---
title: "Estimating storm water quantity"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:storm-water-quantity-estimation"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/storm-water-quantity-estimation
site: BCT Engineering
---

# Estimating storm water quantity

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2076 Baisakh, 4 marks; 2073 Magh, 4 marks; 2068 Bhadra (old course), 6 marks (asked 3 times)
- Video: [Sanitary Engineering Neumerical (Quantity of Storm Water)](https://www.youtube.com/watch?v=S56Md47XMyE) — Everything Foryou (Siddharth)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/storm-water-quantity-estimation
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

How do you determine the quantity of storm water for a community/locality (for a highly populated urban area)? Mention with a suitable example.

## Short answer

Storm water quantity is estimated using the Rational method: Q = CiA/360, where Q is peak runoff (m³/s), C is weighted runoff coefficient, i is rainfall intensity (mm/hr) for the time of concentration, and A is catchment area (ha). For a 40 ha residential-commercial area with C=0.54, time of concentration 20 min, and intensity 25.33 mm/hr, the peak discharge is 1.520 m³/s.

## Answer

The quantity of storm water from a locality is estimated by the **Rational method**, which is the standard method for small and medium urban catchments (up to a few hundred hectares).

$$
Q = \frac{C\,i\,A}{360}
$$

where $Q$ = peak runoff (m³/s), $C$ = coefficient of runoff, $i$ = rainfall intensity (mm/hr) for a duration equal to the time of concentration, and $A$ = catchment area (ha).

#### Steps
1. **Find the catchment area** $A$ from the map, and divide it into surfaces (roofs, roads, lawns, open land).
2. **Find the weighted coefficient of runoff:** $C=\dfrac{\sum C_jA_j}{\sum A_j}$. Typical values: roofs 0.75-0.95, asphalt road 0.70-0.95, gravel 0.25-0.40, lawns 0.05-0.20, parks 0.10-0.25. For future development use values for the fully developed area.
3. **Find the time of concentration:** $t_c=t_e+t_f$, where $t_e$ is the inlet (entry) time (usually 5-20 min) and $t_f=L/v$ is the flow time in the drain.
4. **Find the intensity** $i$ for duration $t_c$ and a chosen return period (2-10 years for urban drains) from the intensity-duration-frequency (IDF) curve or formula of the local Department of Hydrology and Meteorology. A common empirical form is $i=\dfrac{a}{t+b}$.
5. **Compute** $Q=CiA/360$, and design each drain section for its own $t_c$ and area, adding the area of each branch downstream.

#### Example
A residential-commercial locality of 40 ha: 30% roads and pavements ($C=0.85$), 20% roofs ($C=0.90$), 20% yards ($C=0.30$), 30% gardens ($C=0.15$). Entry time = 10 min, flow time in drain = 10 min. Using $i=760/(t+10)$ mm/hr (assumed).

$$
\begin{aligned}
C &= 0.3(0.85)+0.2(0.90)+0.2(0.30)+0.3(0.15) = 0.540 \\
t_c &= 10+10 = 20\ \text{min} \\
i &= \frac{760}{20+10} = 25.33\ \text{mm/hr} \\
Q &= \frac{0.540 \times 25.33 \times 40}{360} = 1.520\ \text{m}^3/\text{s}
\end{aligned}
$$

**Answer: Q = 1.520 m³/s**; the storm drain is designed for this discharge.

## Check yourself

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

1. In Q = CiA/360 with Q in m3/s and i in mm/hr, A is in
   A. m2
   B. km2
   C. hectares
   D. acres
   Answer: C. The factor 360 is for A in hectares and i in mm/hr. (Key point: Rational formula)

2. For which duration is the rainfall intensity i taken?
   A. 1 hour always
   B. Equal to the time of concentration
   C. 24 hours
   D. Equal to the inlet time only
   Answer: B. Peak runoff occurs when the storm lasts for the time of concentration. (Key point: Intensity duration)

3. How is the weighted coefficient of runoff found?
   A. Average of the largest and smallest C
   B. Sum of C values
   C. Sum of (C x area) divided by sum of areas
   D. Largest C of the surfaces
   Answer: C. C = sum(CjAj)/sum(Aj). (Key point: Weighted runoff coefficient)

4. In the example (40 ha, C = 0.540, t_c = 20 min, i = 25.33 mm/hr), what is the peak discharge?
   A. 0.152 m3/s
   B. 15.2 m3/s
   C. 2.43 m3/s
   D. 1.520 m3/s
   Answer: D. Q = 0.540 x 25.33 x 40 / 360 = 1.520 m3/s. (Key point: Worked example)

## Related in Sanitary Engineering

- [Storm water estimation and its limitations](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astorm-water-limitations) — Sanitary Engineering, chapter 2
- [Estimating sewage quantity for a city](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aquantity-sewage-nepal-city) — Sanitary Engineering, chapter 2
- [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
- [DWF, WWF and storm water factors](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-storm-factors) — Sanitary Engineering, chapter 2
- [Recommend a suitable sewerage system](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Arecommend-sewerage-system) — Sanitary Engineering, chapter 1, asked 4×

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