---
title: "Guide bund design, 7000 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bund-q7000-depth5-f11"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q7000-depth5-f11
site: BCT Engineering
---

# Guide bund design, 7000 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2065 Shrawan (old course), 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bund-q7000-depth5-f11
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design a guide bund for a flood discharge of 7000 cumecs, the high flood depth is 5 m and the silt factor is 1.1.

## Short answer

Waterway W=4.75√7000≈397 m, shank 1.25W=496 m, tail 99 m. Pitching t=1.15 m. Scour R=8.71 m. Straight apron width 1.5×8.06=12.10 m, depth below bed 8.06 m; nose width 1.5×12.42=18.63 m, depth 12.42 m; laid thickness √5 t≈2.57 m.

## Answer

#### Data and design basis
- Silt factor given: $f = 1.10$
- Flood depth: $h = 5.00$ m
- Design discharge for scour/apron: $Q = 7000$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{7000} = 397.41$ m. Adopt $W \approx 397$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 397 = 496\ \text{m} \\ L_{ds} &= 0.25W = 99\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 198$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 99$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 198\times 2.094 = 416$ m, so total length on the upstream side $\approx 912$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 7000^{1/3} = 0.06\times 19.13 = 1.15\ \text{m}
$$
Adopt $t \approx 1.2$ m (pitching on the river side slope 2:1, over a 0.15 to 0.3 m graded filter).

#### 3. Scour depth and launching apron
Lacey's normal scour depth:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{7000}{1.10}\right)^{1/3} = 8.71\ \text{m}
$$
Maximum scour below HFL: $2R$ at the upstream nose, $1.5R$ at the shank (straight portion).
The apron must launch to the slope $2H:1V$, so its horizontal width is $1.5D'$ and, when launched, it covers a slope length $\sqrt5\,D'$ with thickness $1.5t$.

| Portion | Scour factor | Scour depth below HFL | Depth below bed $D'$ | Apron length $1.5D'$ |
|---|---|---|---|---|
| Nose (u/s curved head) | 2.0R | 17.42 m | 12.42 m | 18.63 m |
| Shank (straight) | 1.5R | 13.06 m | 8.06 m | 12.10 m |

Thickness: pitching $t = 1.15$ m, so the launched apron thickness is $1.5t = 1.5\times1.15 = 1.72$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.57$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 397.41 m (adopt 397 m) |
| Length of guide bund (u/s) | 496 m; d/s tail 99 m |
| Pitching thickness $t$ | 1.15 m |
| Apron at shank: width $1.5D'$ / thickness | 12.10 m / 1.72 m launched (2.57 m laid) |
| Apron at nose: width / thickness | 18.63 m / 1.72 m launched (2.57 m laid) |

#### Sketches
Plan of guide bunds:
```text
 U/S                                          D/S
                       bridge / weir axis
   Shank (straight)                |
 ________________________________  |  ___
/  1.0-1.5 W upstream            \ | /   \  tail
\________________________________/=|=\___/  0.25 W
 curved head                       |
 R, sweep 120 deg    <-- waterway W -->
 ________________________________  |  ___
/                                \ | /   \
\________________________________/=|=\___/
```
Section at the shank (straight portion):
```text
           top width 6 m
         ___________  <- HFL + 1.5 m
        /           \
 HFL ---/-------------\---  2:1 slopes
       / pitching t    \
 bed__/__ filter ______ \___
      \_ launching apron _/ (launches at 2:1)
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the waterway between the guide bunds found for Q = 7000 m3/s?
   A. Lacey's regime width W = 4.75 sqrt(Q), about 397 m
   B. W = 0.47 (Q/f)^(1/3), about 8.7 m
   C. W = 0.06 Q^(1/3), about 1.15 m
   D. W = 1.5 x flood depth, 7.5 m
   Answer: A. The waterway is the Lacey regime width 4.75 sqrt(7000) = 397 m. The other expressions are for scour depth and pitching thickness. (Key point: Regime waterway)

2. With W = 397 m, what upstream shank length was adopted?
   A. 0.25 W = 99 m
   B. 1.25 W = 496 m
   C. 0.5 W = 198 m
   D. 2.5 W = 993 m
   Answer: B. Upstream shank is 1.0 to 1.5 W, so 1.25 W = 496 m was adopted. The 0.25 W value is the downstream tail. (Key point: Length of bund)

3. How is the thickness of stone pitching found?
   A. t = 0.47 (Q/f)^(1/3), about 8.7 m
   B. t = 0.06 Q, about 420 m
   C. t = 0.06 Q^(1/3), about 1.15 m
   D. t = 2R, about 17 m
   Answer: C. Pitching thickness depends on the cube root of discharge: 0.06 x 19.13 = 1.15 m. (Key point: Pitching thickness)

4. What maximum scour below HFL is used for the nose and the shank?
   A. 1.5R at the nose and 2R at the shank
   B. 1R at both
   C. 2R at both
   D. 2R at the nose and 1.5R at the shank
   Answer: D. The upstream curved nose faces the strongest attack, so 2R; the straight shank uses 1.5R. (Key point: Scour at nose and shank)

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