0.040$); peaks at dunes due to large lee-side separation eddies | **Low** ($n = 0.010\text{–}0.022$); drops sharply when dunes wash out into a flat bed | | **Sediment Concentration** | **Low** ($10\text{–}800\text{ ppm}$); grains roll up luff side and avalanche down lee side | **Very High** ($1,000\text{–}42,000\text{ ppm}$); continuous suspension and bed sheet-flow | **Why Upper-Regime Bedforms Rarely Occur in Natural Rivers (2023 Q3b):** - Upper-regime bedforms require supercritical flow ($Fr \ge 1$, high velocity and steep slope). - Natural alluvial rivers have gentle slopes; whenever velocity rises during a flood, the river widens/scours its bed and form drag absorbs energy, keeping Froude number subcritical ($Fr < 1$). **Grain Roughness ($k_{s1}$) vs. Form Roughness ($k_{s2}$) (2024 Q3c):** - **Total Roughness:** $k_s = k_{s1} + k_{s2}$. - **Grain Roughness ($k_{s1} \approx 3 d_{90}$):** Skin friction directly on individual sediment particles; **dominates in coarse gravel-bed rivers** and flat plane beds. - **Form Roughness ($k_{s2}$):** Pressure drag caused by flow separation and eddies behind ripple/dune crests; **dominates in sand-bed rivers** during lower-regime flow. --- #### Q6.2: Define sphericity and explain how particle shape affects drag coefficient ($C_D$) and fall velocity ($\omega$). - **Appeared in:** 2024 (Q3e, 5m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 6](SA/EWCE-461-Lec6.pdf) p. 25–28 - **Sphericity:** The ratio of the surface area of a sphere having the same volume as the particle to the actual surface area of the particle ($\text{Sphericity} = \frac{A_{\text{sphere}}}{A_{\text{particle}}} \le 1.0$). - **Effect on Drag & Fall Velocity:** 1. Natural sand/silt grains are angular or flaky (typical sphericity $\approx 0.7$), giving them more surface area and wider wakes than a smooth sphere ($1.0$). 2. As **sphericity decreases**, flow separation increases and the **drag coefficient ($C_D$) increases** for any particle Reynolds number ($Re_p$). 3. Higher drag slows the falling grain down, **reducing its terminal fall velocity ($\omega$)** and keeping flatter particles suspended longer in the river. --- ### SA Lecture 7: Suspended Load (Rouse Equation) & Bed Load Formulas - **Source:** [SA/EWCE-461-Lec7.pdf](SA/EWCE-461-Lec7.pdf) --- #### Q7.1: Why is determining suspended sediment load in a river complex and uncertain? - **Appeared in:** 2024 (Q4b, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 7](SA/EWCE-461-Lec7.pdf) p. 1, 5, 17–18, 24 Determining total suspended load ($q_s = \int_a^h c(y) u(y) \, dy$) is difficult for **five reasons**: 1. **Non-linear depth profiles:** Both sediment concentration $c(y)$ and water velocity $u(y)$ change non-linearly from the bed to the surface. 2. **Uncertain reference concentration ($C_a$):** Measuring exact concentration right near the moving bed (at level $y=a$) is physically difficult. 3. **Grain vs. Form shear split:** Total bed shear stress ($\tau_0$) splits between bedform drag and grain friction; only the grain-related shear lifts sediment. 4. **Supply dependence of wash load:** Fine wash load depends on watershed rainfall/erosion rather than local flow hydraulics. 5. **Temperature and grain-size mix:** Water viscosity changes with temperature, and natural sediment contains many grain sizes interacting together (different formulas can disagree by up to $400\%$). --- #### Q7.2: Define shear velocity ($u_*$) and fall velocity ($\omega$). What is the Rouse Number ($R_n$) and how does it control the vertical distribution and transport mode of sediment? - **Appeared in:** 2019 (Q3c, 6m), 2021 (Q2b, 7m), 2023 (Q3d, 5m), 2024 (Q4a, 5m), 2025 (Q8c, 2m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 7](SA/EWCE-461-Lec7.pdf) p. 4, 6–10, 16 - **Shear Velocity ($u_*$):** A velocity scale representing near-bed shear stress ($\tau_b$) and the strength of upward turbulent eddies: $$u_* = \sqrt{\frac{\tau_b}{\rho_w}} = \sqrt{g h S}$$ - **Fall Velocity ($\omega$):** The steady downward terminal speed at which a grain settles through calm water under gravity. - **Difference:** $\omega$ pulls sediment **downward** toward the bed (gravity), whereas $u_*$ pushes sediment **upward** into suspension (turbulence). **Rouse Number ($R_n$) & Transport Regimes:** $$R_n = \frac{\omega}{\kappa u_*} \qquad (\text{where von Kármán constant } \kappa = 0.40)$$ | Rouse Number ($R_n$) | Dominant Force | Mode of Sediment Transport & Vertical Profile | | :--- | :--- | :--- | | **$R_n > 2.5$** | Gravity $\gg$ Turbulence | **Bed Load:** Grains stay on the bed (rolling/sliding). | | **$1.8 < R_n < 2.5$** | Gravity $>$ Turbulence | **Partial Suspension:** Saltation + suspension only near the bottom. | | **$R_n < 1.8$** | Turbulence $>$ Gravity | **Full Suspension:** Grains carried throughout the depth (steeper gradient near bed). | | **$R_n < 0.25$** | Turbulence $\gg$ Gravity | **Wash Load:** Nearly uniform concentration from bed to water surface. | --- #### Q7.3: [Math] Calculate suspended sediment concentration ($C_y$) at elevation $y$ using the Rouse Equation. - **Appeared in:** 2022 (Q3d, 14m), 2023 (Q3e, 10m), 2025 (Q3c, 16m) - **Time & Length Budget:** Methodology Only | [SA: Lec 7](SA/EWCE-461-Lec7.pdf) p. 6–11 **Step-by-Step Methodology:** 1. **Find Flow Depth ($h$) (if not given directly):** - From Manning's equation (wide channel): $Q = \frac{1}{n} B h^{5/3} S^{1/2} \implies h = \left(\frac{Q \cdot n}{B \sqrt{S}}\right)^{3/5}$ - From Chezy's equation: $Q = B C h^{3/2} S^{1/2} \implies h = \left(\frac{Q}{B C \sqrt{S}}\right)^{2/3}$ 2. **Calculate Shear Velocity ($u_*$):** $$u_* = \sqrt{g h S} \qquad (\text{with } g = 9.81\text{ m/s}^2)$$ 3. **Calculate Rouse Number ($R_n$):** $$R_n = \frac{\omega}{\kappa u_*} \qquad (\text{with } \kappa = 0.40 \text{ and fall velocity } \omega \text{ in m/s})$$ 4. **Calculate Suspended Concentration ($C_y$) at Height $y$ from Reference Concentration ($C_a$) at Height $a$:** $$C_y = C_a \left[ \frac{h - y}{y} \times \frac{a}{h - a} \right]^{R_n}$$ --- #### Q7.4: [Math] Estimate bed load transport rate from dune dimensions ($L, \Delta H$) or by Meyer-Peter & Müller (MPM) gravel-bed equation. - **Appeared in:** 2019 (Q3e, 7m), 2021 (Q2d, 13m), 2024 (Q4e, 10m), 2025 (Q8e, 6m) - **Time & Length Budget:** Methodology Only | [SA: Lec 7](SA/EWCE-461-Lec7.pdf) p. 13–15 **Method A — Bed Load from Dune Bedform Migration (2021 Q2d, 2024 Q4e):** 1. **Convert units to SI:** Dune length $L$ ($\text{m}$), dune height $\Delta H$ ($\text{m}$), near-bed dune velocity $v$ ($\text{m/s}$), porosity $p$ (fraction), sediment density $\rho_s = 2650\text{ kg/m}^3$, channel width $W$ ($\text{m}$), and time $T = 1\text{ year} = 365 \times 86400\text{ s}$. 2. **Compute unit bed load transport rate ($q_b$, in $\text{kg}\cdot\text{m}^{-1}\text{s}^{-1}$ or $\text{kg/s}$ per unit width depending on slide convention):** $$q_b = \frac{1}{2} v \rho_s (1 - p) L \Delta H$$ 3. **Multiply by channel width $W$ and time $T$** to get total annual bed load mass (in $\text{kg/yr}$ or $\text{tons/yr}$). **Method B — Meyer-Peter & Müller (MPM) Gravel Bed Load Formula (2019 Q3e, 2025 Q8e):** 1. **Calculate actual bed shear stress ($\tau_0$):** $$\tau_0 = \rho g d S = 1000 \times 9.81 \times d \times S \quad (\text{N/m}^2)$$ 2. **Calculate critical shear stress ($\tau_{cr}$) for coarse gravel ($\tau^* = 0.06$):** $$\tau_{cr} = 0.06 g (\rho_s - \rho) d_{50} = 970 d_{50} \quad (\text{N/m}^2)$$ 3. **Check if motion occurs ($\tau_0 > \tau_{cr}$) and compute unit bed load ($q_b$):** $$q_b = 0.253 (\tau_0 - \tau_{cr})^{3/2} \quad (\text{kg}\cdot\text{m}^{-1}\text{s}^{-1})$$ 4. **Total bed load across river width $W$ over time $T$:** $$Q_{b,\text{total}} = q_b \times W \times T$$ --- ### SA Lecture 8: Dynamic Adjustment of Rivers — Lane's Relationship, Aggradation & Degradation - **Source:** [SA/EWCE-461-Lec8.pdf](SA/EWCE-461-Lec8.pdf) --- #### Q8.1: State Lane's relationship ($Q_s d \propto Q_w S$) and mention its limitations. - **Appeared in:** 2025 (Q8d, 4m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 8](SA/EWCE-461-Lec8.pdf) p. 2–3 - **Lane's Principle (1955):** In an equilibrium alluvial river, the product of sediment load ($Q_s$) and sediment size ($d$) is proportional to the product of water discharge ($Q_w$) and channel slope ($S$): $$Q_s \cdot d \propto Q_w \cdot S$$ - **Three Limitations of Lane's Relationship:** 1. **Qualitative only:** It predicts the *direction* of change (aggradation or degradation) but cannot calculate exact numerical depths or volumes by itself. 2. **No time scale:** It does not tell how many months or years the river will take to reach a new equilibrium. 3. **Ignores cross-section & planform adjustments:** It assumes only bed slope ($S$) adjusts, ignoring simultaneous changes in channel width ($W$), depth ($D$), sinuosity, or braiding. --- #### Q8.2: Explain with sketches the expected morphological changes (aggradation or degradation) in a river due to: (i) Upstream & downstream of a dam / clear-water release, (ii) Water supply diversion, (iii) Increase in flood discharge, (iv) Tributary confluence, and (v) Distributary off-take. - **Appeared in:** 2022 (Q3b, 6m), 2023 (Q4a, 6m), 2025 (Q3b, 8m) - **Time & Length Budget:** 6–8 Minutes | [SA: Lec 8](SA/EWCE-461-Lec8.pdf) p. 5, 11–16, 21–23 *Use Lane's Balance ($Q_s d \propto Q_w S$) for every case:* 1. **Upstream of a Dam:** - Reservoir ponding slows velocity to near zero $\to$ incoming sediment drops out $\to$ **aggradation (bed rises)** and delta forms upstream. 2. **Downstream of a Dam / Release of Clear Water:** - Reservoir traps sediment ($Q_s \downarrow \approx 0$). Released "hungry" clear water has high turbulence (no silt to damp eddies) $\to$ **severe degradation (bed scours downward, slope $S$ flattens)** until coarse grains form a protective **armour layer**. 3. **Downstream of a Water Supply / Irrigation Diversion:** - Clean water is abstracted ($Q_w \downarrow$) while coarse sediment remains in the river $\to$ carrying capacity drops $\to$ **aggradation (bed rises and slope $S$ steepens)**. 4. **Increase in Discharge due to a Flood ($Q_w \uparrow$):** - Higher water flow ($Q_w \uparrow$) raises stream power $\to$ **degradation (general bed scour and slope flattening)** during the rising flood peak, followed by deposition as the flood recedes. 5. **Downstream of a Tributary Confluence:** - If the tributary brings heavy coarse sediment ($Q_s d \uparrow > Q_w \uparrow$), the main river experiences **aggradation and slope steepening** below the junction; if the tributary adds mostly clear water ($Q_w \uparrow$), **degradation** occurs. 6. **Downstream of a Distributary Off-Take:** - A branch channel diverts water ($Q_w \downarrow$) out of the main river faster than coarse bed load $\to$ reduced stream power in the main stem causes **aggradation and bar/shoal formation** just below the off-take. **Quick Sketch for Dam / Diversion / Confluence (45 sec):** ```text (a) Dam Effect on Bed Profile (b) Diversion / Off-take (Qw drops -> Aggradation) \ Aggradation (Silt) | Dam Original Bed \____________________ \....................|____ Water New Steep Bed \....... (Aggradation, S rises) | \ Degradation ----------------------------------------------- | \ (Clear water) (c) Flood / Clear Water (Qw rises -> Degradation) | \_______ New Flat Bed \_______ (Degradation, S drops) ``` --- #### Q8.3: [Math] Predict downstream slope change and aggradation/degradation using Quantitative Modified Lane's Relationship. - **Appeared in:** 2021 (Q3d, 13m), 2022 (Q6e, 10m), 2024 (Q8d, 8m) - **Time & Length Budget:** Methodology Only | [SA: Lec 8](SA/EWCE-461-Lec8.pdf) p. 6–10 **Step-by-Step Methodology:** 1. **Define the 5 dimensionless ratios (after-change subscript $2$ divided by initial subscript $1$):** - Sediment load ratio: $a = \frac{Q_{s2}}{Q_{s1}}$ *(e.g., if $75\%$ is retained in a reservoir, $a = 1 - 0.75 = 0.25$)* - Sediment size ratio: $b = \frac{d_{s2}}{d_{s1}}$ - Flow depth ratio: $c = \frac{y_2}{y_1}$ - Water discharge ratio: $d = \frac{Q_{w2}}{Q_{w1}}$ *(e.g., if $20\%$ water is diverted, $d = 1 - 0.20 = 0.80$)* - Bed slope ratio: $e = \frac{S_{02}}{S_{01}}$ 2. **Apply the Quantitative Lane Formula:** $$a \left(\frac{b}{c}\right)^{1/3} = d \cdot e \quad \implies \quad e = \frac{a \left(b / c\right)^{1/3}}{d}$$ 3. **Calculate the new downstream slope ($S_{02}$):** $$S_{02} = e \times S_{01}$$ 4. **Interpret the Morphological Response:** - **If $e < 1$ ($S_{02} < S_{01}$):** Slope decreases $\implies$ **Degradation (bed erosion)** downstream. - **If $e > 1$ ($S_{02} > S_{01}$):** Slope increases $\implies$ **Aggradation (sediment deposition)** downstream. --- ### SA Lecture 9 & Lecture 10: River Scour, Complex Piers, Lacey–Inglis Method & Countermeasures - **Source:** [SA/EWCE-461-Lec9.pdf](SA/EWCE-461-Lec9.pdf) & [SA/EWCE-461-Lec10.pdf](SA/EWCE-461-Lec10.pdf) --- #### Q9.1: What is local scour? List the types of scour and distinguish between scour and long-term degradation. - **Appeared in:** 2019 (Q3d, 5m), 2025 (Q8c, 2m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 1–4 - **Local Scour:** Localized removal of riverbed sediment right around an obstruction (such as a bridge pier, abutment, or spur) caused by local flow acceleration, downflow, and 3D vortices. - **Three Components of Total Scour:** $$\text{Total Scour} = \text{General Scour (Aggradation/Degradation)} + \text{Contraction Scour} + \text{Local Scour}$$ - **Scour vs. Degradation:** - **Local/Contraction Scour:** Happens **locally** around a structure over a **short time** (during a single flood) due to local vortices/narrowing, and affects only the bridge site. - **Degradation:** Happens **along a long river reach** over **many years** due to a basin-wide sediment deficit (e.g., below a dam), regardless of whether a bridge is present. --- #### Q9.2: What is contraction scour and why is it generally not significant (or when is it neglected) at a bridge site? - **Appeared in:** 2021 (Q1b, 5m), 2022 (Q3c, 5m), 2023 (Q4c, 4m) - **Time & Length Budget:** 4–5 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 4–7 - **Contraction Scour:** General lowering of the riverbed across the entire bridge opening when bridge approaches, abutments, or guide banks narrow the river width, forcing water to speed up ($V \uparrow$) and erode the bed. - **Why Contraction Scour is Generally Insignificant / When Neglected:** 1. **Small reduction in flow area:** Modern bridges span the entire active main channel, blocking only shallow overbank floodplains that carry very little discharge (often $< 3\text{–}4\%$ change in flow area and velocity). 2. **Minimal backwater rise:** Because the velocity increase across the wide opening is tiny (e.g., water level rises by only $\sim 0.01\text{ m}$), the extra bed shear is negligible compared to deep local vortex scour right at the piers. 3. **When ignored:** It is neglected when bridge abutments are set back at or beyond the natural main-channel banks so that the main waterway width is not narrowed. --- #### Q9.3: Explain the scour process around a bridge pier with a figure. Distinguish between horseshoe vortex and wake vortex, and define critical scour velocity ($V_c$) and equilibrium scour depth ($d_s$). - **Appeared in:** 2021 (Q2a, 5m), 2023 (Q4b, 6m), 2025 (Q4a, 8m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 8–11, 14, 34–36 **Sketch of Pier Scour Mechanism (45 sec):** ```text | Pier | Approach Flow ===> | | ===> ( ) ( ) Wake Vortices (Vertical-axis tornadoes) | | | | Downflow Jet v | | | ___________________/ |___|___________________ Original Bed \ (Horseshoe) / \___Vortex___/ <-- Scour Hole (Depth ds) ``` **Scour Process Around a Pier:** 1. Approaching water hits the upstream nose of the pier, comes to a stop, and piles up slightly. 2. Because stagnation pressure is highest near the fast surface water and lowest near the bed, water dives down the front face of the pier like a **vertical downward jet**. 3. The downflow digs a groove in the bed and rolls up into a **horseshoe vortex**, while flow separating past the sides forms **wake vortices**, sweeping sediment out of the scour hole. **Horseshoe Vortex vs. Wake Vortex:** - **Horseshoe Vortex:** A **horizontal-axis** vortex that wraps around the base of the pier nose like a horseshoe; it is the primary driver that scours the front and sides of the pier. - **Wake Vortex:** **Vertical-axis** eddies shed alternately from the downstream corners of the pier; they act like small tornadoes sucking sediment up from the bed behind the pier, but dissipate quickly downstream. **Key Definitions:** - **Critical Scour Velocity ($V_c$):** Minimum approach flow velocity required to initiate movement of bed grains upstream of the bridge ($V_c = 10.95 y_1^{1/6} d_{50}^{1/3}$). - **Equilibrium Scour Depth ($d_s$):** Maximum steady scour depth reached when the scour hole becomes deep enough that vortex energy weakens (or incoming sediment equals outgoing sediment) and net erosion stops. --- #### Q9.4: Explain how flow depth ($y$), pier width ($b_p$) (narrow vs. wide piers), sediment size ($d_{50}$), and angle of attack ($\theta$) affect local pier scour. - **Appeared in:** 2019 (Q4a, 4m), 2021 (Q3a, 6m), 2024 (Q1e, 5m), 2025 (Q1f, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 11–24 1. **Flow Depth ($y$) & Pier Width ($b_p$) — Narrow vs. Wide Piers:** - **Narrow Pier ($y / b_p \ge 1.4$):** Scour depth increases linearly with **pier width ($d_s \propto b_p$)** and is independent of flow depth, because the horseshoe vortex scales with the pier diameter. - **Wide Pier ($y / b_p \le 0.2$):** Scour depth depends only on **flow depth ($d_s \propto y$)** and is independent of pier width, because shallow flow limits the downward jet. - **Intermediate ($0.2 < y / b_p < 1.4$):** Scour depends on both $y$ and $b_p$. 2. **Sediment Size ($d_{50}$):** - Fine sand ($0.1\text{–}0.6\text{ mm}$) scours deepest because it has no cohesion and cannot armor the hole. - Coarse gravel ($>2\text{ mm}$) and well-graded mixtures form a protective **armour layer** in the scour pit, reducing scour depth (captured by factor $K_4$). 3. **Angle of Attack ($\theta$):** - If flow hits a long pier at an angle ($\theta > 0^\circ$), the long side ($L$) of the pier acts like a wide bluff barrier, drastically increasing the effective width blocking the flow and multiplying scour depth by $2\times\text{ to }3\times$ ($K_2 = \left(\cos\theta + \frac{L}{a}\sin\theta\right)^{0.65}$). --- #### Q9.5: Why is pier scour depth generally higher than abutment scour depth? - **Appeared in:** 2019 (Q4c, 4m), 2025 (Q4a, 8m — part) - **Time & Length Budget:** 4 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 12, 25–28 1. **Higher velocity & deeper water:** Piers stand in the middle of the deep main channel where flow velocity, depth, and stream power are maximum, whereas abutments sit on shallow riverbanks or floodplains where flow is much slower. 2. **Two-sided vs. one-sided vortex action:** A pier is surrounded by fast water on all sides, generating full horseshoe and wake vortices on both flanks; an abutment is shielded on one side by the riverbank. 3. **Field evidence:** Typical bridge measurements (e.g., Jamuna/Teesta tables) show pier scour of **$4\text{–}6.7\text{ m}$** compared to abutment scour of only **$0.7\text{–}2.7\text{ m}$**. --- #### Q9.6: Distinguish between clear-water scour and live-bed scour with a graph. Which gives a higher scour depth and why? - **Appeared in:** 2019 (Q4e, 4m), 2022 (Q6a, 6m), 2025 (Q8c, 2m) - **Time & Length Budget:** 4–6 Minutes | [SA: Lec 9](SA/EWCE-461-Lec9.pdf) p. 29–33; [SA: Lec 10](SA/EWCE-461-Lec10.pdf) p. 1–5 **Sketches to Draw (45 sec):** ```text (a) Scour Depth vs Time (b) Scour Depth vs Velocity Ratio (V / Vc) Scour ^ Clear-Water (Slow, High) Scour ^ Peak at V = Vc Depth | . . . . . . . . . . . Depth | /\ ds | . /\/\/\/\/\/\/\/\/\ | / \____/~~~~ Live-Bed | . / Live-Bed (Fast, | / (10% lower) | . / fluctuates with dunes) | / Clear-Water +----------------------------------> Time +------+------------------> V / Vc 1.0 ``` - **Clear-Water Scour ($V \le V_c$):** Upstream riverbed sediment is **at rest** ($V < V_c$); no new sediment enters the scour hole. Scour develops slowly and reaches its **absolute maximum depth at $V = V_c$**. - **Live-Bed Scour ($V > V_c$):** Upstream riverbed is **actively moving**; sediment continuously washes into the scour hole as dunes pass. It reaches equilibrium rapidly and oscillates around an average depth. - **Why Clear-Water Scour Depth is Higher (~10% deeper than Live-Bed):** 1. In live-bed scour, incoming sediment from upstream continuously **refills** the scour hole. 2. Heavy suspended sediment in live-bed flow **absorbs turbulent energy** and damps vortex strength, whereas clear water expends $100\%$ of its vortex energy digging the hole. --- #### Q10.1: What is a complex pier? Sketch Cases I–IV when a normal pier must be treated as a complex pier, and explain why complex pier scour is greater than normal pier scour. - **Appeared in:** 2019 (Q4b, 5m), 2022 (Q6b, 6m), 2023 (Q8b, 4m), 2024 (Q4c, 5m), 2025 (Q4b, 5m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 10](SA/EWCE-461-Lec10.pdf) p. 6–15, 24–25 - **Complex Pier:** A bridge pier made of **three distinct structural components** with different widths: (1) **Pier stem (column)**, (2) **Pile cap**, and (3) **Pile group**. **Four Cases of Pile Cap Position ($Y =$ depth of pile cap top below original riverbed):** ```text Case I (ds < Y) Case II (ds > Y > 0) Case III (Y <= 0) Case IV (Cap at Water Surface) |Stem| |Stem| |Stem| ~~~~[====Cap====]~~~~ Water ___|____|___ Bed ___|____|___ Bed ___[=Cap=]___ Bed || || || || \Scour/ \[=Cap=]/ || || ____ ||_||_||_|| ____ Bed [==Cap==] (Buried) || || (In scour hole) \Scour/ (Cap & piles \ Scour / || || exposed to flow) \_______/ ``` 1. **Case I ($d_s < Y$):** Pile cap stays buried below the bottom of the scour hole $\to$ behaves as a **normal single pier**. 2. **Case II ($d_s > Y > 0$):** Pile cap is below original bed level but gets uncovered inside the scour hole $\to$ wide cap blocks the downward jet and **reduces scour**. 3. **Case III ($Y \le 0$):** Pile cap is at or above the riverbed (exposed directly to approach flow). 4. **Case IV:** Pile cap is at or above the water surface (full pile group + cap exposed to flow). **Why Complex Pier Scour $>$ Normal Pier Scour (and why Effective Diameter $b_e$ is needed):** - When river degradation or Case III/IV placement exposes the **wide pile cap and pile group** to the main current, they block a much larger flow area than the narrow pier stem alone. - This generates a much stronger downward jet and a wider horseshoe vortex, increasing scour depth by **more than $2\times$**. Hence, the three components must be combined into a single **effective pier diameter ($b_e$)** before applying scour formulas. --- #### Q10.2: [Math] Calculate the effective/equivalent pier diameter ($b_e$) for a complex bridge pier with its pile cap at riverbed level. - **Appeared in:** 2023 (Q4e, 8m), 2025 (Q4d, 9m) - **Time & Length Budget:** Methodology Only | [SA: Lec 10](SA/EWCE-461-Lec10.pdf) p. 13, 25, 27 **Step-by-Step Methodology:** 1. **Find uniform flow depth ($d$) from Manning's equation (wide rectangular channel):** $$Q = \frac{1}{n} W d^{5/3} S^{1/2} \implies d = \left(\frac{Q \cdot n}{W \sqrt{S}}\right)^{3/5}$$ 2. **Identify pier geometry variables:** - $b =$ diameter/width of upper pier stem ($\text{m}$) - $b^* =$ diameter/width of lower pile cap ($\text{m}$) - $Y =$ depth to top of pile cap from riverbed ($Y = 0$ when pile cap is placed right at the riverbed level). 3. **Calculate Effective Pier Diameter ($b_e$):** $$b_e = b \left(\frac{d + Y}{d + b^*}\right) + b^* \left(\frac{b^* - Y}{b^* + d}\right)$$ *(For $Y = 0$, this simplifies to $b_e = \frac{b \cdot d + (b^*)^2}{d + b^*}$).* --- #### Q10.3: [Math & Theory] Calculate maximum scour depth around a bridge pier using the Lacey–Inglis method and state its limitations. - **Appeared in:** 2019 (Q4f, 8m) - **Time & Length Budget:** Methodology + 2-Min Theory | [SA: Lec 10](SA/EWCE-461-Lec10.pdf) p. 26–28 **Step-by-Step Methodology:** 1. **Calculate Lacey's silt factor ($f$) from median grain size $d_{50}$ (in $\text{mm}$):** $$f = 1.76 \sqrt{d_{50}}$$ 2. **Calculate Lacey's regime depth ($D_{LQ}$) from design discharge $Q$ ($\text{m}^3/\text{s}$):** $$D_{LQ} = 0.47 \left(\frac{Q}{f}\right)^{1/3}$$ 3. **Calculate maximum equilibrium scour depth below High Flood Level (HFL):** $$D_{se} = 2.0 \times D_{LQ} \quad \text{(below HFL)}$$ *(Local scour depth below existing riverbed $= D_{se} - \text{existing flow depth } d$).* **Limitations of the Lacey–Inglis Method:** 1. It is purely empirical (derived for steady sandy canals in the Indo-Gangetic plain) and **ignores actual pier width ($b_p$), pier shape, angle of attack, and complex pile-cap geometry**. 2. Since $D_{se}$ is measured from the water surface (HFL), it can seriously **underestimate scour in deep rivers** where actual flow depth already approaches $2 D_{LQ}$. --- #### Q10.4: Explain sacrificial piles as a flow-altering countermeasure against bridge pier scour. - **Appeared in:** 2025 (Q4c, 2m — part) - **Time & Length Budget:** 2 Minutes | [SA: Lec 10](SA/EWCE-461-Lec10.pdf) p. 29–31 - **Sacrificial Piles:** A group of $3\text{ to }6$ small dummy piles (diameter $0.2\text{–}0.5 D$, height $\sim 0.6 y$) driven into the riverbed in a triangular wedge upstream of a main bridge pier. - **How they work:** They take the brunt of the oncoming current, deflect high-velocity flow around the pier, and place the bridge pier inside a calm, low-energy wake zone—reducing pier scour by **$20\%\text{ to }60\%$**. --- ### SA Lecture 11: River Training & Bank Protection Works - **Source:** [SA/EWCE- 461-Lec11.pdf](SA/EWCE-%20461-Lec11.pdf) --- #### Q11.1: Explain with examples the differences between active (transverse) and passive (longitudinal) river training structures. - **Appeared in:** 2022 (Q6d, 4m), 2023 (Q8a, 4m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 1, 3–6 | Feature | Active (Transverse) Structures | Passive (Longitudinal) Structures | | :--- | :--- | :--- | | **Orientation to Flow** | Built **perpendicular or angled** out from the bank into the river. | Built **parallel** along the riverbank slope. | | **Working Mechanism** | **Actively modifies flow:** deflects high-velocity current away from the bank, reduces near-bank velocity, and induces sediment deposition (siltation). | **Passively armors the bank:** does not change flow patterns, but shields bank soil against direct current and wave attack. | | **Impact on Channel** | Can reclaim eroded land and build a new bankline; creates deep local scour at the structure nose. | Protects existing bank alignment without reclaiming lost land. | | **Examples** | **Spurs / Groynes, Bandals, Permeable Pile Dykes, Porcupines.** | **Revetments (Riprap, CC blocks, Geobags), Guide Banks, Levees.** | --- #### Q11.2: Discuss the advantages and disadvantages of the soft approach / Nature-Based Solutions (NBS) / bioengineering in riverbank protection. - **Appeared in:** 2022 (Q4a, 8m — part), 2023 (Q8d, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 2, 38–42, 47 - **What it is:** Using deep-rooted plants (e.g., **Vetiver grass** with $3\text{–}4\text{ m}$ roots, Napier grass, bamboo, mangroves, willow stakes) alone or with biodegradable jute geotextiles to bind bank soil and damp flow velocity. - **Advantages:** 1. **Low cost:** Much cheaper than imported boulders or concrete blocks and uses local labor/plants. 2. **Self-healing & strengthens over time:** Roots grow denser each year, increasing soil shear strength and draining pore water. 3. **Eco-friendly:** Captures carbon ($\text{CO}_2$), creates fish/wildlife habitat, and looks natural. - **Disadvantages:** 1. **Not immediately effective:** Plants need months to grow deep roots and can be washed away if a flood hits right after planting. 2. **Cannot resist deep undercutting:** Roots only protect the upper $1\text{–}4\text{ m}$; they fail in deep rivers where toe scour occurs $10\text{–}20\text{ m}$ underwater or where velocities are extreme. --- #### Q11.3: What is a spur/groyne and what is a slack flow zone? Explain with sketches how a series of spurs develops a new bankline, permeable vs. impermeable groynes, and attracting, deflecting, and repelling groynes. - **Appeared in:** 2019 (Q4d, 5m), 2021 (Q3c, 6m), 2024 (Q1f, 5m), 2025 (Q4c, 2m; Q8a, 5m; Q8c, 2m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 4, 6–12 - **Spur / Groyne:** A transverse structure extending from the riverbank into the channel to deflect current away from the bank and trap sediment. - **Slack Flow Zone:** The calm, low-velocity pocket of water created between two adjacent spurs where eddies circulate slowly and suspended silt settles out. - **How a Series of Spurs Develops a New Bankline:** 1. Spurs block fast near-bank flow and push the main current toward the river center. 2. Inside the calm **slack zone** between spurs, sediment continuously deposits during floods, building up sandbars that merge and vegetate into a **new permanent bankline**. **Sketches to Draw (1 min):** ```text (a) Bankline Development by Series of Spurs (b) Groyne Orientations (Flow ===>) Old Eroded Bank ___________________________ Bank _______________ _______________ _______________ | Spur | Slack Zone | Spur | \Attracting |Deflecting Repelling/ | | (Siltation) | | \ (45-60 deg | (90 deg) (10-60 deg New Bankline ->|......|.............|......| v downstream) v v upstream) Main Flow =====>======>=============>======> Flow =======> =======> =======> ``` **Permeable vs. Impermeable Groynes:** - **Impermeable (Solid) Groyne:** Built of sand/earth core armored with rock or CC blocks; completely blocks water and deflects current (best for strong currents and navigation channels, but causes heavy nose scour). - **Permeable Groyne:** Built of bamboo/timber piles or concrete porcupines; lets water pass through slowly, reducing velocity and causing rapid siltation with minimal nose scour (best for rivers rich in suspended sediment). **Classification by Orientation Angle:** 1. **Attracting Groyne (points downstream at $45^\circ\text{–}60^\circ$):** Pulls main current toward itself and keeps the deep channel close to that bank (not good for bank protection alone). 2. **Deflecting Groyne (perpendicular at $90^\circ$):** Diverts current locally away from the bank and creates a slack zone only on its downstream side. 3. **Repelling Groyne (points upstream at $10^\circ\text{–}60^\circ$ to normal):** Pushes current strongly away toward the opposite bank and deposits silt on **both** its upstream and downstream sides (most effective for bank protection). --- #### Q11.4: State the design rules for groynes (length, spacing, number) and their role in flood/erosion management in Bangladesh. - **Appeared in:** 2019 (Q6c, 10m), 2022 (Q8c, 15m) - **Time & Length Budget:** 6–8 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 11–14 1. **Groyne Length ($L_g$):** - Should not exceed **one-fifth of the river width** ($L_g \le W / 5$) so it does not over-constrict the channel. - Must be at least **$2.5\times$ the maximum scour depth** ($L_g \ge 2.5 d_s$, where $d_s = 1.35 (q^2/f)^{1/3}$) to keep the nose scour hole safely away from the riverbank. 2. **Groyne Spacing ($S_g$):** - Set at **$2\text{ to }2.5\times$ the groyne length** ($S_g = 2 L_g \text{ to } 2.5 L_g$) so the deflected current does not re-attach to the bank between spurs (wider spacing on convex banks, closer on concave bends). 3. **Number of Groynes ($N$):** - Installed in a **series of $2\text{ to }5$ groynes** so a continuous slack flow zone forms between them. 4. **Head / Nose Protection & Launching Apron:** - The riverward head must be streamlined (T-head, hockey-stick, or sloping round nose) and surrounded by a flexible rock/CC-block **launching apron** ($1.5 d_s$ wide) that settles automatically into the nose scour hole. 5. **Application in Bangladesh:** Used along the Jamuna (e.g., Sirajganj, Sariakandi, Betil, Enayetpur), Padma, and Teesta to deflect monsoon currents away from flood embankments and towns. --- #### Q11.5: Differentiate between open-joint (flexible) and close-joint (rigid) revetments and state where each is suitable. - **Appeared in:** 2023 (Q8c, 5m) - **Time & Length Budget:** 4 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 29 | Feature | Open-Joint (Flexible) Revetment | Close-Joint (Rigid) Revetment | | :--- | :--- | :--- | | **Joint Construction** | Gaps between stones/CC blocks are left **unsealed** over a geotextile/gravel filter. | Joints between blocks/slabs are **sealed tightly** with cement mortar or asphalt. | | **Drainage & Uplift Pressure** | Allows bank groundwater to **drain freely** when river water level drops, preventing hydrostatic uplift pressure. | **Traps groundwater** behind the slab; high uplift pressure can crack or blow out the revetment when river level falls rapidly. | | **Flexibility** | Can settle slightly to follow minor bank movement without breaking. | Brittle; cracks if underlying soil washes out or settles. | | **Where Suitable** | **Below High Water Level (HWL)** and in rivers with **large seasonal or tidal water-level fluctuations**. | **Only above High Water Level (HWL)** or where groundwater table fluctuations are negligible. | --- #### Q11.6: Define and differentiate between Riprap and Gabion revetments. - **Appeared in:** 2022 (Q4a, 8m — part), 2024 (Q1f, 5m — part) - **Time & Length Budget:** 4 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 30–32 - **Riprap:** A flexible armor layer of **loose, heavy angular boulders/stones** dumped or hand-placed over a filter layer on a riverbank slope. - **Gabion:** Rectangular **galvanized or PVC-coated wire-mesh cages/mattresses** filled with smaller stones and wired together on the bank. | Feature | Riprap | Gabion | | :--- | :--- | :--- | | **Stone Size Needed** | Requires **large, heavy rocks ($> 30\text{ cm}$)** so individual stones are not washed away. | Uses **smaller, cheaper stones ($8\text{–}20\text{ cm}$)** held together inside the wire cage. | | **Bank Slope** | Needs a **gentle slope ($\le 1V:2H$)** so loose rocks do not roll down. | Can be stacked on **steep banks** (up to $1V:1H$ or stepped walls) for velocities of $2\text{–}5\text{ m/s}$. | | **Cost & Risk** | Expensive in Bangladesh (large boulders must be imported), but never rusts. | Cheaper stone cost, but wire mesh can corrode or tear over time under heavy abrasion. | --- #### Q11.7: What are Bandals and how do they protect riverbanks and improve dry-season navigation channels? - **Appeared in:** 2025 (Q4c, 2m — part) - **Time & Length Budget:** 2 Minutes | [SA: Lec 11](SA/EWCE-%20461-Lec11.pdf) p. 43–46 - **Bandal:** A low-cost indigenous bamboo-and-mat screen erected at an angle ($30^\circ\text{–}40^\circ$) to the flow during the falling flood stage, blocking the **upper half of the water depth** while leaving the **bottom half open**. - **Working Principle:** The upper mat slows surface water and forces suspended silt to settle near the bank (protecting the bank), while bottom water accelerates under the screen, scouring a **single deeper navigation channel** for the dry season. --- ### SA Lecture 12: Inland Navigation, Meander Cut-Offs & River Dredging - **Source:** [SA/EWCE 461-Lec12.pdf](SA/EWCE%20461-Lec12.pdf) --- #### Q12.1: Why is inland water transport the cheapest mode of transport? State its advantages over road and rail. - **Appeared in:** 2022 (Q4c, 8m — part), 2024 (Q4d, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 1–8 1. **Lowest Energy / Fuel Resistance:** Water friction is very low at moderate ship speeds—**$1\text{ horsepower}$ moves $4\text{ tons}$ on water**, compared to only $0.5\text{ tons}$ on rail and $0.15\text{ tons}$ on road (fuel consumption ratio Water : Rail : Road $= 1 : 1.5 : 4$). 2. **Lowest Freight Cost:** Relative transport cost is **Water : Rail : Road $= 1 : 2 : 5$** (in Bangladesh: $\text{Tk }1.7/\text{ton-km}$ by water vs. $\text{Tk }5.6$ by rail and $\text{Tk }6.5$ by road). 3. **Massive Payload Capacity:** A single barge tow carries as much cargo as dozens of rail cars or hundreds of trucks. 4. **Minimal Land & Infrastructure Cost:** Rivers are natural highways requiring no land acquisition, paving, or rail tracks. 5. **Lowest Pollution & Highest Safety:** Emits the least $\text{CO}_2$, causes minimal noise, and has the lowest accident rate ($7\times$ lower external social cost than road transport). --- #### Q12.2: Explain the morphological and flow factors that affect the navigability of a river. - **Appeared in:** 2022 (Q4b, 8m), 2023 (Q4d, 6m), 2025 (Q4c, 2m) - **Time & Length Budget:** 6 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 12–16 **A. Morphological (Channel Geometry) Factors:** 1. **Water Depth & Under-Keel Clearance:** Channel depth must exceed vessel draft plus safety clearance (raising clearance from $0.6\text{ m}$ to $1.5\text{ m}$ cuts fuel use by $25\%$). 2. **Channel Width:** Must allow two-way vessel passing plus bank clearance; must be **wider at river bends** because ships swing out at a drift angle when turning. 3. **Radius of Curvature at Bends ($r_c$):** Sharp bends stop long barges from turning safely; minimum bend radius should be **$r_c \ge 3 L_{\text{fleet}}$** (and never $< 2 L_{\text{fleet}}$). 4. **Cross-Sectional Blockage Factor ($A_c / A_v$):** Ratio of wet channel area ($A_c$) to submerged vessel cross-section ($A_v$). If $A_c / A_v < 6$, water cannot easily pass around the hull, creating high drag resistance and waves (**requires $A_c / A_v \ge 6$**). **B. Flow (Hydraulic) Factors:** 1. **Longitudinal Velocity:** High velocity ($> 3\text{ m/s}$) makes upstream travel slow and fuel-heavy, and makes stopping/steering dangerous when going downstream. 2. **Transverse & Helical Currents:** Cross-currents at bends, confluences, or lock entrances ($> 0.3\text{ m/s}$) push vessels sideways toward banks or piers. 3. **Shoals, Rapids & Eddies:** Shifting sandbars (chars) and whirlpools block the fairway during the dry season. --- #### Q12.3: Explain the three methods of achieving or improving the navigability of a river. - **Appeared in:** 2024 (Q8a, 6m) - **Time & Length Budget:** 5–6 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 16–21 1. **Channel Regulation (Open-River Training & Dredging):** - Uses spurs, bandals, and revetments to concentrate low flows into a single deep channel so the river scours its own bed, supplemented by annual **maintenance dredging** across shallow crossings. - *Best for:* Large rivers with sufficient discharge and moderate slope. 2. **Navigation Lock and Dam (Slack-Water Impoundment):** - Series of low dams/barrages create deep, slow pools behind them; gated **navigation locks** lift or lower ships step-by-step from one pool to the next by filling or emptying the lock chamber by gravity. - *Pros/Cons:* Works even with small discharge and steep slopes, but is very expensive, delays ships at locks, and silts up rapidly in high-sediment rivers. 3. **Canalization (Bypass Canal):** - Excavating an artificial cut or canal through land to bypass rock rapids, sharp meander bends, or shallow reaches, or to link two separate rivers. --- #### Q12.4: Explain with a figure the morphological consequences of a meander cut-off. - **Appeared in:** 2023 (Q8e, 5m) - **Time & Length Budget:** 5 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 22–25 **Sketches to Draw (1 min):** ```text (a) Plan View of Cut-Off (b) Longitudinal Bed Profile Response /~~~~~~\ Abandoned Loop Elev ^ Upstream Erosion | | / Oxbow \ (Silts up) | (Headcutting) |Cut-Off ======A ======== B======> | \...................| | Cut-Off Channel | \ Original Slope \Steep | \......... Deposition (Short path -> Steep slope) | \__________________\Slope|__\________ Downstream +-----------------------A-----B-----------> Distance ``` **Morphological Consequences:** 1. **Sudden Slope Steepening between A and B:** Cutting straight across the narrow neck between upstream point **A** and downstream point **B** shortens the travel distance while the elevation drop ($\Delta Z$) stays the same, **sharply increasing bed slope ($S$), velocity, and stream power**. 2. **Upstream Erosion (Headcutting):** Fast flow at the entrance **A** scours the bed, creating a steep drop (nickpoint) that migrates **upstream**, lowering upstream bed levels and water tables. 3. **Downstream Deposition (Aggradation):** The heavy load of sediment scoured from the cut-off dumps below **B** where the river flattens out again, raising the downstream bed and flood levels. 4. **Oxbow Lake Formation:** Slow water at the entrances of the old curved loop silts up, sealing it off into an **oxbow lake**. --- #### Q12.5: Explain the four phases of dredging, compare hydraulic vs. mechanical dredgers (and cutter suction dredger), and distinguish between capital and maintenance dredging. - **Appeared in:** 2022 (Q4c, 4m; Q4d, 6m), 2023 (Q8f, 7m), 2024 (Q7a, 2.5m; Q8b, 8m), 2025 (Q4c, 2m; Q8b, 7m) - **Time & Length Budget:** 6–7 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 24, 26–31; [SB: Lec 5](SB/Lecture-5.pdf) p. 10–20 **Four Phases of the Dredging Process:** 1. **Pretreatment:** Breaking hard rock or stiff cemented soil mechanically or chemically before excavation. 2. **Extraction:** Dislodging sediment from the riverbed and lifting it vertically to the water surface. 3. **Transportation:** Moving the dredged mixture horizontally via floating slurry pipelines, barges, or self-propelled hoppers. 4. **Disposal / Placement:** Dumping the spoil in deep water, inside diked containment areas, or onto low land for reclamation. **Hydraulic vs. Mechanical Dredging:** - **Hydraulic Dredger:** Mixes loose sand/silt with water into a liquid slurry and sucks it up through a centrifugal pump and pipeline (or hopper). **High production rate, low unit cost, and transports spoil directly via pipeline**, but cannot dig stiff boulders/debris. - **Mechanical Dredger:** Uses buckets, clamshell grabs, or dipper shovels to scoop stiff cohesive clay or gravel onto barges. **Works in tight spaces and hard soil**, but is slow and expensive for large river reaches. - **Cutter Suction Dredger (CSD):** Combines **both**—a rotating mechanical cutter head at the suction intake breaks up compacted soil, and a heavy centrifugal pump sends the slurry through floating pipes up to $10\text{ km}$ away. **Capital vs. Maintenance Dredging:** - **Capital Dredging:** One-time deep excavation to create a brand-new channel, deepen a port, or revive a dead/silted river (e.g., BWDB dredging the Gorai River off-take). - **Maintenance Dredging:** Routine, periodic (annual) removal of newly deposited monsoon silt to keep existing navigation channels and ferry routes open (e.g., BIWTA dredging ferry ghats). --- #### Q12.6: Explain with longitudinal profile figures why capital/navigation dredging is generally not sustainable in alluvial rivers. - **Appeared in:** 2022 (Q4d, 6m), 2024 (Q8b, 8m) - **Time & Length Budget:** 6–8 Minutes | [SA: Lec 12](SA/EWCE%20461-Lec12.pdf) p. 37–46; [SB: Lec 5](SB/Lecture-5.pdf) p. 22 **Longitudinal Profile Sketch of a Dredged Trench (1 min):** ```text Flow ===> Upstream Nickpoint (Steep) Downstream Edge Original Bed ______________| |___________________ | Erodes back |\ 1. Headcutting | \ 3. Hungry-water | (Upstream) | \ 2. Siltation in Deep Pit | \ bed scour | \................................| \ (Downstream) |___________________________________| Dredged Trench (Velocity drops) ``` **Why Navigation / Capital Dredging is Unsustainable:** 1. **Upstream Nickpoint Erosion (Headcutting):** Digging a trench creates a sudden steep drop at the upstream end. Local slope and stream power spike here, causing the edge to erode backward upstream. 2. **Rapid Siltation Inside the Dredged Pit:** Inside the deepened trench, flow cross-sectional area ($A$) is larger, so water velocity ($V = Q/A$) and bed shear stress drop sharply. Incoming bed load and suspended sediment immediately settle in the trench, filling it back up. 3. **Downstream "Hungry-Water" Erosion:** Because the trench traps incoming sediment, the water leaving the downstream end of the pit is starved of sediment and scours the downstream bed and banks. 4. **Spoil Resuspension & Monsoon Refilling:** Dredged sand dumped on nearby banks or shallow water washes right back into the cut during rain and floods. In high-sediment rivers of Bangladesh (e.g., the $1999$ Gorai River capital dredging), **a single monsoon flood completely refills the dredged channel** back to its original bed level. --- ## Section B: Flood Management (`EWCE 461/SB`) --- ### SB Lecture 1: Overview of Floods, Types & Causes of Floods in Bangladesh - **Source:** [SB/Lecture-1.pdf](SB/Lecture-1.pdf) --- #### Q1.1: Why are normal floods considered a blessing ("Borsha") rather than a disaster ("Bonna")? What are the positive outcomes of normal and flash floods? - **Appeared in:** 2022 (Q8b, 5m) - **Time & Length Budget:** 5 Minutes | [SB: Lec 1](SB/Lecture-1.pdf) p. 9, 12–14 In Bangladesh, normal seasonal inundation (**"Borsha"**) covers about $20\%\text{–}25\%$ of the land and is essential for rural life, whereas only extreme damaging floods are called **"Bonna"**: 1. **Soil Fertility Renewal:** Floodwaters deposit fresh mineral-rich **alluvial silt** across floodplains, naturally fertilizing agricultural fields without chemical fertilizers. 2. **Soil Moisture & Groundwater Recharge:** Recharges shallow aquifers and stores vital soil moisture for dry-season (*Rabi* and *Boro*) crops. 3. **Open-Water Fisheries:** Connects rivers with floodplains, *beels*, and *haors*, creating spawning and feeding grounds for inland fish. 4. **Flushing & Navigation:** Flushes out stagnant pollutants and salinity, and provides deep water for cheap rural boat transport. --- #### Q1.2: Explain the four major types of floods in Bangladesh with their locations, timing, and impacts. - **Appeared in:** 2023 (Q6a, 10m — part) - **Time & Length Budget:** 6 Minutes | [SB: Lec 1](SB/Lecture-1.pdf) p. 10–24, 40 | Flood Type | Where It Occurs | Timing & Duration | Cause & Main Impact | | :--- | :--- | :--- | :--- | | **1. Flash Flood** | North-eastern (*Haor* basin: Sylhet, Sunamganj), northern, and south-eastern hill borders (Chattogram Hill Tracts). | **Pre-monsoon (April–May)**; rises and falls within hours to a few days. | Intense cross-border hill rain; destroys standing ripe **Boro rice** just before harvest and breaches submersible dykes. | | **2. Monsoon River Flood** | Major river floodplains of the **Ganges–Brahmaputra–Meghna (GBM)** system ($25\%\text{–}68\%$ of country). | **Monsoon (June–September)**; lasts weeks to months. | Prolonged monsoon rain + Himalayan snowmelt overtop riverbanks; damages *Aman* crops, homes, and roads when extreme. | | **3. Rain-Fed / Urban Drainage Congestion** | Low-lying inland depressions (*beels*) and major cities (**Dhaka, Chattogram, Khulna**). | **Monsoon (May–October)** after heavy local downpours. | High-intensity local rainfall exceeds choked storm drains and filled-in canals; floods streets, slums, and ground floors. | | **4. Coastal Storm Surge Flood** | Southern coastline and offshore islands along the **Bay of Bengal** ($~800\text{ km}$ coast). | **Pre-monsoon (Apr–May)** & **Post-monsoon (Oct–Nov)**. | Tropical cyclones push sea water $3\text{–}9\text{ m}$ high over coastal polders; causes loss of life and long-term **soil/water salinity**. | --- #### Q1.3: Explain the geographical, physiographic, and hydro-meteorological factors responsible for recurring floods in Bangladesh. - **Appeared in:** 2019 (Q5b, 8m), 2022 (Q7a, 10m), 2023 (Q5a, 10m), 2024 (Q5b, 10m), 2025 (Q6a, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 1](SB/Lecture-1.pdf) p. 25–39, 41 **A. Geographical & Physiographic Factors:** 1. **Lowermost Riparian Funnel ($7\%$ vs. $93\%$):** Bangladesh occupies only **$7\%$** of the massive $1.72\text{ million km}^2$ GBM catchment, yet must drain **$93\%$ of the runoff** generated outside its borders through a narrow coastal outlet. 2. **Flat Deltaic Topography:** Over **$80\%$ of Bangladesh is a flat floodplain** lying less than $10\text{ m}$ above mean sea level, so any rise in river stage spreads miles across the land. 3. **Steep Mountain Wall at the Border:** The Himalayas to the north and Meghalaya/Tripura hills right at the border force moist monsoon winds upward, dumping world-record rainfall (e.g., Cherrapunji) that rushes directly onto flat plains. 4. **Riverbed Siltation & Deforestation:** Upstream deforestation in Nepal/India washes **$>1\text{ billion tons/year}$ of sediment** into Bangladesh's flat rivers, raising riverbeds and shrinking channel conveyance capacity. 5. **Unplanned Human Interventions:** Upstream dams/barrages (e.g., Farakka) and unplanned floodplain roads, embankments, and urban canal encroachment block natural drainage. **B. Hydro-Meteorological & Tidal Factors:** 1. **Intense Monsoon Rainfall:** $80\%$ of the heavy annual rainfall ($1,200\text{–}5,500\text{ mm}$) falls within just four months (June–September). 2. **Synchronization of GBM Flood Peaks:** When flood peaks of the **Brahmaputra-Jamuna** (July–August) and **Ganges-Padma** (August–September) arrive at the same time (as in $1988$ and $1998$), catastrophic nationwide flooding occurs. 3. **Himalayan Snow & Glacier Melt:** Summer melting adds high baseflow to rivers before the monsoon rains even start. 4. **Bay of Bengal Backwater Effect:** High spring tides, strong south-westerly monsoon winds, and long-term sea-level rise pile seawater up in the Meghna estuary, slowing down river outflow into the sea. --- #### Q1.4: Briefly describe the river system network of Bangladesh and the dominance of the Brahmaputra–Jamuna system. - **Appeared in:** 2019 (Q5c, 6m), 2024 (Q8c, 8m — part) - **Time & Length Budget:** 5 Minutes | [SB: Lec 1](SB/Lecture-1.pdf) p. 29–36 - **Dense Interlinked Network:** Bangladesh is crisscrossed by **about $230$ active rivers** (including **57 transboundary rivers**—54 from India and 3 from Myanmar) that are tributaries and distributaries of three major systems: 1. **Brahmaputra–Jamuna System:** Enters from the north; feeds distributaries like the **Old Brahmaputra** and **Dhaleshwari**, and receives tributaries like the **Teesta, Dharla, Dudhkumar, and Karatoya–Atrai**. 2. **Ganges–Padma System:** Enters from the west; feeds south-western distributaries (**Gorai–Madhumati, Arial Khan, Mathabhanga, Kobadak**) and receives the **Mahananda**. 3. **Surma–Kushiyara–Meghna System:** Drains the north-eastern Sylhet *haor* basin and south-eastern hills (**Gumti, Muhuri, Khowai, Manu**). - **Dominance of the Brahmaputra–Jamuna:** - It is one of the world's largest braided sand-bed rivers ($10\text{–}15\text{ km}$ wide, peak discharge up to $100,000\text{ m}^3/\text{s}$), carrying the largest share of Bangladesh's monsoon floodwater and over $500\text{ million tons/year}$ of sediment. --- ### SB Lecture 2: Flood Process in Urban and Rural Areas - **Source:** [SB/Lecture-2.pdf](SB/Lecture-2.pdf) --- #### Q2.1: Define urban flooding. How does urban flooding differ from rural flooding? Explain the major impacts of urban flooding. - **Appeared in:** 2021 (Q7b, 12m — part), 2024 (Q6b, 10m), 2025 (Q7a, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 2](SB/Lecture-2.pdf) p. 8, 14–19 - **Urban Flooding:** Inundation of built-up city land when intense rainfall generates **stormwater** runoff faster than concrete surfaces can absorb it or urban storm sewers, box culverts, and canals can drain it away. **Differences Between Urban Flooding and Rural Flooding:** | Feature | Urban Flooding (Stormwater) | Rural Flooding (Floodwater) | | :--- | :--- | :--- | | **Water Source** | **Stormwater:** Local rain falling directly on paved city roofs, roads, and parking lots. | **Floodwater:** River water overflowing banks from a large upstream watershed. | | **Infiltration & Runoff** | **Impervious concrete/asphalt** blocks infiltration ($\sim 55\%$ surface runoff vs. $10\%$ in nature); **very fast, sharp peak**. | **Pervious soil & vegetation** soak up $\sim 50\%$ of rain; runoff rises slowly over days or weeks. | | **Main Causes** | Filling of natural wetlands/canals (*khals*), clogged storm drains (plastic/solid waste), and pump failures. | Heavy basin rainfall, river siltation, embankment breaches, and tidal backwater. | | **Benefit vs. Harm** | **Purely destructive** (filthy water mixed with sewage and industrial waste). | Moderate rural floods (**Borsha**) bring fertile silt, moisture, and fish. | **Five Major Impacts of Urban Flooding:** 1. **Economic Losses:** Floods ground-floor shops, factories, power substations, and telecom networks, halting daily business and wage earning. 2. **Traffic & Transport Paralysis:** Submerges city roads and intersections, stalls vehicles, damages asphalt pavements, and causes hours-long gridlock. 3. **Human Health Hazards:** Stormwater mixes with overflowing **sanitary sewers and garbage**, spreading waterborne diseases (**cholera, diarrhea, typhoid, skin infections**) and breeding **dengue mosquitoes**. 4. **Environmental Contamination:** Washes toxic heavy metals, oils, and industrial chemicals into urban lakes and rivers. 5. **Social & Livestock Distress:** Forces low-income slum residents to live in knee-deep dirty water without clean drinking water or sanitation, and drowns urban poultry/livestock. --- ### SB Lecture 3: Flood Management Measures, Dams & Hydraulic Structures - **Source:** [SB/Lecture-3.pdf](SB/Lecture-3.pdf) --- #### Q3.1: Outline the four phases of planning for flood management. - **Appeared in:** 2025 (Q5c, 8m — part) - **Time & Length Budget:** 4 Minutes | [SB: Lec 3](SB/Lecture-3.pdf) p. 7–8 1. **Pre-Disaster Mitigation (Long-Term Prevention):** Building embankments/drainage, enforcing floodplain zoning and building codes, watershed afforestation, and buying flood insurance to reduce future risk. 2. **Preparedness (Before the Flood Season):** Operating flood forecasting and early warning systems, stocking food/medicine in flood shelters, and training local rescue volunteers. 3. **Disaster Response (During the Flood):** Evacuating stranded people and livestock to raised shelters, sandbagging weak dykes, and distributing emergency drinking water, food, and medical aid. 4. **Post-Flood Recovery (After Waters Recede):** Repairing breached embankments, roads, and homes; pumping out stagnant water; and supplying farmers with seeds, fertilizer, and low-interest loans. --- #### Q3.2: Differentiate between "Flood Control" and "Flood Management." Illustrate short-term and long-term measures for flood management. - **Appeared in:** 2021 (Q5a, 13m), 2024 (Q6c, 8m), 2025 (Q7b, 5m) - **Time & Length Budget:** 6–8 Minutes | [SB: Lec 3](SB/Lecture-3.pdf) p. 9–11, 16 **Flood Control vs. Flood Management:** - **Flood Control (Old Concept):** Attempts to **completely stop or contain floodwaters** away from land using heavy engineering structures (high levees, dams, floodwalls). It gives a false sense of $100\%$ safety, harms floodplain ecology/fisheries, and causes catastrophic damage if a levee breaches. - **Flood Management (Modern Integrated Concept):** Accepts that **absolute protection from all floods is impossible**; instead, it combines structural defenses with non-structural measures (forecasting, zoning, flood proofing, watershed care) to **"live with floods," minimize damage, and preserve ecological benefits**. **Short-Term vs. Long-Term Flood Management Measures:** 1. **Short-Term (Emergency & Immediate) Measures:** - Real-time **flood forecasting and early warning** broadcasts. - Temporary **sandbagging** and patrolling to stop embankment overtopping or piping. - **Evacuation** of people and cattle to raised flood shelters (*killas*). - Emergency **pumping** of waterlogged areas and distribution of relief (clean water, oral saline, food). 2. **Long-Term (Permanent & Strategic) Measures:** - **Structural:** Constructing and maintaining reservoirs, detention basins, levees/polders, diversion floodways, and channel dredging. - **Non-Structural:** Enforcing **floodplain zoning** and land-use laws, **flood-proofing** homes (raising plinths above HFL), **watershed reforestation**, and crop-calendar adaptation. --- #### Q3.3: On what factors do damages from a flood event depend? - **Appeared in:** 2019 (Q5c, 4m), 2025 (Q7b, 5m — part) - **Time & Length Budget:** 4 Minutes | [SB: Lec 3](SB/Lecture-3.pdf) p. 12; [SB: Lec 9](SB/Lecture-9.pdf) p. 13–17 Flood damage depends on **six key factors**: 1. **Land Use & Location Affected:** Flooding a densely populated city or industrial zone causes far higher monetary loss than flooding open pasture. 2. **Depth of Inundation:** Deeper water submerges raised plinths, machinery, and tall crops (primary variable in stage-damage curves). 3. **Duration of Inundation:** Standing water lasting more than $1\text{–}2\text{ weeks}$ rots crop roots, softens earthen road/house foundations, and halts livelihoods. 4. **Flow Velocity:** High velocity ($> 1.5\text{ m/s}$) sweeps away people, erodes riverbanks, and collapses buildings. 5. **Rate of Water Level Rise:** Rapid rise (flash flood) leaves no time for evacuation or moving goods to safety. 6. **Timing / Seasonality (Crop Calendar):** A flood hitting in late April ruins mature *Boro* rice right before harvest, whereas the same water level in an empty post-harvest field causes minor crop loss. --- #### Q3.4: Explain structural and non-structural measures for flood management and how they are applied in Bangladesh. - **Appeared in:** 2019 (Q5a, 12m), 2023 (Q6a, 10m — part) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 3](SB/Lecture-3.pdf) p. 13–18, 46–47 **A. Structural Measures (Physical Engineering Works that Keep Water Away from People):** 1. **Storage Reservoirs & Detention Basins:** Trap flood peaks upstream and release water gradually after the storm passes. 2. **Levees / Embankments & Polders:** Earthen dykes built parallel to rivers or around coastal islands to block overflow (Bangladesh has $> 10,000\text{ km}$ of embankments and $139$ coastal polders). 3. **Submersible Embankments:** Low dykes in the north-east *Haor* basin built just high enough to block April–May pre-monsoon flash floods until *Boro* rice is harvested, then intentionally overtopped by June monsoon floods to let fish and silt in. 4. **Floodwalls:** Vertical reinforced-concrete walls protecting cramped city riverfronts (e.g., parts of Dhaka/Narayanganj). 5. **Floodways & Diversion Channels:** Bypass channels that divert excess peak flow around cities or into depressions. 6. **Channel Improvement & Dredging:** Straightening bends, removing obstructions, and dredging to increase discharge capacity. **B. Non-Structural Measures (Policy, Planning & Adaptation that Keep People Out of Harm's Way):** 1. **Flood Forecasting & Early Warning System (FFWS):** Operated by BWDB across major rivers to give $3\text{–}10\text{ days}$ lead time. 2. **Floodplain Zoning & Land-Use Regulation:** Restricting high-value construction in high-risk floodways. 3. **Flood Proofing:** Raising homestead earthen plinths (*vita*) above the $20\text{–}100\text{ year}$ flood level (e.g., Chars Livelihoods Programme). 4. **Watershed & Land Management:** Reforestation, contour farming, and wetland conservation to slow surface runoff. 5. **Emergency Preparedness, Shelters & Insurance:** Multi-purpose cyclone/flood shelters, raised earthen *killas* for cattle, and post-flood agricultural rehabilitation. --- #### Q3.5: Draw a typical dam/reservoir layout showing storage zones, classify dams according to use, differentiate between Weir, Barrage, and Dam, and list the causes of dam failure. - **Appeared in:** 2019 (Q6a, 10m; Q6b, 10m), 2021 (Q6a, 10m), 2022 (Q8a, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 3](SB/Lecture-3.pdf) p. 19–25, 31–38 **Sketch of Reservoir Storage Zones (1 min):** ```text | Dam Crest Max Water Level (MWL) |~~~~~~~~~~~~~~~~~| <-- 1. Flood / Surcharge Storage (Spillway) Normal Pool Level |-----------------| <-- 2. Active / Useful Storage (Irrigation, Power, Supply) Min Pool Level |.................| <-- 3. Dead Storage (Reserved for Sediment Accumulation) Riverbed |_________________|______ ``` **Classification of Dams According to Use:** 1. **Storage Dam:** Impounds surplus wet-season runoff for dry-season irrigation, municipal water supply, hydroelectric power, and recreation (e.g., Kaptai Dam). 2. **Diversion Dam (Weir / Barrage):** Raises river water level slightly without large storage so water can flow by gravity into irrigation canals (e.g., Teesta Barrage). 3. **Detention Dam:** Temporarily holds back flash flood peaks to prevent downstream flooding and releases water slowly through outlets; a **Dry Dam** holds no permanent pool and stays dry between floods. 4. **Coffer Dam:** A temporary enclosure built in a river and pumped dry so workers can construct a permanent dam or bridge pier inside. 5. **Check Dam & Saddle Dam:** **Check dams** are small barriers across gullies to trap soil erosion; **saddle dams** are auxiliary dykes plugging low spots along a reservoir rim. **Differences Between Dam, Weir, and Barrage:** | Feature | Dam | Weir | Barrage | | :--- | :--- | :--- | :--- | | **Primary Purpose** | Creates a **deep storage reservoir** for flood control, hydropower, and water supply. | Raises water level across a river using a **fixed solid masonry crest** for diversion. | Raises and regulates water level using **adjustable gates across the full river width**. | | **Flow Control** | Controlled by high-level spillways and sluice outlets. | **No gates** over the main crest; water simply spills over the top (causes high upstream afflux/silting). | **Fully gated** from bank to bank; gates open flat during floods to flush silt and pass peak flow safely. | **Main Causes of Dam Failure:** 1. **Overtopping ($\sim 35\%$):** Inadequate spillway capacity during extreme floods causes water to pour over the dam crest and wash it away. 2. **Foundation Defect & Piping/Seepage ($\sim 30\%\text{–}38\%$):** Water seeping under or through an earth dam washes out fine soil particles (**piping**) or causes foundation sliding/settlement. 3. **Slope Instability & Structural Cracking:** Earthquakes, rapid reservoir drawdown, or poor compaction causing embankment slumping. --- #### Q3.6: Write short notes on: (i) Floodwall and Levee, (ii) Gravity Dam, and (iii) Diversion Dam. - **Appeared in:** 2024 (Q6a, 6m), 2025 (Q5a, 6m) - **Time & Length Budget:** 6 Minutes (2 mins each) | [SB: Lec 3](SB/Lecture-3.pdf) p. 14, 27–28, 35–36; [SB: Lec 4](SB/Lecture-4.pdf) p. 21; [SB: Lec 7](SB/Lecture-7.pdf) p. 36–38 1. **Floodwall and Levee:** - Both are longitudinal barriers built parallel to a river to stop floodwaters from entering protected land. - A **levee (embankment)** is a wide, compacted **earthen dyke** with gentle side slopes ($1V:2H\text{ to }1V:3H$) that requires a wide strip of land but is cheap to build. - A **floodwall** is a narrow, vertical **reinforced-concrete or masonry wall** built where riverside land is scarce or expensive (such as crowded cities); it includes movable **floodgates** for road access and interior drainage pumps. 2. **Gravity Dam:** - A massive dam built of concrete or stone masonry designed to hold back water **purely by its own downward weight ($W$)** resisting the horizontal push of water pressure. - It has a roughly triangular cross-section (vertical upstream face, sloping downstream face) and requires a **strong, solid rock foundation** so it cannot slide or overturn. 3. **Diversion Dam:** - A relatively low barrier (such as a **barrage** or **weir**) built across a river not to store a large volume of water, but to **raise the upstream water level (head)** enough to redirect water into off-taking irrigation canals, power flumes, or flood bypass channels (e.g., Teesta and Ganges/Farakka barrages). --- ### SB Lecture 4: Diversion Channels, Floodways & Floodwalls - **Source:** [SB/Lecture-4.pdf](SB/Lecture-4.pdf) --- #### Q4.1: Define flood diversion channel (floodway). Explain its importance, advantages, and disadvantages for flood management, and distinguish between SPF and PMF. - **Appeared in:** 2022 (Q5a, 10m), 2023 (Q6c, 10m), 2024 (Q5d, 5m) - **Time & Length Budget:** 5–10 Minutes | [SB: Lec 4](SB/Lecture-4.pdf) p. 6–10 **Sketch of a Flood Diversion Channel / Bypass (45 sec):** ```text [Protected City / Town] Main River =====>[Control Gate]=======>======>[Rejoins Downstream]===> \ / \==== Flood Bypass =====/ (Carries excess peak flow around town) ``` - **Flood Diversion Channel (Floodway):** A natural or artificial bypass channel that branches off from a main river upstream of a vulnerable town/area to carry away excess flood peak discharge and return it safely downstream (or into a lake/sea). - **Advantages / Importance:** 1. **Lowers peak flood levels** in the main river right through crowded cities where raising levees higher is impossible. 2. **Reduces main-channel flow velocity**, protecting riverbanks and bridge piers from severe scour. 3. Dry floodways can be used as **farmland, pasture, or parks** during the $9\text{–}10$ dry months of the year. - **Disadvantages:** 1. **Requires huge land acquisition** and resettlement along the bypass route. 2. **Morphological imbalance (by Lane's relation):** Drawing water ($Q_w \downarrow$) out of the main river reduces its sediment-carrying power, causing **siltation and bar formation in the main river** just below the intake while the steep bypass channel itself may scour. 3. **High cost** of gated headworks, highway bridges across the floodway, and annual dredging. - **SPF vs. PMF:** - **Standard Project Flood (SPF):** The flood discharge resulting from the most severe combination of meteorological and hydrological conditions considered **reasonably characteristic** of the region (typically $40\%\text{–}60\%$ of PMF; used for urban levees/bypass channels). - **Probable Maximum Flood (PMF):** The theoretical **largest possible flood** that could ever occur under the worst physical combination of Probable Maximum Precipitation (PMP) and saturated snowmelt/runoff conditions (used for high-hazard dam spillways so they never overtop). --- ### SB Lecture 5: Improvement of Channel Capacity by Dredging - **Source:** [SB/Lecture-5.pdf](SB/Lecture-5.pdf) *(Note: Dredging phases, hydraulic vs. mechanical dredgers, and morphological unsustainability are also covered in **SA Lecture 12 Q12.5 & Q12.6**).* --- #### Q5.1: Explain the purposes of river dredging and the morphological, hydraulic, and environmental impacts of dredging. - **Appeared in:** 2019 (Q7a, 10m), 2021 (Q7c, 10m), 2022 (Q7d, 5m), 2024 (Q5c, 5m) - **Time & Length Budget:** 6–8 Minutes | [SB: Lec 5](SB/Lecture-5.pdf) p. 7–9, 21–23 **Five Main Purposes of Dredging:** 1. **Flood Capacity Improvement:** Deepening and widening silted rivers and off-takes (e.g., Gorai, Old Brahmaputra) so they carry more monsoon flow without overtopping banks. 2. **Navigation Maintenance:** Keeping minimum draft depth open in shipping fairways, ports, and ferry ghats. 3. **Land Reclamation:** Pumping dredged river sand onto low-lying floodplains to raise land for townships, industrial zones, or homestead plinths. 4. **Environmental Cleanup & Mining:** Removing contaminated bottom sludge near industrial cities and mining construction sand. **Impacts of Dredging Across Three Perspectives:** 1. **Hydraulic & Morphological Impacts (2024 Q5c):** - Lowers water surface slope and flow velocity inside the dredged cut, which causes **rapid re-siltation** inside the trench. - Triggers **upstream bed erosion (nickpoint headcutting)** and **downstream "hungry-water" scour**. - Digging too close to riverbanks steepens the underwater toe slope, triggering **massive riverbank collapse**. 2. **Environmental & Ecological Impacts:** - **High Turbidity Plumes:** Stirred-up fine silt blocks sunlight, halting aquatic plant photosynthesis and clogging fish gills. - **Habitat Destruction:** Destroys bottom-dwelling (*benthic*) organisms and fish spawning beds. - **Toxic Resuspension:** Stirs up heavy metals, pesticides, and organic waste trapped in bottom mud, lowering dissolved oxygen (DO). 3. **Spoil Disposal Impacts:** Unplanned dumping of dredged sand on fertile agricultural land or wetlands ruins crops and washes back into the river with the next rain. --- ### SB Lecture 6: Land & Watershed Management for Flood Mitigation - **Source:** [SB/Lecture-6.pdf](SB/Lecture-6.pdf) --- #### Q6.1: What is watershed management and what are its main components? Why does land/watershed management play a key role compared to structural flood measures? - **Appeared in:** 2021 (Q5b, 5m; Q7b, 6m), 2023 (Q7c, 10m), 2024 (Q7a, 2.5m) - **Time & Length Budget:** 6–8 Minutes | [SB: Lec 6](SB/Lecture-6.pdf) p. 8–9, 12–17, 58 - **Watershed:** A topographically bounded drainage area from which all surface runoff flows to a single common river outlet. - **Watershed Management:** The integrated planning and use of land, vegetation, soil, and water resources within a drainage basin to reduce surface runoff, stop soil erosion, recharge groundwater, and sustain livelihoods. **Three Core Components of Watershed Management:** 1. **Land Management:** Contour farming, terracing on hill slopes, strip cropping, vegetative buffer strips, and reforestation. 2. **Water Management:** Rainwater harvesting (RWH), farm ponds, check dams across gullies, and wetland/beel conservation. 3. **Biomass / Socio-Economic Management:** Agro-forestry, controlled grazing, community participation, and sustainable crop selection. **Why Land / Watershed Management Plays a Key Role Compared to Structural Defenses:** 1. **Treats the Root Cause at the Source:** Structural works (levees, floodwalls) only fight floodwater *after* it reaches the river; land management **absorbs rain where it falls**, reducing and delaying the flood peak before it ever enters the channel. 2. **Stops Riverbed Siltation:** Forest cover and terracing hold topsoil on hills, preventing the heavy sediment load that silts up reservoirs, clogs rivers, and makes dredging fail. 3. **No Risk of Catastrophic Failure:** Unlike levees or dams that can breach suddenly and transfer worse flooding downstream, watershed conservation safely lowers basin-wide flood peaks. 4. **Multiple Year-Round Co-Benefits:** Recharges dry-season groundwater (preventing drought), improves soil fertility, supports biodiversity, and costs far less to maintain than concrete structures. --- #### Q6.2: Explain the effect of land-use change processes (urbanization, floodplain roads/embankments, and agricultural practices) on flood hazards, and outline the land-use & flood management planning process. - **Appeared in:** 2025 (Q5c, 8m — part; Q5d, 8m) - **Time & Length Budget:** 8 Minutes | [SB: Lec 6](SB/Lecture-6.pdf) p. 35–46, 49–55 **Effects of Land-Use Changes on Flood Hazards (2025 Q5d):** 1. **Urbanization & Wetland Filling:** - Replacing natural soil, forests, and retention ponds (*khals/beels*) with impervious roofs and asphalt cuts infiltration from $\sim 50\%$ to $<15\%$ and increases surface runoff from $10\%$ to **$55\%$**. - Runoff reaches drains much faster (shorter time of concentration), creating a **higher, sharper flood hydrograph peak**. 2. **Unplanned Floodplain Roads & Embankments:** - Rural roads and railways built across the floodplain direction without enough bridges/culverts act as **solid dams**, trapping floodwater on the upstream side and causing prolonged waterlogging. - Confining rivers between narrow embankments prevents silt from spreading onto floodplains, forcing sediment to settle on the riverbed and raising flood stages over time. 3. **Deforestation & Intensive Agriculture:** - Clearing hill forests (e.g., *Jhum* shifting cultivation) and plowing up-and-down slopes strips away protective plant cover. - Heavy rain compacts bare soil, accelerates sheet/gully erosion, and washes massive silt loads into rivers, reducing channel capacity. **Integrated Land-Use & Flood Management Planning Process (2025 Q5c):** 1. **Baseline Assessment:** Map watershed topography, soil types, land use, and historical flood hazard zones. 2. **Impact & Scenario Modeling:** Use hydrological models (lumped or distributed, e.g., HEC-HMS/SWAT) to test how proposed roads, towns, or forests will change runoff and flood peaks. 3. **Zone Allocation & Regulation:** Preserve natural floodways, wetlands, and retention basins as no-build zones; place cities and industry on higher ground. 4. **Implementation & Monitoring:** Enforce building codes, rainwater harvesting, and green buffers, and update plans as climate and land use evolve. --- ### SB Lecture 7: Flood Proofing Strategies - **Source:** [SB/Lecture-7.pdf](SB/Lecture-7.pdf) --- #### Q7.1: Define flood proofing and state its advantages and disadvantages. - **Appeared in:** 2021 (Q7a, 8m), 2024 (Q7d, 7m) - **Time & Length Budget:** 6–7 Minutes | [SB: Lec 7](SB/Lecture-7.pdf) p. 6, 11–12 - **Flood Proofing:** Any combination of structural adjustments, site changes, or emergency practices applied to **individual buildings and properties** to reduce or eliminate flood damage. - **Advantages:** 1. **Avoids Resettlement:** Allows families and businesses to stay safely on their own land and keep their livelihoods without relocating. 2. **Affordable When Community Levees Are Too Costly:** Ideal for scattered rural homes or river islands (*chars*) where building a massive embankment is economically impossible. 3. **Extra Layer of Safety:** Protects buildings even if a nearby river levee is overtopped or breached. 4. **Individual Control:** Homeowners can implement and maintain it themselves without waiting for large government projects. - **Disadvantages:** 1. **Does Not Protect Outside Areas:** Roads, crops, power lines, and yards around the house still get flooded, isolating residents during long floods. 2. **Requires Accurate Hazard Data & Warnings:** Contingent measures (flood shields, gates) fail if the owner is away or warning time is too short. 3. **Structural Failure Under Deep Floods:** Dry flood-proofed walls can crack or float (buoyancy uplift) if flood depth exceeds $\sim 3\text{ ft}$ ($1\text{ m}$). 4. **Can Create False Security:** Residents may refuse to evacuate during an extreme cyclone or deep flood, risking their lives. --- #### Q7.2: Briefly explain the seven considerations for developing a flood proofing strategy. - **Appeared in:** 2021 (Q6b, 10m), 2022 (Q5b, 10m), 2023 (Q6b, 10m), 2024 (Q5a, 10m), 2025 (Q6c, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 7](SB/Lecture-7.pdf) p. 13–20 1. **Flood Hazard Analysis (Hydraulic Forces):** - Determine **flood depth** (below $3\text{ ft}$ allows dry proofing; $>3\text{ ft}$ requires elevation or relocation), **flow velocity** (high velocity scours foundations and carries debris), **rate of rise / warning time**, and **duration**. 2. **Site Characteristics:** - Check ground elevation, lot size (whether there is room for an earthen ring levee), and **soil type** (permeable sandy soil lets water seep up under floors; clay swells/shrinks). 3. **Building Characteristics:** - Inspect foundation type (**slab-on-grade** vs. **crawlspace/basement**), wall material (**brick/masonry** resists water better than wood/tin frame), building age, and structural condition. 4. **Codes and Regulations:** - Ensure the chosen method complies with local floodplain management laws, building codes, and Base Flood Elevation (BFE) requirements. 5. **Appearance (Aesthetics):** - Consider how high foundation stilts, blank waterproof walls, or ring dykes will look in the neighborhood and whether they can be blended with landscaping. 6. **Accessibility & Human Intervention:** - Raising a house adds stairs, requiring ramps/lifts for elderly or disabled residents; also consider whether someone will be physically present and able to install heavy flood shields before water arrives. 7. **Cost & Economic Feasibility:** - Compare upfront construction and maintenance costs against the expected reduction in future flood damages and insurance premiums. --- #### Q7.3: Explain the six flood proofing strategies outlined by FEMA and how flood proofing is practiced in vulnerable char/coastal areas of Bangladesh. - **Appeared in:** 2022 (Q5b, 10m — part), 2024 (Q6a, 2m), 2025 (Q5a, 2m; Q7c, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 7](SB/Lecture-7.pdf) p. 21–39, 42–50 **Six FEMA Flood Proofing Strategies:** 1. **Elevation (Raising the Building):** Lifting the entire house so its lowest floor sits above the Design Flood Level on **compacted earthen fill, concrete piers, posts, columns, or piles** (best for deep floods and coastal waves). 2. **Wet Flood Proofing:** Allowing floodwater to **freely enter and exit** uninhabited lower areas (garages, crawlspaces, storage) through flood vents so inside and outside water pressures cancel out, while moving electrical panels, furnaces, and living spaces to upper floors. 3. **Dry Flood Proofing:** Sealing a building's exterior walls with waterproof membranes, sealants, and removable door/window **flood shields** plus sump pumps and backflow valves so **no water enters** (safe **only for strong masonry buildings where flood depth is $< 3\text{ ft}$**; deeper water causes wall collapse or slab flotation). 4. **Relocation:** Moving an existing building intact onto a trailer and placing it on a new foundation on permanent high ground outside the floodplain (most dependable permanent solution). 5. **Floodwalls and Levees (Ring Dykes):** Building a small local earthen levee or concrete wall around a single home or group of buildings with a closure gate and interior drain pump (used up to $\sim 6\text{ ft}$ depth). 6. **Demolition:** Tearing down severely damaged or structurally weak buildings and rebuilding a new elevated structure on the same site or elsewhere. **Flood Proofing in Char & Coastal Areas of Bangladesh (e.g., CLP Case Study):** - Under programs like the **Chars Livelihoods Programme (CLP)** along the Jamuna, rural homesteads are flood-proofed by **raising compacted earthen plinths (*vita*) at least $60\text{ cm}$ ($2\text{ ft}$) above the highest known flood level** (such as the $1998$ flood mark). - Plinth side slopes are planted with **Vetiver grass and fodder crops** to stop wave erosion, and **raised tube-wells and sanitary latrines** are installed on top of the plinth so families have clean water, sanitation, and safe shelter for cattle during monsoon floods. --- ### SB Lecture 8: Floodplain Zoning & Flood Mapping - **Source:** [SB/Lecture-8.pdf](SB/Lecture-8.pdf) --- #### Q8.1: Define floodplain, flood map (FIRM, FBFM, FHBM), and floodplain zoning. Explain the advantages and disadvantages of floodplain zoning ("Floodplain zoning reduces the risk and cost of damages"). - **Appeared in:** 2019 (Q7b, 7m), 2021 (Q6c, 10m), 2022 (Q7b, 5m), 2023 (Q7b, 4m), 2024 (Q6d, 2m) - **Time & Length Budget:** 6–8 Minutes | [SB: Lec 8](SB/Lecture-8.pdf) p. 6–11 - **Floodplain:** Low-lying flat land bordering a river or coast that is periodically inundated by floods. - **Floodplain Zoning:** A **non-structural legal and land-use planning tool** that divides a floodplain into zones based on flood hazard probability and restricts what types of buildings or activities are allowed in each zone. - **Types of Flood Maps:** - **FHBM (Flood Hazard Boundary Map):** Preliminary map showing rough flood-prone boundaries. - **FIRM (Flood Insurance Rate Map):** Official map showing $100\text{-year}$ and $500\text{-year}$ flood risk zones and Base Flood Elevations (used for flood insurance). - **FBFM (Flood Boundary & Floodway Map):** Engineering map delineating the active **regulatory floodway** vs. the **floodway fringe**. **Advantages (How It Reduces Risk & Cost of Damages):** 1. **Prevents New Damage at Zero Structural Cost:** Stops factories, hospitals, and dense housing from being built in dangerous floodways in the first place, slashing future disaster relief and repair bills. 2. **Preserves Natural Flood Storage & Conveyance:** Keeps floodways open as parks, wetlands, or farmland so flood peaks are not pushed higher onto neighboring communities. 3. **Guides Safe Development & Insurance:** Informs citizens of exact flood risk before buying land and sets fair flood insurance rates. 4. **Protects Ecology:** Conserves riverside wetlands, groundwater recharge zones, and fish habitats. **Disadvantages / Limitations:** 1. **Does Not Protect Existing Buildings:** Does nothing to reduce flooding for homes and towns already built inside the floodplain. 2. **Political & Social Resistance:** Landowners oppose zoning laws because restricting construction lowers the market value of their land. 3. **Difficult Enforcement in Developing Countries:** High population pressure and land scarcity in Bangladesh make it hard to stop informal settlements in flood-prone fringes. --- #### Q8.2: Describe the different flood zones classified based on annual flood occurrence probability (Zones 1, 2, 3a, 3b & FEMA zones), their roles in flood management, and the approaches and data required for flood zoning. - **Appeared in:** 2019 (Q7b, 3m), 2022 (Q7b, 5m), 2023 (Q7b, 6m), 2024 (Q6d, 4m) - **Time & Length Budget:** 6–8 Minutes | [SB: Lec 8](SB/Lecture-8.pdf) p. 12–19 **Sketch of Floodplain Zones Across a River Valley (45 sec):** ```text Zone 1 (Low Risk) | Zone 2 (Med) | Zone 3a (High) | Zone 3b (Active Floodway) | Zone 3a | Zone 2 (<0.1% / >1000-yr)| (0.1%-1% / | (>1% / <100-yr | (>5% / <20-yr flood; carries| | Safe for hospitals| 100-1000 yr) | Flood Fringe) | fast flow -> Parks/Agri only| | -------------------+--------------+----------------+ [Main River] +---------+-------- ``` **1. Probability-Based Flood Zones & Permitted Land Uses:** - **Zone 1 — Little or No Risk ($P < 0.1\%$ annually; $> 1000\text{-year}$ return period):** Safe for **all development**, including critical lifelines (hospitals, power plants, emergency command centers). - **Zone 2 — Low to Medium Risk ($0.1\% \le P \le 1.0\%$ annually; $100\text{ to }1000\text{-year}$ flood):** Suitable for general housing and commercial buildings (with flood-resistant design), but **avoid critical facilities**. - **Zone 3a — High Risk / Flood Fringe ($P > 1.0\%$ annually; $< 100\text{-year}$ flood):** New residential development is discouraged unless elevated above Base Flood Elevation with strict flood proofing. - **Zone 3b — Functional / Active Floodway ($P > 5.0\%$ annually; $< 20\text{-year}$ flood):** Carries fast, deep flood currents; **no new buildings allowed**—reserved strictly for agriculture, parks, playgrounds, and water-compatible uses. - *(FEMA equivalents: **Zone A** = $100\text{-yr}$ river flood; **Zone V** = $100\text{-yr}$ coastal wave velocity zone; **Zone B/X-shaded** = $500\text{-yr}$ moderate risk; **Zone C/X-unshaded** = minimal risk).* **2. Three Approaches to Flood Zoning:** 1. **Historic Approach:** Maps flood boundaries from past extreme flood marks, aerial photos, and satellite images (simple, but misses changes from new roads/urbanization). 2. **Conventional Geomorphological Approach:** Maps natural landforms (terraces, natural levees, backswamps) from soil and topographic surveys. 3. **Hydrological & Hydraulic Modeling Approach:** Combines flood frequency analysis ($Q_T$ for $20, 100, 1000\text{ yrs}$) with 1D/2D hydrodynamic models (HEC-RAS/MIKE) and GIS DEMs to map exact water depths and velocities. **3. Seven Data Types Required for Flood Zoning:** - (1) Historical flood stage/discharge records, (2) Rainfall & climate data, (3) High-resolution topographic DEM & river cross-sections, (4) Soil & geology maps, (5) Land-use / building inventory maps, (6) Satellite flood imagery, and (7) Hydraulic structure locations (bridges, levees, barrages). --- ### SB Lecture 9: Flood Hazard Mapping, Risk Analysis & Frequency Analysis - **Source:** [SB/Lecture-9.pdf](SB/Lecture-9.pdf) --- #### Q9.1: Define and relate Flood Hazard ($H$), Exposure ($E$), Susceptibility, Vulnerability ($V$), Flood Damage ($D$), and Flood Risk ($R$). Explain how flood damage is assessed using stage-damage curves. - **Appeared in:** 2019 (Q8b, 5m), 2022 (Q7c, 10m — part) - **Time & Length Budget:** 5–6 Minutes | [SB: Lec 9](SB/Lecture-9.pdf) p. 7–12, 18–22 **Core Definitions & Mathematical Link:** 1. **Flood Hazard ($H$):** The **annual probability of occurrence** ($H = 1 / T$, where $T$ is return period in years) of a potentially damaging flood of a given depth, velocity, and duration. 2. **Exposure ($E$):** The total value or count of **elements at risk** (people, houses, crops, roads, factories) located inside the flooded zone. 3. **Susceptibility & Vulnerability ($V$):** **Susceptibility** is the lack of resistance of an exposed asset to water; **Vulnerability ($V$, from $0$ to $1$)** is the fraction or degree of loss suffered by an exposed element at a given flood depth. 4. **Flood Damage ($D$):** Total monetary loss caused by a single flood event: $$D = E \times V = \sum E_i V_i$$ 5. **Flood Risk ($R$):** The **expected annual flood damage** (probability multiplied by consequence): $$R = \text{Hazard} \times \text{Exposure} \times \text{Vulnerability} = H \times D = \frac{D}{T}$$ *(If any one of Hazard, Exposure, or Vulnerability is zero, **Flood Risk is zero**).* **Flood Damage Assessment Process (Stage–Damage Curves):** - Group floodplain assets into categories (residential homes, commercial shops, agriculture, roads). - Use **Stage–Damage (Depth–Damage) Curves**, which plot the percentage damage ($V$) or monetary loss ($D$) against water depth above floor/ground level for each asset type. - Overlay the flood depth map (from GIS/hydraulic modeling) onto the land-use asset map and sum damages across all grid cells: $D_{\text{total}} = \sum E_i V_i(h)$. --- #### Q9.2: [Math] Calculate annual flood risk ($R$) for island/floodplain communities and determine the economic risk-reduction benefit of building an embankment. - **Appeared in:** 2019 (Q8c, 6m), 2025 (Q5b, 8m) - **Time & Length Budget:** Methodology Only | [SB: Lec 9](SB/Lecture-9.pdf) p. 23–25 **Step-by-Step Methodology:** 1. **For a Single Flood Return Period $T$ (2019 Q8c):** - Annual exceedance probability (Hazard): $H = \frac{1}{T}$ ($\text{yr}^{-1}$). - Total damage during one flood event: $D = E \times V$ (e.g., $\text{Number of households} \times \text{Asset value per household} \times \text{Vulnerability fraction}$). - **Annual Flood Risk ($R$):** $$R = H \times D = \frac{E \times V}{T} \quad (\text{Currency/year})$$ - Compare $R$ between communities: Two communities can have the **exact same annual risk $R$** even if one has frequent small losses (low $T$, low $D$) and the other has rare catastrophic losses (high $T$, high $D$). 2. **For Multiple Return Periods & Embankment Benefit (2025 Q5b — Weighted Annual Risk):** - Order the $N$ flood scenarios from most frequent ($T_1$) to rarest ($T_N$) with annual probabilities $P_i = 1 / T_i$ and damages $D_i$. - Calculate **incremental probability ($\Delta P_i$)** or weighted hazard ($H_n$) for each interval and sum to find **Baseline Mean Annual Risk ($R_{\text{without}}$)**: $$R_{\text{without}} = \sum_{n=1}^{N} H_n D_n \quad \text{or} \quad \sum \Delta P_i \bar{D}_i$$ - **With an Embankment protecting up to a $T_{\text{design}}$-year flood (e.g., $T = 20\text{ yrs}$):** Set damage $D_n = 0$ for all floods with $T_n \le T_{\text{design}}$, and recalculate the remaining residual annual risk ($R_{\text{with}}$) for floods exceeding $T_{\text{design}}$. - **Annual Economic Benefit of the Embankment:** $$\text{Annual Benefit} = R_{\text{without}} - R_{\text{with}}$$ --- #### Q9.3: [Math] Perform flood or rainfall frequency analysis using Normal and Log-Normal probability distributions. - **Appeared in:** 2021 (Q5c, 12m), 2022 (Q5c, 10m), 2023 (Q7a, 10m) - **Time & Length Budget:** Methodology Only | [SB: Lec 9](SB/Lecture-9.pdf) p. 35–38 **Step-by-Step Methodology:** **Case A — Normal Distribution (using raw data $X_i$ directly):** 1. **Compute sample mean ($\bar{X}$) and standard deviation ($S_x$) for $n$ years of peak records:** $$\bar{X} = \frac{1}{n}\sum_{i=1}^{n} X_i, \qquad S_x = \sqrt{\frac{\sum (X_i - \bar{X})^2}{n - 1}}$$ 2. **To find the $T$-year Flood/Rainfall ($X_T$):** - Exceedance probability: $P = \frac{1}{T}$ (non-exceedance $= 1 - P$). - Read the standard normal variate $z_T$ (or frequency factor $K_T = z_T$) from the Normal table, or use Abramowitz & Stegun's rational approximation with $w = \sqrt{\ln(1/P^2)}$: $$z_T = K_T = w - \frac{2.515517 + 0.802853 w + 0.010328 w^2}{1 + 1.432788 w + 0.189269 w^2 + 0.001308 w^3}$$ - Compute $T$-year magnitude: $$X_T = \bar{X} + K_T S_x$$ 3. **To find the Probability and Return Period ($T$) of a given threshold $X_0$ (2023 Q7a):** - Compute $z = \frac{X_0 - \bar{X}}{S_x}$, find exceedance probability $P(Z \ge z)$ from the standard normal table, and compute $T = \frac{1}{P}$. **Case B — Log-Normal Distribution (2021 Q5c, 2022 Q5c):** 1. **Transform each data point by taking logarithm:** $Y_i = \ln X_i$ (or $\log_{10} X_i$). 2. **Compute mean ($\bar{Y}$) and standard deviation ($S_y$) of the log-transformed series:** $$\bar{Y} = \frac{1}{n}\sum_{i=1}^{n} Y_i, \qquad S_y = \sqrt{\frac{\sum (Y_i - \bar{Y})^2}{n - 1}}$$ *(Or if only $\bar{X}$ and $S_x$ are given, use $CV = S_x/\bar{X}$, $S_y = \sqrt{\ln(1 + CV^2)}$, and $\bar{Y} = \ln\bar{X} - \frac{1}{2}S_y^2$).* 3. **Find the log-domain $T$-year value ($Y_T$) using the same normal factor $K_T = z_T$:** $$Y_T = \bar{Y} + K_T S_y$$ 4. **Take the antilog to obtain the $T$-year flood/rainfall ($X_T$):** $$X_T = e^{Y_T} \qquad (\text{or } 10^{Y_T} \text{ if } \log_{10} \text{ was used})$$ --- ### SB Lecture 10: Flood Forecasting and Warning System (FFWS) - **Source:** [SB/Lecture-10.pdf](SB/Lecture-10.pdf) --- #### Q10.1: Define flood forecasting and flood warning. Explain the role of flood forecasting in flood management, and describe the data requirements, models, and limitations of flood forecasting in Bangladesh. - **Appeared in:** 2019 (Q8a, 10m), 2023 (Q5c, 10m), 2024 (Q7b, 8m), 2025 (Q6b, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 10](SB/Lecture-10.pdf) p. 6–23 **Definitions:** - **Flood Forecasting:** The scientific process of estimating and predicting **future river water levels, discharges, inundation extent, and timing of flood peaks** at specific locations before they occur, using real-time rainfall and upstream gauge data. - **Flood Warning:** The actionable **public alert message** broadcast ahead of an approaching flood so authorities and citizens can evacuate, protect property, and operate flood gates. **Role of Flood Forecasting in Flood Management:** 1. **Saves Human Lives & Livestock:** Gives residents hours or days of lead time to evacuate to flood shelters and move cattle to raised *killas*. 2. **Reduces Household & Business Property Loss:** Allows people to lift furniture, grain, and machinery onto high platforms and install temporary flood shields. 3. **Protects Agricultural Harvests:** Early flash-flood warnings in the *Haor* basin allow farmers to harvest mature *Boro* paddy before dykes overtop. 4. **Guides Reservoir & Gate Operations:** Helps engineers pre-release water from reservoirs, close urban floodgates, and mobilize sandbagging crews at weak embankment points. 5. **Most Cost-Effective Non-Structural Measure:** Delivers high damage reduction across the entire country at a tiny fraction of the cost of building levees. **Salient Features of Flood Forecasting in Bangladesh (FFWC / BWDB):** - **Data Requirements (4 Categories):** 1. **Hydrological:** Real-time water level and discharge at boundary and internal river gauges (BWDB network). 2. **Meteorological:** Real-time rainfall, satellite cloud imagery, and weather radar (BMD network). 3. **Morphological & Topographical:** River cross-sections, rating curves, embankment alignments, and floodplain DEM. - **Forecasting Models Used:** 1. **Gauge-to-Gauge Correlation:** Simple statistical curves relating upstream water level + travel time to downstream water level. 2. **Rainfall–Runoff (Precipitation-Driven) Models:** Convert basin rainfall into runoff hydrographs (used for flash floods). 3. **Hydrodynamic Flood Routing Models:** 1D/2D MIKE 11 / HEC-RAS models routing flows through Bangladesh's river network ($3\text{–}10\text{ day}$ deterministic and probabilistic forecasts). - **Major Limitations in Bangladesh:** 1. **Lack of Upstream Cross-Border Data ($93\%$ Outside BD):** Limited real-time gauge and rainfall data from upstream India, Nepal, and China restricts lead time at border stations. 2. **Very Short Lead Time for Flash Floods:** Steep hilly rivers in Sylhet/Chattogram peak within a few hours of rain. 3. **Rapidly Shifting Riverbeds:** Braided rivers (Jamuna/Padma) erode and silt up constantly, making stage-discharge rating curves outdated. 4. **Warning Dissemination Gap:** Technical forecasts (in "cm above Danger Level") are often hard for rural farmers to translate into local homestead water depth. --- ### SB Lecture 11: Flood Damage Assessment & Economic Evaluation of Flood Mitigation Projects - **Source:** [SB/Lecture-11.pdf](SB/Lecture-11.pdf) --- #### Q11.1: Differentiate between tangible (direct and indirect) and intangible flood damages with examples for urban and rural areas. - **Appeared in:** 2019 (Q8c, 4m), 2022 (Q7c, 10m — part) - **Time & Length Budget:** 4–5 Minutes | [SB: Lec 11](SB/Lecture-11.pdf) p. 6–11 | Damage Category | Definition | Urban Examples | Rural Examples | | :--- | :--- | :--- | :--- | | **1. Tangible — Direct Damage** | Losses that can be **measured in money** caused by **direct physical contact** with floodwater. | Damage to houses, shops, factory machines, cars, roads, and power sub-stations. | Destruction of standing crops (*Aman/Boro* paddy), drowned cattle/fish ponds, washed-out earthen homes and rural dykes. | | **2. Tangible — Indirect Damage** | Monetary losses caused by **disruption of economic and social activities** outside or after the physical flood contact. | Lost factory production, lost daily wages, traffic detour fuel costs, and emergency sandbagging/pumping costs. | Loss of seasonal farm labor income, food price inflation, and cost of replanting seeds. | | **3. Intangible Damage** | Losses that **cannot be accurately priced in money** (human, social, psychological, ecological). | **Loss of human life**, drowning, cholera/dengue outbreaks, mental trauma/stress, and loss of cultural heritage. | Loss of human life, malnutrition, disruption of children's schooling, and displacement distress. | --- #### Q11.2: Briefly explain the five approaches for the economic evaluation of flood mitigation projects. - **Appeared in:** 2023 (Q5b, 10m), 2024 (Q7c, 10m) - **Time & Length Budget:** 8–10 Minutes | [SB: Lec 11](SB/Lecture-11.pdf) p. 12–18 1. **Cost–Benefit Analysis (CBA) Approach:** - Converts all project benefits (expected annual flood damage prevented: $B = R_{\text{without}} - R_{\text{with}}$) and all project costs (capital construction + annual O&M: $C$) into monetary present/annual values. - Selects the design scale that **maximizes Net Benefit ($B - C$)** or achieves a **Benefit–Cost Ratio ($B / C > 1.0$)** and highest Internal Rate of Return (IRR). 2. **Fixed-Cost (Budget-Constrained) Approach:** - Used when the government has a **strict, fixed budget ceiling**. - Engineers compare different combinations of measures that cost that exact budget and choose the option that **maximizes total flood damage reduction** within the fixed sum. 3. **Cost-Effectiveness (Fixed Protection Target) Approach:** - Used when a **minimum protection standard is set by law or policy** (e.g., "protect the city against a $100\text{-year}$ flood") regardless of monetary benefit calculations. - Engineers compare alternative designs (e.g., levee vs. bypass channel vs. upstream reservoir) that all meet the $100\text{-year}$ target and select the **least-cost option**. 4. **Multi-Objective / Multi-Criteria Evaluation Approach:** - Evaluates projects across **both monetary and non-monetary goals** at the same time—weighting economic return alongside **loss-of-life reduction, environmental impact, fisheries preservation, and social equity**. 5. **Non-Economic (Social / Political / Humanitarian) Approach:** - Decisions driven purely by **humanitarian duty, public safety, or national security**—such as building emergency cyclone/flood shelters to save human lives in poor coastal/char areas even where monetary $B/C < 1$. --- #### Q11.3: [Math] Recommend the optimal design river stage (height) for a flood levee from flood stage, damage, return period, and construction cost data. - **Appeared in:** 2019 (Q7c, 10m) - **Time & Length Budget:** Methodology Only | [SB: Lec 11](SB/Lecture-11.pdf) p. 14, 19–20 **Step-by-Step Methodology (Expected Annual Damage & Net Benefit Optimization Table):** 1. **Set up a calculation table ordered by increasing River Stage ($H_i$) and Return Period ($T_i$):** - Column 1: Flood Stage $H_i$ ($\text{m}$) - Column 2: Total Flood Damage $D_i$ ($\text{\$}$ or $\text{Tk}$) if water reaches stage $H_i$ - Column 3: Return Period $T_i$ ($\text{years}$) - Column 4: Annual Exceedance Probability $P_i = \frac{1}{T_i}$ 2. **Calculate Incremental Expected Annual Damage ($\Delta \text{EAD}_i$) for each stage interval ($i \to i+1$):** - Incremental probability of a flood falling between stage $i$ and $i+1$: $$\Delta P_i = P_i - P_{i+1} = \frac{1}{T_i} - \frac{1}{T_{i+1}}$$ - Average damage in that interval: $$\bar{D}_i = \frac{D_i + D_{i+1}}{2}$$ - Incremental annual damage: $$\Delta \text{EAD}_i = \bar{D}_i \times \Delta P_i$$ 3. **Compute Annual Flood Benefit ($B_k$) for each candidate Levee Design Stage ($H_k$):** - A levee built to stage $H_k$ eliminates all flood damages up to stage $H_k$, so its **Annual Benefit ($B_k$)** is the cumulative sum of $\Delta \text{EAD}_i$ from the threshold bankfull stage up to $H_k$: $$B_k = \sum_{i=1}^{k} \Delta \text{EAD}_i$$ 4. **Compare Annual Benefit ($B_k$) against Annualized Levee Cost ($C_k$) for each stage:** - Calculate **Net Annual Benefit** for each candidate stage: $$\text{Net Benefit}_k = B_k - C_k \qquad \left(\text{and Benefit–Cost Ratio } \text{BCR}_k = \frac{B_k}{C_k}\right)$$ 5. **Select Optimal Design Stage:** - Recommend the river stage $H_k$ where **Net Annual Benefit ($B_k - C_k$) is MAXIMUM** (equivalently, where marginal benefit equals marginal cost: $\Delta B = \Delta C$, and total annual cost $[C_k + \text{Residual Damage}]$ is minimum).