---
title: "Side intake sketch; trash rack loss"
subject: "Hydropower Engineering"
chapter: 5
key: "semester_7/hydropower_engineering:intake-side-sketch-trashrack-loss"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/intake-side-sketch-trashrack-loss
site: BCT Engineering
---

# Side intake sketch; trash rack loss

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

- Asked in: 2073 Shrawan, 3+2 marks
- 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-sketch-trashrack-loss
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-5

## Question

Draw a neat sketch of a side intake with all components. How do you calculate the hydraulic loss at the trash rack?

## Short answer

A side intake includes trash boom, inclined rack, sill, gate, hoist deck, undersluice and transition. Trash rack loss is Kirschmer's formula h_t = beta(s/b)^(4/3)(V^2/2g) sin theta, with beta 2.42 rectangular, 1.79 circular; add 25-50% for clogging.

## Answer

#### Sketch of a side intake
```text
PLAN
 river ==========================>
   weir |||||||  undersluice
   bank +---------------------+
        |trash rack  | gate   |--> gravel trap --> canal
        +---------------------+

SECTION
  pond ~~~~~~~~~~~~~~~~~~~~~~~~
   boom o  |  rack (75°)  | gate | hoist deck
        ###|##\           |      |
        ###|###\__________|______|======> canal
   sill (1.0-1.5 m above undersluice)
 /////////////////////////////////////////// bed
```
Components: trash boom, trash rack with rake, intake sill, gate with stop-log slot, hoist deck, flushing undersluice and the transition to the canal.

#### Head loss at the trash rack (Kirschmer's formula)
$$
h_t=\beta\left(\frac{s}{b}\right)^{4/3}\frac{V^2}{2g}\sin\theta
$$
where $h_t$ is the head loss (m), $\beta$ the bar shape factor (2.42 for rectangular bars, 1.83 for rectangular bars with a rounded upstream edge, 1.79 for circular bars, 0.76 for streamlined bars), $s$ the bar thickness, $b$ the clear spacing between bars, $V$ the approach velocity (m/s) and $\theta$ the angle of the rack from the horizontal.

A rack in service is partly clogged, so the clean-rack loss is increased by a clogging allowance of 25-50%, and this increased loss is used in the design of the intake.

**Example**: circular bars ($\beta=1.79$) 10 mm thick at 100 mm clear spacing, $V=0.8$ m/s, $\theta=75^\circ$:
$$
h_t=1.79\,(0.1)^{4/3}\frac{0.8^2}{19.62}\sin75^\circ=1.79\times0.0464\times0.0326\times0.966=0.0026\ \text{m}
$$
This is very small, so the loss at a clean rack is negligible.

## Check yourself

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

1. Which formula gives trash rack loss?
   A. ht = f L V^2/(2 g D)
   B. Kirschmer: ht = beta (s/b)^(4/3) V^2/2g sin(theta)
   C. ht = n^2 V^2 L / R^(4/3)
   D. ht = 0.5 V^2/2g
   Answer: B. Kirschmer relates loss to bar shape, thickness-to-spacing ratio, velocity and rack inclination. (Key point: Rack loss formula)

2. Which bar shape gives the lowest loss?
   A. Streamlined bars (beta 0.76)
   B. Rectangular bars (2.42)
   C. Circular bars (1.79)
   D. All same
   Answer: A. Streamlined bars have the smallest beta. (Key point: Bar shape factor)

3. How is a clogged rack handled in design?
   A. Ignore it
   B. Halve the loss
   C. Add an allowance of 25-50% to the loss
   D. Set the loss to zero
   Answer: C. Clogging raises loss, so design includes an extra allowance. (Key point: Clogging)

## Related in Hydropower Engineering

- [Length of bottom intake rack](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aintake-bottom-rack-length-8cumec) — Hydropower Engineering, chapter 5
- [Hydropower intake sketch; minimizing head loss](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Ahydropower-intake-sketch-minimise-headloss) — Hydropower Engineering, chapter 5
- [Design of side intake with trashrack](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aintake-side-design-3315masl) — Hydropower Engineering, chapter 5
- [Design of orifice type intake](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aintake-orifice-design-5cumec) — Hydropower Engineering, chapter 5
- [RoR headworks sketch; intake importance and location](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-headworks-arrangement-intake-location) — Hydropower Engineering, chapter 5
- [Vortex formation in intake](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aintake-vortex-formation-no-vortex) — Hydropower Engineering, chapter 5

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