Weirs, Barrages and River Diversion Structures

Introduction

River-diversion structures raise or control river level so that water can enter canals, intakes, hydropower systems or environmental channels. A weir generally has a fixed crest over which water passes, whereas a barrage uses gated bays to regulate upstream level and discharge. A complete diversion headworks may also include canal head regulators, undersluices, divide walls, sediment-control works, fish passages, energy dissipators, cutoffs and river-training works.

Their design is multidisciplinary. Hydraulic capacity, afflux, sediment movement, seepage, uplift, scour, gate loading, ecological continuity and operational reliability must be considered together. A structure that passes the design flood may still fail through piping beneath its floor, excessive downstream scour, gate vibration, sediment blockage or adverse river morphology.

Learning Objectives

After studying this chapter, the reader should be able to:

• distinguish weirs, barrages, regulators and undersluices;
• estimate crest and gate discharge under free and submerged conditions;
• assess afflux, hydraulic jumps, scour and energy dissipation;
• undertake preliminary hydraulic and structural sizing of gates;
• design regulators, seepage-control floors, cutoffs and bed protection;
• plan sediment-control arrangements and fish passages; and
• formulate an integrated diversion-headworks design and operating strategy.

1. Functions and Selection of Diversion Structures

A diversion structure commonly performs five functions:

1. raises the river level to provide gravity diversion;

2. regulates discharge entering an off-taking canal;

3. passes floods without unacceptable afflux;

4. controls sediment entering the canal; and

5. protects the riverbed and banks against local erosion.

1.1 Weirs

Fixed weirs are comparatively simple and robust. They are suitable where upstream-level variation is acceptable, sediment loads are moderate and precise flood regulation is unnecessary. Common forms include sharp-crested, broad-crested, ogee and rockfill weirs.

1.2 Barrages

A barrage consists of gated waterway bays separated by piers. Gates can be raised during floods and lowered to maintain pond level during low flows. Barrages offer superior control but require reliable mechanical equipment, power, operating rules and maintenance.

1.3 Regulators

A canal head regulator controls water entering a canal. Cross-regulators maintain canal levels or distribute flow between reaches. Regulators must provide adequate capacity without creating excessive entry velocity, sediment ingestion or unsafe gate differentials.

1.4 Undersluices

Undersluices, or scouring sluices, are low-level gated openings near the canal offtake. Their sill is normally below the main weir crest. They pass sediment-laden lower layers, provide flood capacity and create a relatively clear-water pocket near the canal entrance. A divide wall separates this pocket from the main river waterway and stabilizes approach flow.

2. Site Investigation and Design Basis

Design inputs should include:

• flood-frequency estimates and uncertainty bounds;
• low-flow and environmental-flow requirements;
• rating curves and downstream tailwater levels;
• channel cross-sections, planform change and bank stability;
• sediment size distribution, concentration and seasonal load;
• bed-material mobility and potential degradation or aggradation;
• foundation permeability, stratification and liquefaction risk;
• canal demand, pond level and permissible afflux;
• navigation, fisheries, land acquisition and social constraints; and
• debris, ice and seismic conditions where relevant.

A one-dimensional model may be adequate for initial afflux assessment. Two-dimensional modelling is preferable where approach flow is oblique, floodplains are active, sediment pathways are important or training works strongly alter the flow. Physical models or validated computational fluid dynamics may be justified for major structures with complex pier, gate or stilling-basin flows.

3. Crest Hydraulics and Waterway Sizing

3.1 Broad-Crested Weir

For a rectangular broad-crested weir, neglecting approach velocity and losses, the specific energy above the crest is

H = y + v²/(2g).

At critical flow, v² = gy and therefore

H = 3y_c/2, so y_c = 2H/3.

The ideal discharge per unit width is

q = y_c√(gy_c) = √g(2H/3)^(3/2).

Consequently,

Q = C_d b√g(2H/3)^(3/2) = 1.705C_d bH^(3/2),

where Q is discharge in m³/s, b is effective width in metres, H is total head above the crest in metres, g is gravitational acceleration and C_d is a dimensionless coefficient accounting for losses and non-ideal geometry. The numerical coefficient 1.705 applies when g = 9.81 m/s².

The equation requires free overflow and critical control on the crest. Significant submergence, approach velocity, non-rectangular geometry or rapidly varying flow requires appropriate corrections or numerical analysis.

3.2 Other Crest Forms

For a sharp-crested rectangular weir,

Q = (2/3)C_d b√(2g)H^(3/2).

For an ogee crest, the practical relation is often written

Q = C L_e H_e^(3/2),

where C is a calibrated coefficient, L_e is effective crest length and H_e is total design head including approach velocity. The coefficient is not universal; it depends on crest profile, head ratio, pier shape and submergence.

Pier and abutment contractions reduce effective crest length. A commonly used form is

L_e = L − 2(NK_p + K_a)H_e,

where N is the number of piers and K_p and K_a are experimentally or code-derived contraction coefficients. This relation should not be applied outside the geometric conditions for which its coefficients were established.

Worked Example 1: Clear Waterway and Structural Width

A preliminary broad-crested diversion weir must pass 1,000 m³/s at an effective head of 1.63 m. Assume ideal broad-crested control, so the coefficient is 1.705.

b = Q/[1.705H^(3/2)]

H^(3/2) = 1.63^(3/2) = 2.0810

b = 1,000/(1.705 × 2.0810) = 281.8 m.

Adopt twenty bays, each 14.10 m clear:

Clear waterway = 20 × 14.10 = 282.0 m.

With nineteen intermediate piers, each 1.50 m thick:

Width between outer abutment faces = 282.0 + 19(1.50) = 310.5 m.

If each abutment is 2.0 m thick, the overall concrete footprint is approximately 314.5 m. Detailed design must subsequently account for contraction, approach velocity, gate geometry and blockage.

4. Afflux and Flood Conveyance

Afflux is the increase in upstream flood level caused by a structure relative to the corresponding unobstructed condition. It arises from reduced flow area, pier drag, contractions, gate restrictions and energy losses.

Between an upstream section and a control section, the energy equation is

z₁ + y₁ + α₁V₁²/(2g) = z₂ + y₂ + α₂V₂²/(2g) + h_L,

where z is bed elevation, y is depth, α is the kinetic-energy coefficient and h_L is head loss. Afflux cannot generally be inferred from area reduction alone because the downstream rating curve and backwater profile influence the result.

Professional assessment should compare existing and proposed water-surface profiles over a suitable range of floods. Sensitivity tests should include debris blockage, one or more inoperative gates, floodplain roughness, sediment aggradation and uncertainty in boundary conditions. Permissible afflux is normally governed by flood-risk regulation, embankment freeboard and impacts on upstream property and ecosystems.

5. Flow Under Gates

5.1 Free Gate Discharge

For a vertical gate of width b and opening a, when the contracted jet is free and not controlled by tailwater,

Q = C_d ba√(2gh_c),

where h_c is the upstream free-surface elevation above the centre of the opening. This is an approximate orifice expression; C_d incorporates contraction and velocity-distribution effects.

5.2 Submerged Gate Discharge

When the downstream water submerges the jet, discharge is governed by the differential head across the gate:

Q = C_d ba√(2gΔH).

Here ΔH is the difference between upstream and downstream piezometric heads, with velocity-head corrections where significant. Using only the upstream water level above the opening centre for a submerged gate is incorrect because downstream pressure directly opposes the flow.

Worked Example 2: Submerged Regulator Gate

A submerged opening is 10 m wide and 2 m high. The differential head is 3.34 m and C_d = 0.62.

Q = 0.62(10)(2)√(2 × 9.81 × 3.34)

Q = 100.4 m³/s.

This result applies to submerged flow. A free-discharge calculation using upstream depth alone would represent a different hydraulic condition.

6. Regulator and Undersluice Design

6.1 Regulator Sizing

For N equal submerged bays,

Q_d = C_dNba√(2gΔH),

so that the required total clear width is

Nb = Q_d/[C_da√(2gΔH)].

The opening should be checked at minimum operating head, maximum demand and partial-gate operation. Entry velocity must be compatible with sediment behaviour, trash-rack losses and canal-bank stability.

For example, if Q_d = 250 m³/s, C_d = 0.68, a = 2.5 m and ΔH = 1.2 m,

Nb = 250/[0.68(2.5)√(2 × 9.81 × 1.2)] = 30.3 m.

Six bays of approximately 5.1 m clear width would meet the hydraulic requirement before allowing for operational margins.

6.2 Undersluice Capacity and Layout

Undersluice design should address:

• low sill elevation to attract near-bed sediment;
• sufficient capacity to pass the required sediment-laden and flood flow;
• approach geometry that avoids stagnant zones;
• a divide wall extending far enough upstream and downstream to separate flow fields;
• safe downstream energy dissipation; and
• gate redundancy during sediment flushing.

Capacity is determined using free- or submerged-gate equations according to tailwater conditions. Designers should test the full operating envelope rather than assume one discharge coefficient. Excessively high velocity may damage the floor, while insufficient velocity may not mobilize deposited sediment.

7. Structural Design of Gates

Gate design includes the skin plate, stiffeners, horizontal or vertical girders, end posts, wheels or trunnions, seals, hoist and embedded parts. Hydraulic, dead, impact, debris, seismic and operational loads should be combined in accordance with the governing structural standard.

7.1 Hydrostatic Loading

For a rectangular vertical gate retaining water to depth h on one side, the resultant force is

F = γ_wbh²/2,

acting h/3 above the bottom. Here γ_w is the unit weight of water. With water on both sides, the net pressure diagram must be formed by subtracting downstream pressure from upstream pressure at every elevation.

For an 8 m wide gate retaining 4 m of water against a dry downstream face,

F = 9.81(8)(4²)/2 = 627.8 kN,

acting 1.33 m above the sill.

7.2 Skin-Plate Sizing

A conservative preliminary model treats a plate strip between stiffeners as simply supported. For pressure p and stiffener spacing s,

M = ps²/8

per metre width. With allowable bending stress σ_a, the required plate thickness is

t = √(6M/σ_a).

At p = 39.24 kPa, s = 0.75 m and σ_a = 150 MPa,

M = 39.24(0.75²)/8 = 2.759 kN·m per metre,

t = √[6(2,759)/(150 × 10⁶)] = 0.0105 m.

Thus the structural thickness is approximately 10.5 mm. A nominal selection might be 14 mm after adding 3 mm corrosion allowance, subject to plate-buckling, fatigue, fabrication and code checks. Two-way plate action may reduce bending but should be established using an appropriate plate model rather than assumed.

7.3 Girders, Wheels and Hoists

Each girder is loaded by the pressure acting over its tributary plate area. Girder strength, shear, lateral stability, deflection and fatigue must be verified. Wheel loads are obtained from gate reactions, accounting for load-sharing imperfections and guide friction.

The hoist must overcome gate weight, seal friction, bearing resistance and possible hydraulic downpull. A simplified lifting estimate is

T_h ≥ FS[W_g + μN_s + F_b ± F_h],

where W_g is effective gate weight, μN_s is seal friction, F_b is bearing resistance and F_h is the vertical hydraulic force. The factor of safety and load combinations must follow the applicable machinery and gate standard. Dual drives require synchronization and protection against gate racking. Emergency closure, manual backup and position indication are essential operational provisions.

8. Seepage, Uplift and Piping

Water flowing beneath an impervious floor creates uplift pressure and exit gradients. Failure can occur through excessive uplift, piping, heave or loss of foundation material.

Modern design should use two-dimensional seepage analysis, flow nets or finite-element models representing soil layers, cutoffs, drains and anisotropy. Historical creep theories may be useful for preliminary comparison but should not replace rational seepage analysis for major works.

At a point beneath the floor,

p_u = γ_wh_p,

where h_p is pressure head. If floor weight alone resists uplift, a preliminary thickness requirement is

t ≥ FS·p_u/(γ_c − γ_w),

where γ_c − γ_w is the submerged unit weight of concrete. This expression assumes a uniformly loaded floor segment and excludes structural continuity, anchors and drainage. Actual floors should be designed as slabs or blocks under spatially varying uplift.

Cutoffs lengthen the seepage path and reduce exit gradient. A downstream cutoff is particularly important against piping, while an upstream cutoff reduces seepage entry. Pressure-relief drains can reduce uplift, but only if filters prevent soil migration and inspection confirms that the drains remain functional.

9. Hydraulic Jump and Energy Dissipation

For a hydraulic jump in a horizontal rectangular channel, conservation of momentum gives the sequent-depth relation

y₂/y₁ = 0.5[√(1 + 8Fr₁²) − 1],

where

Fr₁ = V₁/√(gy₁).

The relation assumes hydrostatic pressure at the approach and downstream sections, negligible bed friction through the short jump and a rectangular channel.

The energy loss is

ΔE = (y₂ − y₁)³/(4y₁y₂).

Worked Example 3: Sequent Depth

Supercritical flow enters a stilling basin at y₁ = 1.20 m and V₁ = 20.0 m/s.

Fr₁ = 20/√(9.81 × 1.20) = 5.83.

Therefore,

y₂ = 1.20 × 0.5[√(1 + 8 × 5.83²) − 1] = 9.31 m.

The required sequent depth is approximately 9.3 m, not 18.6 m. The available tailwater should be compared with this value. If it is too low, the jump may sweep downstream; if too high, the jump may drown. Basin length is commonly selected from validated stilling-basin standards, often of the order of four to six sequent depths for preliminary planning, but appurtenances and final dimensions must be based on the applicable basin type and Froude-number range.

10. Scour and Riverbed Protection

10.1 Regime Scour Estimate

For alluvial rivers, a commonly used preliminary regime-scour relation is

R = 0.473(Q/f)^(1/3),

where R is regime scour depth below high flood level in metres, Q is discharge in m³/s and f is the dimensionless Lacey silt factor. An equivalent discharge-intensity form is approximately

R = 1.35(q²/f)^(1/3),

under the regime-waterway assumptions from which the equivalence is derived. These empirical relations should not be treated as universal local-scour laws.

Local design scour is commonly estimated by multiplying regime depth by a location-dependent factor to reflect bends, constrictions, noses, downstream turbulence or oblique flow. The factor must be selected from regional evidence, physical modelling or the governing design standard.

For Q = 1,000 m³/s and f = 1.0,

R = 0.473(1,000)^(1/3) = 4.73 m below high flood level.

If a downstream local-scour factor of 2.0 is justified, the design depth becomes 9.46 m below high flood level. This is an empirical design estimate, not a prediction of the exact post-flood bed profile.

10.2 Cutoffs, Aprons and Filters

The impervious floor should extend beyond the principal pressure and turbulence zones. End cutoffs should penetrate below credible scour surfaces with allowance for foundation uncertainty. Rigid floors are supplemented by flexible launching aprons that settle into developing scour holes while maintaining a protective blanket.

If D is the vertical depth of anticipated scour below the existing apron level, a preliminary downstream apron width may be about 1.5D, subject to local standards and model evidence. For a launched slope of 1 vertical to 2 horizontal, its length is D√5. A volume-based sizing check is

BT ≥ λt_lD√5,

where B and T are initial apron width and thickness, t_l is required launched blanket thickness and λ allows for voids, losses and non-uniform launching.

Stone size must be checked against hydraulic tractive forces or an accepted rock-stability relation using local velocity, turbulence, relative density and placement condition. A graded granular filter or geotextile-compatible filter is required beneath the stone to prevent loss of bed material. Geotextiles must be protected from puncture, ultraviolet exposure and undermining.

11. Sediment Management

Diversion structures alter sediment continuity by reducing velocity upstream and concentrating flow through gates. Sustainable design combines geometry and operation.

• Undersluices pass sediment-rich lower flow layers.
• Sediment excluders installed in the approach to a canal divert bed-load and lower-layer sediment before it enters.
• Sediment ejectors remove sediment after it has entered a canal.
• Guide banks and divide walls control approach alignment and reduce cross-currents.
• Flushing operations temporarily increase low-level velocity to remobilize deposits.

Sediment management should be evaluated over seasonal hydrographs rather than at only one design discharge. Closing main barrage bays and opening undersluices may improve flushing but can also cause severe local scour or downstream sediment pulses. Operating rules should therefore specify gate sequences, allowable differentials and monitoring thresholds.

12. Fish Passage and Environmental Connectivity

A diversion structure can obstruct fish migration, alter habitat and trap sediment and nutrients. Fish-passage selection must reflect target species, swimming ability, migration season, attraction flow and river variability.

Options include pool-and-weir fishways, vertical-slot fishways, nature-like bypass channels, fish lifts and eel passes. Vertical-slot systems tolerate moderate water-level variation and permit passage without requiring fish to leap. Pool drops and slot velocities must remain within species-specific limits.

For an unsubmerged slot or opening, discharge may be estimated with a calibrated orifice relation. Submerged slots require differential head across each slot. Hydraulic capacity alone is insufficient: the entrance must be located where fish can detect attraction flow, and debris-control and maintenance access are essential.

Environmental-flow gates should operate independently where possible. Fish passage must not be used to justify eliminating downstream environmental releases.

13. Integrated Illustrative Case: Nadi Diversion Barrage

Background

This fictional application concerns a proposed barrage supplying a 220 m³/s irrigation canal from an alluvial river. The design flood is 3,000 m³/s, the bed carries medium sand, and migratory fish are present.

Problem

A fixed weir would create unacceptable wet-season afflux and rapid sediment deposition near the canal intake. The foundation consists of 18 m of permeable sand over dense gravel. Tailwater varies sufficiently to produce both free and submerged gate flow.

Analysis

Hydraulic modelling indicates that a gated waterway limits design-flood afflux to the regulatory threshold when one bay is assumed unavailable. Sediment-transport modelling shows that a canal intake placed immediately beside the main barrage bays would capture excessive near-bed sediment. Seepage analysis identifies a high downstream exit gradient unless a downstream cutoff and filtered relief system are provided.

The canal regulator is checked at the minimum differential head using the submerged-opening equation. Barrage gates are checked for maximum one-sided hydrostatic head, debris loading, seal friction and emergency closure. Hydraulic-jump calculations show that low-tailwater operation is the critical stilling-basin condition.

Approach and Solution

The selected arrangement comprises gated barrage bays, four low-level undersluice bays, an upstream divide wall, a recessed canal head regulator and sediment-excluder tunnels discharging through the undersluice pocket. The floor includes upstream and downstream sheet-pile cutoffs, filtered drains and an instrumented pressure-relief system.

A stilling basin is designed for the governing Froude-number range and supplemented by a downstream launching apron. A vertical-slot fishway is placed near the riverbank with a controllable attraction-flow outlet. Gate operating rules require staged opening to prevent cross-flow and prohibit large adjacent-gate differentials.

Expected Outcome

The arrangement provides canal supply while limiting afflux, sediment entry, uplift and downstream erosion. Piezometers, gate-load sensors and repeated bathymetric surveys provide evidence for adaptive operation. The project includes a sediment budget and funds for long-term fishway and gate maintenance.

Learning Points

• Hydraulic capacity cannot be separated from sediment and morphological behaviour.
• Gate redundancy must be included in flood assessment.
• Seepage drains require filters, monitoring and maintenance.
• Fish passage depends on attraction and operation as well as geometry.
• Instrumentation enables operating rules to be revised using observed performance.

14. Governance, Safety and Contemporary Practice

Diversion structures affect upstream flood risk, downstream sediment supply, fisheries and community access to water. Transparent allocation rules, environmental-flow commitments and emergency-action plans are therefore integral to engineering design. Dam- or barrage-safety reviews should identify gate-jamming, power failure, foundation piping, extreme flood and unauthorized operation scenarios.

Remote sensing, acoustic Doppler measurements and repeated drone or sonar surveys increasingly support scour and sediment monitoring. Digital twins and machine-learning tools may assist anomaly detection, gate scheduling and predictive maintenance. They should not autonomously override safety constraints without validated models, human supervision and reliable sensors. Data quality, cybersecurity and explainability are important where automated decisions affect flood safety or water allocation.

15. Design Best Practices

A defensible design should:

• define hydraulic and structural limit states before selecting dimensions;
• distinguish clearly between free and submerged gate flow;
• evaluate flood conveyance with blocked-bay and sensitivity scenarios;
• use geotechnically representative seepage models;
• match stilling-basin performance to the full tailwater envelope;
• combine rigid floors, cutoffs, filters and flexible protection;
• treat sediment management as an operational as well as structural problem;
• provide safe inspection, maintenance and emergency access;
• validate empirical scour estimates using local evidence; and
• incorporate ecological passage and environmental releases from the outset.

Chapter Summary

Weirs and barrages raise and regulate river levels for diversion, but their safe design requires integrated treatment of hydraulics, structures, foundations, sediment and ecology. Crest-flow equations apply only under their stated control and submergence conditions. Free gate flow depends on upstream head above the opening, whereas submerged flow depends on the differential head across the gate.

Afflux should be evaluated through water-surface modelling and sensitivity analysis. Gates require structural checks of plates, stiffeners, girders, bearings and hoists under hydrostatic and operational loads. Seepage analysis determines uplift, exit gradients, cutoff requirements and drainage provisions. Hydraulic jumps, stilling basins, cutoffs and launching aprons protect the downstream riverbed. Undersluices, excluders and operating rules manage sediment, while properly located fish passages preserve ecological connectivity. Monitoring and adaptive operation are essential because river morphology and mechanical condition change throughout the structure’s life.

Key Terms

Afflux: Increase in upstream water level caused by a hydraulic structure.

Barrage: A gated structure used to regulate river level and discharge.

Clear waterway: Sum of unobstructed widths available for flow.

Cutoff: Impermeable vertical element used to control seepage and undermining.

Differential head: Difference in piezometric or total head across a structure.

Divide wall: Wall separating undersluice flow from the main waterway.

Effective crest length: Crest length remaining after contraction effects are considered.

Exit gradient: Hydraulic gradient where seepage emerges from the foundation.

Fish passage: Structure that enables aquatic organisms to cross a barrier.

Hydraulic jump: Rapid transition from supercritical to subcritical flow.

Launching apron: Flexible protection designed to settle over an eroding slope.

Piping: Progressive removal of foundation soil by seepage.

Regulator: Gated structure controlling flow into or within a canal.

Sequent depth: Downstream depth conjugate to a specified supercritical depth.

Stilling basin: Engineered basin in which excess flow energy is dissipated.

Submerged gate flow: Gate flow controlled by head difference between upstream and downstream pools.

Undersluice: Low-level gated opening used for sediment and flood passage.

Uplift pressure: Upward water pressure acting beneath a floor or foundation.

Weir: Overflow structure used to raise water level or measure/control discharge.

Review Questions

Short Answer Questions

1. Distinguish a fixed weir from a gated barrage.

2. Why must differential head be used for submerged gate flow?

3. State the principal functions of an undersluice and divide wall.

4. What conditions are assumed in the rectangular hydraulic-jump equation?

5. Explain the purpose of a launching apron.

Long Answer Questions

1. Derive the ideal discharge equation for a rectangular broad-crested weir and discuss its limitations.

2. Explain how afflux should be assessed for a barrage under design and blocked-gate conditions.

3. Describe the structural design process for a vertical-lift gate, including plate, girder, wheel and hoist checks.

4. Discuss seepage, uplift and piping control beneath an alluvial diversion structure.

5. Develop an integrated sediment-management strategy using undersluices, excluders and gate-operation rules.

Analytical / Application Questions

1. Design the clear waterway and preliminary bay arrangement for a barrage passing 1,500 m³/s under a specified design head, including contraction and blocked-bay checks.

2. For a range of upstream and downstream levels, determine whether a regulator operates under free or submerged flow and prepare a gate-opening schedule.

3. Given foundation strata, tailwater levels and scour estimates, develop a preliminary floor, cutoff, stilling-basin and launching-apron arrangement and justify each component.

Multiple Choice Questions with Answers

Question 1

Which feature most clearly distinguishes a barrage from a conventional fixed-crest weir?

A. A barrage uses controllable gated bays

B. A barrage cannot pass floodwater

C. A barrage is always founded on rock

D. A barrage has no downstream protection

Correct Answer: A. A barrage uses controllable gated bays

Question 2

A submerged gate opening is 10 m wide and 2 m high. If Cd = 0.62 and the differential head is 3.34 m, what is the discharge?

A. 82.1 m³/s

B. 92.7 m³/s

C. 100.4 m³/s

D. 119.0 m³/s

Correct Answer: C. 100.4 m³/s

Question 3

Supercritical flow has y₁ = 1.20 m and V₁ = 20.0 m/s in a rectangular channel. What is the approximate sequent depth?

A. 4.66 m

B. 9.31 m

C. 13.97 m

D. 18.60 m

Correct Answer: B. 9.31 m

Question 4

Using Q = 1.705bH^(3/2), what clear width is required for Q = 1,000 m³/s and H = 1.63 m?

A. 281.8 m

B. 283.5 m

C. 294.1 m

D. 310.5 m

Correct Answer: A. 281.8 m

Question 5

What is the hydrostatic force on an 8 m wide vertical gate retaining 4 m of water against a dry downstream face?

A. 156.9 kN

B. 313.9 kN

C. 502.3 kN

D. 627.8 kN

Correct Answer: D. 627.8 kN

Question 6

Which head should be used in the orifice equation for a fully submerged regulator opening?

A. Upstream depth above the sill only

B. Upstream head above the opening centre only

C. Differential piezometric head across the opening

D. Downstream depth above the sill only

Correct Answer: C. Differential piezometric head across the opening

Question 7

Using R = 0.473(Q/f)^(1/3), what is the regime scour depth for Q = 1,000 m³/s and f = 1.0?

A. 3.15 m

B. 4.73 m

C. 7.10 m

D. 9.46 m

Correct Answer: B. 4.73 m

Question 8

Why is attraction flow important at a fish-passage entrance?

A. It eliminates the need for environmental flow

B. It helps fish locate the passage entrance

C. It prevents all sediment deposition

D. It increases structural uplift resistance

Correct Answer: B. It helps fish locate the passage entrance

Question 9

Which measure most directly reduces uplift beneath an impervious floor while retaining soil particles?

A. Increasing gate opening

B. Filtered pressure-relief drains

C. Reducing fishway discharge

D. Removing the downstream cutoff

Correct Answer: B. Filtered pressure-relief drains

Question 10

A one-metre plate strip carries 39.24 kPa between stiffeners spaced 0.75 m apart. Using M = ps²/8 and an allowable stress of 150 MPa, what structural plate thickness is required before corrosion allowance?

A. 7.5 mm

B. 10.5 mm

C. 14.0 mm

D. 18.5 mm

Correct Answer: B. 10.5 mm