---
title: "Guide bank design, 4000 m3/s, d 0.40 mm"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:guide-bank-q4000-hfl205-bed202"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/guide-bank-q4000-hfl205-bed202
site: BCT Engineering
---

# Guide bank design, 4000 m3/s, d 0.40 mm

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2078 Poush, 5 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-bank-q4000-hfl205-bed202
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design a guide bank required for a bridge on a river having following particulars: maximum flood discharge = 4000 m³/s, HFL = 205 m, river bed level = 202 m, average diameter of river bed material = 0.40 mm.

## Short answer

Lacey waterway 300 m; upstream guide bund 375 m, downstream tail 75 m. Pitching thickness 0.95 m. Normal scour depth R=7.20 m. Nose apron width 17.10 m, shank apron 11.70 m; launched thickness 1.43 m, laid thickness 2.13 m.

## Answer

#### Data and design basis
- Silt factor (Lacey): $f = 1.76\sqrt{d_{mm}} = 1.76\sqrt{0.4} = 1.11$
- Flood depth above bed: $h = 205 - 202 = 3.00$ m
- Design discharge for scour/apron: $Q = 4000$ m³/s

#### 1. Waterway and length of guide bund
Lacey's regime waterway: $W = 4.75\sqrt{Q} = 4.75\sqrt{4000} = 300.42$ m. Adopt $W \approx 300$ m.
Length of guide bund (Garg): upstream shank $=1.0$ to $1.5\,W$, downstream tail $=0.2$ to $0.5\,W$. Adopt
$$
\begin{aligned} L_{us} &= 1.25W = 1.25\times 300 = 375\ \text{m} \\ L_{ds} &= 0.25W = 75\ \text{m} \end{aligned}
$$
Curved heads: upstream radius $R_u \approx 0.5W = 150$ m with sweep $120^\circ$ to $145^\circ$ (adopt $120^\circ$); downstream radius $R_d \approx 0.25W = 75$ m with sweep about $45^\circ$.
Arc length of upstream head $= R_u\theta = 150\times 2.094 = 314$ m, so total length on the upstream side $\approx 689$ m.

#### 2. Thickness of stone pitching
$$
t = 0.06\,Q^{1/3} = 0.06\times 4000^{1/3} = 0.06\times 15.87 = 0.95\ \text{m}
$$
Adopt $t \approx 1.0$ 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:
$$
R = 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\left(\frac{4000}{1.11}\right)^{1/3} = 7.20\ \text{m}
$$
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 | 14.40 m | 11.40 m | 17.10 m |
| Shank (straight) | 1.5R | 10.80 m | 7.80 m | 11.70 m |

Thickness: pitching $t = 0.95$ m, so the launched apron thickness is $1.5t = 1.5\times0.95 = 1.43$ m. Equating volumes per metre, the thickness as laid over $1.5D'$ width is $T = \dfrac{1.5t\times\sqrt5 D'}{1.5D'} = \sqrt5\,t = 2.13$ m (about $2.25t$).

#### Summary
| Item | Value |
|---|---|
| Lacey waterway $W$ | 300.42 m (adopt 300 m) |
| Length of guide bund (u/s) | 375 m; d/s tail 75 m |
| Pitching thickness $t$ | 0.95 m |
| Apron at shank: width $1.5D'$ / thickness | 11.70 m / 1.43 m launched (2.13 m laid) |
| Apron at nose: width / thickness | 17.10 m / 1.43 m launched (2.13 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. For d = 0.40 mm the Lacey silt factor is:
   A. f = 1.76 sqrt(0.4) = 1.11
   B. f = 1.76 x 0.4 = 0.70
   C. f = 1.76 / sqrt(0.4) = 2.78
   D. f = 0.47 sqrt(0.4) = 0.30
   Answer: A. f = 1.76 sqrt(d_mm), giving 1.11. (Key point: Silt factor)

2. For a design flood of 4000 m3/s, the guide bund design is based on the Lacey regime width. Which value is obtained?
   A. W = 4.75 Q^(1/3) = much smaller (about 75 m)
   B. W = 4.75 sqrt(Q) = 300 m
   C. W = 2.71 sqrt(Q) (about 171 m)
   D. W = 0.47 (Q/f)^(1/3) (about 7 m)
   Answer: B. Lacey's regime waterway is W = 4.75 sqrt(Q); it fixes the guide bund lengths and radii. (Key point: Lacey regime waterway)

3. With waterway W = 300 m, how are the upstream shank and downstream tail lengths chosen?
   A. Upstream shank about 0.25W; downstream tail about 1.25W
   B. Both equal to W
   C. Upstream shank about 1.25W = 375 m; downstream tail about 0.25W = 75 m
   D. Upstream shank about 3W; downstream tail about W
   Answer: C. Garg's rule: upstream shank 1.0 to 1.5 W and downstream tail 0.2 to 0.5 W; the upstream side is longer to guide flow smoothly into the waterway. (Key point: Length of guide bund)

4. Lacey scour depth R = 7.2 m and flood depth h = 3.0 m. What is the apron horizontal width at the nose (scour 2.0R below HFL, launching slope 2H:1V)?
   A. D' = 2.0R - h = 11.4 m, so width 1.5D' = 17.1 m
   B. Width = 2.0R = 14.4 m (forgetting to subtract the flood depth)
   C. Width = 2D' = 22.8 m
   D. Width = D' = 11.4 m
   Answer: A. The apron must cover scour below the bed: D' = (factor x R) - h. At 2H:1V the apron width laid is 1.5D'. (Key point: Depth below bed and apron width)

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