---
title: "Glacis fall: crest, cistern, HGL; Q 118"
subject: "Irrigation and Drainage Engineering"
chapter: 7
key: "semester_6/irrigation_and_drainage_engineering:glacis-fall-q118-fluming"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/glacis-fall-q118-fluming
site: BCT Engineering
---

# Glacis fall: crest, cistern, HGL; Q 118

Irrigation and Drainage Engineering · Chapter 7: Regulating Structures

- Asked in: 2075 Bhadra, 15 marks
- Video: [DESIGN STEPS OF GLACIS FALL PART 1 BY AMIT SINGH water resources engineering](https://www.youtube.com/watch?v=hbBIrvABtKA) — Study with Civil buddy
- 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/glacis-fall-q118-fluming
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-7

## Question

Design crest elements, cistern elements and draw HGL line of a designed glacis fall on a canal waterway with the following data: Full supply discharge = 118 cumec; Full supply level of canal (U/S) = 209.7 m; Full supply level of canal (D/S) = 208.2 m; Safe exit gradient of canal material = 1/6; Canal bed level (U/S) = 207.5 m; Canal bed level (D/S) = 206.0 m; Canal bed width (U/S & D/S) = 62 m; Fluming ratio = 75%.

## Short answer

Flumed crest width 46.5 m, crest level 208.43 m, head 1.304 m, crest width 3.0 m. Cistern depth 0.25 m, floor level 205.75 m, length 8.28 m. Total impervious floor length 38 m. HGL drops from 209.70 m at u/s end to 206.85 m at d/s pile, then to 206.00 m exit.

## Answer

A glacis fall passes the flow over a raised crest, down a sloping glacis, and into a cistern in which a hydraulic jump forms. The canal is flumed (contracted) at the crest to reduce the cost and to increase the discharge intensity, so that the jump is more stable. Here the canal width is 62 m and the fluming ratio is 75 %.

#### Data
$Q = 118$ m³/s, FSL u/s $209.70$ m and d/s $208.20$ m, bed u/s $207.50$ m and d/s $206.00$ m, so $D_u = D_d = 2.2$ m, bed width 62 m, fluming ratio 75 %, safe exit gradient 1/6.

#### 1. Crest elements
- Fluming: throat width $L = 0.75\times62 = 46.5$ m.
- Approach velocity: $v = \dfrac{118}{62\times2.2} = 0.865$ m/s, $h_a = 0.0381$ m.
- Head over a broad crest: $Q = 1.705\,L\,H^{3/2}$
$$
H = \left(\frac{118}{1.705\times46.5}\right)^{2/3} = 1.304\ \text{m}
$$
- Crest level $= FSL_u + h_a - H = 209.70 + 0.038 - 1.304 = 208.43$ m, which is 0.93 m above the u/s bed.
- Crest width: about $2H$ to $3H$ for a broad crest; adopt **3.0 m**. U/s approach slope 1:1 (ramp), glacis slope 3:1 from the crest to the cistern.

#### 2. Cistern elements
Discharge per metre of flumed crest $q = 118/46.5 = 2.538$ m²/s. Energy above the d/s bed $E_1 = 209.70 + 0.038 - 206.00 = 3.738$ m. The conjugate depth without depression is 1.911 m, which is less than the tail water 2.20 m, so the jump is submerged and a nominal depression is adopted:
- **Cistern depth $X = 0.25$ m**; cistern floor level $= 206.00 - 0.25 = 205.75$ m.
- $y_1 = 0.298$ m, $F_1 = 4.97$, $y_2 = 1.954$ m.
- **Cistern length** $L_c = 5(y_2 - y_1) = 8.28$ m.
- Fall height from crest to cistern floor $= 208.43 - 205.75 = 2.68$ m, so the horizontal length of the 3:1 glacis $= 3\times2.68 = 8.05$ m.

#### 3. Floor length and HGL
Maximum static head $H_s = 209.70 - 206.00 = 3.70$ m. Lacey's scour (f = 1): $R = 1.35(q^2/f)^{1/3} = 2.51$ m, so the d/s cut-off need only reach 204.43 m; adopt $d = 2.5$ m (practical minimum) below the floor.
$$
\frac{1}{\pi\sqrt{\lambda}} = \frac{1}{6}\frac{d}{H_s} = 0.1126 \Rightarrow \lambda = 7.99,\ \alpha = 14.95,\ b = 37.4\ \text{m}
$$
Provide a total impervious floor of **38 m**.

**HGL (Khosla)** at the d/s pile: $\phi_E = 23.0\,\%$ and $\phi_D = 16.1\,\%$ of $H_s$. Pressure head levels:

| Point | Residual head | HGL level (m) |
|---|---|---|
| U/s end of floor | 100 % of 3.70 m | 209.70 |
| Junction at d/s pile | 23.0 % | 206.85 |
| Bottom of d/s pile | 16.1 % | 206.60 |
| D/s exit | 0 | 206.00 |

The HGL is a straight line from 209.70 m at the u/s end to 206.85 m at the d/s pile, then falls to the exit.

```text
 209.7 ~~~~~~~~~~
        \__ crest 208.43
 207.5 ___\___ glacis 3:1
              \_cistern_______ 206.0
          HGL: 209.7 ------------> 206.85
                                 |  d/s pile
```

**Answer:** crest level 208.43 m, crest width 3.0 m, flumed length 46.5 m; cistern depth 0.25 m, length 8.28 m; floor length 38 m.

## Check yourself

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

1. Why is the canal flumed at the crest of the glacis fall?
   A. To reduce the cost and increase discharge intensity q so the jump is more stable
   B. To lower the crest level
   C. To reduce the cistern length to zero
   D. To increase the upstream velocity of approach
   Answer: A. Contracting the width cuts the structure cost and raises q, which gives a stable hydraulic jump in the cistern. (Key point: Purpose of fluming)

2. What is the throat (flumed) width for a 75% fluming ratio on a 62 m canal?
   A. 62 m
   B. 46.5 m
   C. 83 m
   D. 15.5 m
   Answer: B. Throat width = fluming ratio x canal width = 0.75 x 62 = 46.5 m. (Key point: Crest elements)

3. How is the cistern length found?
   A. L_c = 5 x (y2 - y1), the hydraulic jump length
   B. L_c = 3 x glacis height
   C. L_c = 5 x crest width
   D. L_c = 2 x H
   Answer: A. The cistern must contain the jump, taken as 5 times the difference of conjugate depths: 5(1.954 - 0.298) = 8.28 m. (Key point: Cistern length)

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