---
title: "Design of side intake with trashrack"
subject: "Hydropower Engineering"
chapter: 5
key: "semester_7/hydropower_engineering:intake-side-design-3315masl"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/intake-side-design-3315masl
site: BCT Engineering
---

# Design of side intake with trashrack

Hydropower Engineering · Chapter 5: Headworks of Run-of- River (RoR) Plants

- Asked in: 2078 Bhadra, 6 marks
- Video: [Side intake | Headworks | Part 3](https://www.youtube.com/watch?v=grnkJo7DPXw) — EdifyingYOU
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/intake-side-design-3315masl
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-5

## Question

Design and draw a section of a side intake for a project in which the river bed level is 3315.0 masl. Weir crest level is fixed at 3317.5 masl. The highest flood level in 100 years return period is 3319.55 masl. The canal water level is fixed at 3317.3 masl. The turbine discharge of a period is 1.45 m³/s. Assume other suitable data. Take cylindrical trashrack bar with 10 mm thickness and 100 mm spacing.

## Short answer

Set sill at RL 3316.00 m and opening 1.6 m wide by 1.3 m high, top RL 3317.30 m. Actual rack velocity 0.767 m/s, rack loss 0.0024 m; at the 100-year flood throttle gate to about 0.23 m opening; deck at RL 3320.15 m.

## Answer

**Given**: river bed RL 3315.00 m; weir crest RL 3317.50 m; 100-year flood level RL 3319.55 m; canal water level RL 3317.30 m; $Q=1.45$ m³/s; circular bars 10 mm thick at 100 mm clear spacing. Assumed: rack velocity 0.8 m/s, rack inclined at $75^\circ$, $C_d=0.6$ for the gate orifice.

#### Step 1: Level of the sill and opening
- Sill level: 1.0 m above the river bed, i.e. **RL 3316.00 m**, to keep bed load out.
- Opening height $h=1.3$ m, so the **top of the opening is at RL 3317.30 m**. This is the canal water level and 0.2 m below the weir crest, so the opening stays submerged at low flow.

#### Step 2: Rack area and width
$$
A_{net}=\frac{Q}{V}=\frac{1.45}{0.8}=1.812\ \text{m}^2
$$
$$
A_{gross}=A_{net}\times\frac{s+b}{b}=1.812\times\frac{110}{100}=1.994\ \text{m}^2
$$
$$
\text{width }=\frac{A_{gross}}{h}=\frac{1.994}{1.3}=1.53\ \text{m}\ \Rightarrow\ \text{adopt }1.6\ \text{m}
$$
Adopted gross opening $1.6\times1.3=2.08$ m²; net area $=2.08\times\frac{100}{110}=1.891$ m²; actual velocity $V=\dfrac{1.45}{1.891}=0.767$ m/s (< 1 m/s, so acceptable).

#### Step 3: Orifice check at normal pond level
Head difference between the pond (crest RL 3317.50 m) and the canal (RL 3317.30 m) $=0.2$ m.
$$
Q_{cap}=C_dA\sqrt{2g\Delta h}=0.6\times2.08\times\sqrt{19.62\times0.2}=2.47\ \text{m}^3/\text{s}>1.45
$$
Required head $\Delta h=\dfrac{1}{2g}\left(\dfrac{1.45}{0.6\times2.08}\right)^2=0.069$ m. The gate is throttled to keep the canal level at RL 3317.30 m.

#### Step 4: Trash rack loss (Kirschmer)
$$
h_t=\beta\left(\frac{s}{b}\right)^{4/3}\frac{V^2}{2g}\sin\theta=1.79\,(0.1)^{4/3}\frac{0.767^2}{19.62}\sin75^\circ=0.0024\ \text{m}
$$
which is negligible.

#### Step 5: Flood condition and levels
At the 100-year flood (RL 3319.55 m) the opening is submerged on both sides (canal WL 3317.30 m), so the head across it is $\Delta h=3319.55-3317.30=2.25$ m. Without control the intake would pass $0.6\times2.08\sqrt{2g\times2.25}=8.3$ m³/s, so the gate must be throttled or closed. To pass only 1.45 m³/s, the opening area is $\dfrac{1.45}{0.6\sqrt{2g\times2.25}}=0.36$ m², i.e. a gate opening of about $0.36/1.6=0.23$ m.
Top of the intake wall and hoist deck: flood level + 0.6 m freeboard $=$ **RL 3320.15 m**.

```text
SECTION (levels in m)
 3320.15 ____ deck
 3319.55 ~~~~~ HFL
 3317.50 ~~~~~~~~~~ weir crest (pond level)
          | rack  |gate|
 3317.30  |###\   |    |~~ canal WL 3317.30
          |###     |    |
 3316.00  |_____sill____|   (1.0 m above bed)
 3315.00 /////////////// river bed
```

**Answer**: sill RL 3316.00 m; opening 1.6 m wide x 1.3 m high (top RL 3317.30 m); rack velocity 0.767 m/s; rack loss 0.0024 m; gate opening about 0.23 m at the design flood; deck RL 3320.15 m.

## Check yourself

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

1. With bed RL 3315.00 and canal water level RL 3317.30, what are the sill level and the top of the 1.3 m high opening?
   A. Sill RL 3316.00 (1.0 m above bed); top RL 3317.30 (canal water level)
   B. Sill RL 3315.00 (on the bed); top RL 3316.30
   C. Sill RL 3316.00; top RL 3317.50 (weir crest)
   D. Sill RL 3317.30; top RL 3318.60
   Answer: A. The sill is 1.0 m above the bed to keep bed load out, and sill + 1.3 m gives the top at the canal level so the opening stays submerged. (Key point: Sill and opening levels)

2. The net rack area is 1.812 m^2 for 10 mm bars at 100 mm clear spacing. How is the gross area found?
   A. A_gross = A_net x 100/110 = 1.647 m^2
   B. A_gross = A_net x (s + b)/b = 1.812 x 110/100 = 1.994 m^2
   C. A_gross = A_net + 0.1 = 1.912 m^2
   D. A_gross = A_net = 1.812 m^2
   Answer: B. Bars block part of the width, so the gross opening must exceed the net flow area by the factor (bar + clear)/clear. (Key point: Gross area with bars)

3. At the 100-year flood the head on the opening centre is 2.90 m and the open gate would pass about 9.4 m^3/s. What is done?
   A. The gate is left fully open since the canal can carry 9.4 m^3/s
   B. The sill is lowered to reduce the flood head
   C. The gate is closed or throttled to a small opening (about 0.20 m) so only 1.45 m^3/s enters
   D. The rack is made finer to cut the flow
   Answer: C. Orifice discharge grows with head, so in flood the gate opening must be reduced (about 0.32 m^2 area) to admit only the turbine discharge. (Key point: Flood control)

4. How is the top of the intake wall and hoist deck level fixed?
   A. Weir crest level + 0.6 m = RL 3318.10
   B. Canal water level + 0.6 m = RL 3317.90
   C. Flood level - 0.6 m = RL 3318.95
   D. Flood level + 0.6 m free board = RL 3320.15
   Answer: D. The deck must stay above the 100-year flood level, so freeboard is added to the highest flood level: 3319.55 + 0.6 = 3320.15. (Key point: Top of intake wall)

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