---
title: "Street inlets and catch basins"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:street-inlets-catch-basins"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/street-inlets-catch-basins
site: BCT Engineering
---

# Street inlets and catch basins

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2071 Bhadra, 4 marks
- Video: [Sewer Inlets | street inlets | Curb inlets | Gutter inlets with diagram |  IN HINDI by EASY CIVIL](https://www.youtube.com/watch?v=KQPGvbz4KE4) — EASY CIVIL
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/street-inlets-catch-basins
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Describe street inlets and catch basins with neat sketches.

## Short answer

Street inlets are openings in the kerb or gutter that admit storm water into sewers. Catch basins are chambers below the inlet that trap silt and grit; velocity drops, solids settle in the sump, and a hooded outlet retains floating matter and forms a water seal.

## Answer

#### Street inlets
Street inlets are openings in the road kerb or gutter that let storm water from the road surface enter the underground storm drain or combined sewer. Types: **curb inlets** (vertical opening in the kerb), **gutter inlets** (horizontal grating in the gutter) and **combined inlets** (both together).

```text
 Curb inlet          Gutter inlet
   |                 ________
   |____ opening     |=grating=|
   |   |             |         |
   |   v chamber     |  chamber|
```

Inlets are placed at 30-60 m spacing, at corners and at low points of the road, and they connect to the sewer by a 150-300 mm pipe.

#### Catch basin
A catch basin is a small masonry or RCC chamber, usually 0.6-0.9 m in diameter and 0.9-1.5 m deep below the outlet, built in the line from a street inlet to the sewer to **collect silt, sand, grit and floating rubbish** which would otherwise settle in the sewer.

```text
      Street inlet grating
          |
     _____v_____
    |           |  ground level
    |           |
    |      ___  |-- hooded outlet (dips in water)
    |     |   | |
    |_____|silt|_|
```

**Construction:** brick or concrete walls plastered inside, a concrete floor, a cast iron cover, and an outlet pipe placed 0.3 m or more above the floor so that the heavy matter settles below it. A hood (bend) on the outlet traps floating matter and gives a water seal against foul gas.
**Working:** the velocity falls in the basin, the silt settles, and the clear water goes into the sewer. The basin is cleaned regularly (every few weeks, and more often in the rainy season). It is not provided in modern sewers where velocity can carry the silt and where cleaning is difficult; instead gully traps are used.

## Check yourself

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

1. Which is a gutter inlet?
   A. A vertical opening in the kerb
   B. A pipe in the manhole
   C. A silt pit
   D. A horizontal grating in the gutter
   Answer: D. Gutter inlets are horizontal gratings; curb inlets are vertical kerb openings. (Key point: Inlet types)

2. What is the main purpose of a catch basin?
   A. Collect silt, sand, grit and floating rubbish before they enter the sewer
   B. Increase the sewage flow
   C. Measure flow
   D. Ventilate the sewer
   Answer: A. It retains solids that would settle in the sewer. (Key point: Catch basin purpose)

3. Why is the catch basin outlet placed at least 0.3 m above the floor?
   A. To make pipe cheaper
   B. So heavy matter settles below it
   C. To give faster flow
   D. To avoid hood
   Answer: B. Heavy material collects in the pit below the outlet. (Key point: Outlet)

4. What is used in modern sewers instead of catch basins where velocity carries silt?
   A. Drop manholes
   B. Flush tanks
   C. Gully traps
   D. Inverted siphons
   Answer: C. Gully traps replace catch basins because basins are hard to clean. (Key point: Modern practice)

## Related in Sanitary Engineering

- [Street inlets and their types](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astreet-inlets-types) — Sanitary Engineering, chapter 4, asked 2×
- [Street inlets: curb vs gutter inlet](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astreet-inlets-curb-gutter) — Sanitary Engineering, chapter 4

## Related in other subjects

- [On-street vs off-street parking; kerb parking geometry](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aparking-onstreet-offstreet-comparison-geometry) — Transportation Engineering II, chapter 1