---
title: "Guide bund and apron, 13,100 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:barrage-waterway-guide-bund-q13100"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/barrage-waterway-guide-bund-q13100
site: BCT Engineering
---

# Guide bund and apron, 13,100 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2078 Baisakh, 8 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/barrage-waterway-guide-bund-q13100
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design guide bunds and launching apron required to be provided for a bridge across a river whose total waterway is 658.88 m. The design flood discharge is 13,100 m³/s which may be increased by 20% for the design of the launching apron. The mean size of the river bed material is 0.3 mm.

## Short answer

Total waterway 659 m; upstream guide bund 824 m, downstream tail 165 m. Pitching thickness 1.41 m. Apron design uses 1.2×Q=15720 m³/s. Lacey's normal scour depth R=11.92 m. At nose, apron width 20.71 m; at shank, 11.77 m; launched thickness 2.12 m, laid thickness 3.16 m.

## Answer

#### Data and design basis
- Total waterway $W = 658.88$ m (given). The flood depth is not given, so Lacey's regime depth for the intensity $q = Q/W = 13100/658.88 = 19.88$ m²/s is used: $R_L = 1.35(q^2/f)^{1/3} = 10.03$ m. $f = 1.76\sqrt{0.3} = 0.964$.
- Discharge is increased by 20 % for the apron design: $Q_a = 1.2\times13100 = 15720$ m³/s.
- Silt factor (computed, not given): $f \approx 0.96$
- Flood depth (assumed Lacey regime depth): $h = 10.03$ m
- Design discharge for scour/apron: $Q = 15720$ m³/s; for pitching, $Q = 13100$ m³/s

#### 1. Waterway and length of guide bund
Waterway between abutments $W = 658.88$ m (given).
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 659 = 824\ \text{m} \\ L_{ds} &= 0.25W = 165\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 330$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 165$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 330\times 2.094 = 690$ m, so total length on the upstream side $\approx 1514$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 13100^{1/3} = 0.06\times 23.57 = 1.41\ \text{m}
$$
Adopt $t \approx 1.5$ 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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{15720}{0.96}\right)^{1/3} = 11.92\ \text{m}
$$
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 | 23.84 m | 13.81 m | 20.71 m |
| Shank (straight) | 1.5R | 17.88 m | 7.85 m | 11.77 m |

Thickness: pitching $t = 1.41$ m, so the launched apron thickness is $1.5t = 1.5\times1.41 = 2.12$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 3.16$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Waterway $W$ (given) | 658.88 m (adopt 659 m) |
| Length of guide bund (u/s) | 824 m; d/s tail 165 m |
| Pitching thickness $t$ | 1.41 m |
| Apron at shank: width $1.5D'$ / thickness | 11.77 m / 2.12 m launched (3.16 m laid) |
| Apron at nose: width / thickness | 20.71 m / 2.12 m launched (3.16 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. The total waterway is given as 658.88 m. How is it used in the guide bund design?
   A. It is recomputed as 4.75 sqrt(Q) and the given value is ignored
   B. It is used directly as W to set the guide bund lengths and head radii
   C. It is divided by 2 to obtain W
   D. It is used only for the apron thickness
   Answer: B. Since W is given, the guide bund lengths and radii are based on it (1.25W = 824 m upstream, 0.25W = 165 m tail). (Key point: Waterway given)

2. Why is the discharge taken as 1.2 x 13100 = 15720 m3/s for the apron?
   A. The problem says to increase the design flood by 20% for the launching apron design
   B. Because Lacey's formula needs 20% extra for any flow
   C. To convert m3/s to cumecs
   D. Because the apron is 1.2 m thick
   Answer: A. The extra 20% gives a safety margin on scour; R = 0.47 (15720/0.96)^(1/3) = 11.92 m. (Key point: Discharge for apron)

3. The thickness of stone pitching on the river-side slope is found from t = 0.06 Q^(1/3). For Q = 13100 m3/s it is about:
   A. 2.82 m (doubling the value)
   B. 0.7 m (halving the value)
   C. 1.41 m
   D. 6.9 m (using sqrt(Q) instead of Q^(1/3))
   Answer: C. Pitching thickness uses the cube root of Q: t = 0.06 Q^(1/3). (Key point: Thickness of stone pitching)

4. How is the thickness of the launching apron obtained?
   A. Launched thickness equals t; laid thickness equals t
   B. Laid thickness 1.5t and launched thickness sqrt5 t
   C. Laid thickness is found from the slope 1:1 giving 2t
   D. Launched thickness 1.5t = 2.12 m; laid thickness T = sqrt5 t = 3.16 m by equating volumes
   Answer: D. The stones on the 2:1 slope length sqrt5 D' at thickness 1.5t come from a flat apron of width 1.5D'; equal volumes give T = sqrt5 t (about 2.25t). (Key point: Launched and laid apron thickness)

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