---
title: "Spur design for concave bank, 5000 m3/s"
subject: "Irrigation and Drainage Engineering"
chapter: 6
key: "semester_6/irrigation_and_drainage_engineering:spur-concave-bank-q5000"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/spur-concave-bank-q5000
site: BCT Engineering
---

# Spur design for concave bank, 5000 m3/s

Irrigation and Drainage Engineering · Chapter 6: River Training Works

- Asked in: 2079 Chaitra, 10 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/spur-concave-bank-q5000
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-6

## Question

Design a spur for concave bank of the river if maximum flood discharge is 5,000 m³/sec, river bed level = 1000.00 masl, high flood level = 1010.00 masl, and Lacey's silt factor = 1.0. Design length spacing, total height and side slope of spur and launching apron for Bank, middle part and nose.

## Short answer

Spur length 65 m, spacing 130 m, total height 11 m, side slopes 2:1 (shank/bank) and 3:1 (nose). Apron widths: bank 3.1 m, middle 6.1 m, nose 12.1 m.

## Answer

A spur on the concave bank deflects the current away from the eroding bank. It is designed from the regime width, the scour depth and the severity of the bend. The design uses Lacey and standard spur practice (Garg).

#### Data
$Q = 5000$ m³/s, bed level 1000.00 m, HFL 1010.00 m (flood depth $h = 10$ m), $f = 1.0$.

#### Step 1. Regime width and scour depth
$$
\begin{aligned} W &= 4.75\sqrt{Q} = 4.75\sqrt{5000} = 335.9\ \text{m} \\ R &= 0.47\left(\frac{Q}{f}\right)^{1/3} = 0.47\times17.10 = 8.04\ \text{m} \\ t &= 0.06\,Q^{1/3} = 1.03\ \text{m} \end{aligned}
$$

#### Step 2. Length and spacing
- Length of spur is limited to about 20 % of the regime width: $L_s = 0.2\times336 = 67$ m; adopt $L_s = 65$ m (measured from the bank line at HFL).
- Spacing on a concave bank $S = 2$ to $2.5\,L_s$; adopt $S = 2L_s = 130$ m.
- Spurs are set pointing slightly upstream (about $100^\circ$ to $110^\circ$ with the bank), with a T-head or round head at the nose.

#### Step 3. Height and side slopes
- Top level $=$ HFL $+ 1.0$ m $= 1011.0$ m (bank end); nose end is kept about 0.5 to 1.0 m lower, but not below HFL.
- Total height at the root above bed $= 1011.0 - 1000.0 = 11.0$ m.
- Top width 4 m (3 to 6 m).
- Side slopes: 2:1 on the shank (middle) and bank part, 3:1 at the nose; stone pitching thickness $t = 1.03$ m (adopt 1.1 m) on a 0.3 m filter.

#### Step 4. Launching apron at bank, middle and nose
Scour below HFL is taken as 1.5R at the bank (root), 1.75R in the middle and 2.25R at the nose. The apron slope is 2:1 and its width is $1.5D'$.

| Part | Scour factor | Scour below HFL (m) | $D'$ below bed (m) | Apron width $1.5D'$ (m) | Launched thickness $1.5t$ (m) |
|---|---|---|---|---|---|
| Bank (root) | 1.5R | 12.06 | 2.06 | 3.08 | 1.54 |
| Middle | 1.75R | 14.06 | 4.06 | 6.10 | 1.54 |
| Nose (head) | 2.25R | 18.08 | 8.08 | 12.12 | 1.54 |

Thickness of apron laid $\approx \sqrt5\,t = 2.29$ m (launched thickness $1.5t = 1.54$ m).

**Answer:** $L_s \approx 65$ m, spacing $\approx 130$ m, total height 11 m, side slopes 2:1 (3:1 at the nose), apron widths 3.1 m (bank), 6.1 m (middle) and 12.1 m (nose).

```text
 bank ====root====|========== shank ===========\ nose
 HFL+1 --------------------------------------    \
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 bed ____apron 3.1m___apron 6.1m_______apron 12.1m__
```

## Check yourself

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

1. How is the spur length limited?
   A. Equal to the full regime width, 336 m
   B. About 2R, 16 m
   C. About 20% of the regime width: 0.2 x 336, about 65 m
   D. 1.5D' of the apron
   Answer: C. Spur length is limited to about 20% of the Lacey regime width 4.75 sqrt(Q). (Key point: Spur length)

2. What spacing is adopted on a concave bank?
   A. 0.5 Ls = 32 m
   B. Equal to the regime width
   C. 10 Ls = 650 m
   D. 2 to 2.5 times the spur length; adopted 2Ls = 130 m
   Answer: D. A concave bank is under heavy attack, so spurs are closely spaced: S = 2 to 2.5 Ls. (Key point: Spacing)

3. Which scour factors are used below HFL at the bank, middle and nose?
   A. 1.5R, 1.75R and 2.25R
   B. 2.25R, 1.75R and 1.5R
   C. 1.5R at all
   D. R, 1.5R and 2R
   Answer: A. Scour increases toward the nose, so the factors rise from the bank root to the nose. (Key point: Scour factors)

4. What top level is used and how do apron widths vary?
   A. HFL level exactly; widths all 12.1 m
   B. HFL + 1 m = 1011 m (height 11 m); widths 3.1, 6.1 and 12.1 m rising to the nose
   C. HFL + 3 m; widths decrease toward the nose
   D. HFL + 1 m; widths all 3.1 m
   Answer: B. Apron width is 1.5D', and D' grows toward the nose because scour is deeper there. (Key point: Height and apron)

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