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

# Storm water estimation and its limitations

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2074 Bhadra, 4 marks
- Video: [Estimation of storm runoff](https://www.youtube.com/watch?v=s3lu6DMvLZg) — Environmental engg
- 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-limitations
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

As a sanitation engineer how would you determine the quantity of storm water for a highly populated urban area? What type of limitations exist in storm water quantity determination for such an area? Discuss in detail.

## Short answer

Use the rational method Q=CiA/360 for small catchments; for large complex areas use hydrographs or SWMM. Limitations include nonuniform rainfall, uncertain runoff coefficient, difficult time of concentration, scarce rainfall data, peak-only result, blockage/backwater effects, small catchment validity, changing land use, and sewage interactions.

## Answer

#### Determination of storm water quantity
For a densely built urban area the **rational method** is used:

$$
Q=\frac{C\,i\,A}{360}\ \ \text{(m}^3/\text{s)}
$$

1. Measure the catchment area $A$ (ha) of each drain, from a contour map and drainage layout.
2. Find the weighted runoff coefficient $C$ from the land use (roofs, paved roads, courtyards, open land). For a dense urban area $C$ is high, about 0.7-0.9.
3. Find the time of concentration $t_c=t_e+t_f$ (entry time 5-10 min in dense areas plus flow time in the drain).
4. Read the intensity $i$ for $t_c$ and the design return period (5-10 years for main drains) from the IDF curve of the nearest rain gauge station.
5. Compute $Q$ for each section and design the drains using Manning's formula.

For large or complex areas, hydrograph methods or computer models (unit hydrograph, SWMM) are used.

#### Limitations in a highly populated urban area
1. **Assumption of uniform rainfall** over the whole catchment and uniform intensity during $t_c$ is not true for large areas.
2. **Runoff coefficient is uncertain.** It changes with soil moisture, storm intensity and quick urban growth. A fixed $C$ may under- or overestimate.
3. **Time of concentration is hard to find** in a complex network with many inlets, varied slopes and encroached drains.
4. **Lack of rainfall data:** short-duration records are often few or missing for Nepali towns, so IDF curves are doubtful.
5. **The method gives only the peak,** not volume or hydrograph; it cannot account for storage in ponds or detention.
6. **Backwater and blockage:** the method ignores blocked inlets and drains, solid waste in drains, river backwater and surcharging, so actual flow differs.
7. **Valid only for small catchments** (generally below 400-500 ha).
8. **Changing land use:** future urbanisation, filling of ponds and drains, climate change increasing rainfall, are difficult to predict.
9. **Interaction with the sewage flow and illegal connections** are not included.

To reduce errors a factor of safety is used, local data are collected, and drains are checked after construction.

## Check yourself

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

1. Which method is used for storm water in a dense urban area?
   A. Harman's formula
   B. Rational method Q = CiA/360
   C. Geometric increase
   D. Manning's formula for Q
   Answer: B. Manning's formula is then used to design drains. (Key point: Method)

2. The rational method is valid mainly for
   A. very large basins of thousands of ha
   B. any catchment
   C. small catchments, generally below 400-500 ha
   D. volume estimation
   Answer: C. It does not suit large complex catchments. (Key point: Method limit)

3. The rational method gives only
   A. the full hydrograph
   B. storage volume
   C. the peak discharge, not volume or hydrograph
   D. groundwater levels
   Answer: C. It cannot account for storage in ponds or detention. (Key point: Output limit)

4. Why are IDF curves doubtful for Nepali towns?
   A. Short-duration rainfall records are few or missing
   B. Rainfall is uniform
   C. Rainfall is too low
   D. No catchment maps exist
   Answer: A. Lack of data makes intensity uncertain. (Key point: Data limitation)

## Related in Sanitary Engineering

- [Estimating storm water quantity](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Astorm-water-quantity-estimation) — Sanitary Engineering, chapter 2, asked 3×
- [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
- [Water carriage vs conservancy sanitation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asanitation-water-carriage-conservancy) — Sanitary Engineering, chapter 1, asked 4×
- [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

## Related in other subjects

- [Water supply and sanitation management](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-sanitation) — Building Technology, chapter 12
- [T/F: loop arrangement in urban area economical](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-loop-urban-economical) — Transmission and Distribution System Design, chapter 5
- [Water sources: quality and quantity](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asources-quality-quantity) — Water Supply Engineering, chapter 2, asked 3×