---
title: "Runoff from a 20 ha built-up area"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:runoff-builtup-area-20ha"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/runoff-builtup-area-20ha
site: BCT Engineering
---

# Runoff from a 20 ha built-up area

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2070 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/runoff-builtup-area-20ha
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

A built-up area of 20 ha consisting of 40% roof and pavements with runoff coefficient as 0.8 and 60% lawns and gardens with runoff coefficient as 0.2 has a rainfall intensity of 50 mm/hr. What will be the runoff from that area?

## Short answer

Weighted runoff coefficient is 0.44. Using the rational formula Q=CiA/360 with A=20 ha and i=50 mm/hr gives runoff = 1.222 m3/s (about 1222 L/s).

## Answer

The runoff from the area is found by the Rational formula $Q = CiA/360$, with $A$ in hectares, $i$ in mm/hr and $Q$ in m³/s.

#### Weighted runoff coefficient
$$
C=\frac{0.40(0.8)+0.60(0.2)}{1.00}=0.32+0.12=0.44
$$

#### Runoff
$$
Q=\frac{C\,i\,A}{360}=\frac{0.44\times50\times20}{360}=1.222\ \text{m}^3/\text{s}
$$

**Answer: Runoff = 1.222 m³/s (about 1222 l/s).**

## Check yourself

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

1. Which formula gives the runoff?
   A. Q = CiA/360
   B. Q = CiA x 360
   C. Q = Ci/A
   D. Q = C + i + A
   Answer: A. A in ha, i in mm/hr, Q in m3/s. (Key point: Formula)

2. The weighted runoff coefficient is
   A. 0.50
   B. 0.80
   C. 0.44
   D. 0.20
   Answer: C. 0.40 x 0.8 + 0.60 x 0.2 = 0.44. (Key point: Weighted C)

3. The runoff is
   A. 12.22 m3/s
   B. 1.222 m3/s
   C. 0.1222 m3/s
   D. 122.2 m3/s
   Answer: B. 0.44 x 50 x 20 / 360 = 1.222 m3/s. (Key point: Result)

4. 1.222 m3/s equals about
   A. 12.2 l/s
   B. 122 l/s
   C. 1222 l/s
   D. 12220 l/s
   Answer: C. 1 m3/s = 1000 l/s. (Key point: Unit)

## Related in Sanitary Engineering

- [Storm water from a 45 ha catchment](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astorm-water-catchment-surfaces) — Sanitary Engineering, chapter 2
- [Circular combined sewer, 60 ha](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-60ha) — Sanitary Engineering, chapter 3
- [Wet weather discharge, combined sewer](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Awet-weather-discharge-small-town) — Sanitary Engineering, chapter 2
- [Combined sewer section, 85 ha area](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-85ha) — Sanitary Engineering, chapter 3
- [Combined circular sewer, 35 ha](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Acombined-sewer-35ha) — Sanitary Engineering, chapter 3
- [Grit chamber for 60 MLD, 0.23 mm](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Agrit-60mld-0p23) — Sanitary Engineering, chapter 7, asked 2×

## Related in other subjects

- [Runoff and effective rainfall, 4-hr storm, 80 km2](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arunoff-80km2-subbasins-phi) — Engineering Hydrology, chapter 3, asked 2×
- [Maximum rainfall intensity for 1, 3, 5 hr](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Amax-intensity-1-3-5hr-7am) — Engineering Hydrology, chapter 2
- [Phi-index, excess rainfall hyetograph, runoff volume](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aphi-hyetograph-5km2) — Engineering Hydrology, chapter 3
- [24-hr maximum rainfall for 50-year return period](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-24hr-max-50yr-semilog) — Engineering Hydrology, chapter 6