---
title: "DWF, WWF and storm water factors"
subject: "Sanitary Engineering"
chapter: 2
key: "semester_6/sanitary_engineering:dwf-wwf-storm-factors"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dwf-wwf-storm-factors
site: BCT Engineering
---

# DWF, WWF and storm water factors

Sanitary Engineering · Chapter 2: Quantity of Wastewater

- Asked in: 2080 Chaitra, 1+3 marks
- Video: [QUANTITY OF SEWAGE: DRY WEATHER FLOW AND STORM WATER FLOW, PART 35, ENVIRONMENTAL ENGINEERING](https://www.youtube.com/watch?v=j2oP6DvTj24) — Vidhya Focus-2020
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/dwf-wwf-storm-factors
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-2

## Question

Define dry and wet weather flow. Describe the factors affecting storm water.

## Short answer

Dry weather flow is sewage flow in absence of rain; wet weather flow adds storm runoff. Storm water is affected by rainfall intensity and duration, catchment area and imperviousness, slope and shape, time of concentration, antecedent moisture, depression storage, vegetation interception, and drain condition.

## Answer

#### Definitions
- **Dry weather flow (DWF):** the flow of sewage in a sewer in dry weather, consisting of domestic and industrial sewage and groundwater infiltration, without any storm water.
- **Wet weather flow (WWF):** the flow in a sewer during rain: DWF plus storm water runoff.

#### Factors affecting storm water (runoff)
1. **Rainfall intensity and duration:** heavier and longer storms give more runoff. Peak runoff happens when the duration equals the time of concentration.
2. **Area of the catchment:** runoff is directly proportional to the area drained.
3. **Imperviousness (type of surface):** roofs, roads and pavements give 70-95% runoff, but lawns and forests only 5-20%. Urbanisation increases runoff.
4. **Slope and shape of catchment:** steep and short catchments give quick, high peaks; flat and long catchments give delayed, flat peaks.
5. **Time of concentration:** depends on size, slope and surface roughness; a shorter time gives higher intensity.
6. **Initial wetness of soil and infiltration:** wet or saturated soil gives more runoff.
7. **Depression storage, evaporation and interception** by vegetation reduce runoff.
8. **Distribution of rainfall:** a storm that is uneven over the area or moves along the drainage direction changes the peak.
9. **Condition and capacity of drains:** blocked drains, back water and ponding delay and reduce the flow.

## Check yourself

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

1. Dry weather flow differs from wet weather flow in that DWF
   A. includes storm runoff
   B. has no storm water
   C. is always larger
   D. occurs only in rain
   Answer: B. DWF is sewage plus infiltration in dry weather. (Key point: DWF definition)

2. Why does urbanisation increase storm runoff?
   A. Roofs and pavements are impervious and give 70-95% runoff
   B. Rainfall intensity increases
   C. Soil becomes more porous
   D. Drains become wider
   Answer: A. Impervious surfaces reduce infiltration, so more rain becomes runoff. (Key point: Imperviousness)

3. Which catchment gives a quick, high peak?
   A. Flat and long
   B. Steep and short
   C. Flat and wide
   D. Heavily vegetated
   Answer: B. Short, steep catchments have a small tc. (Key point: Slope and shape)

4. How does initial soil condition affect runoff?
   A. Wet or saturated soil gives more runoff
   B. Wet soil gives less runoff
   C. No effect
   D. Dry soil gives more runoff
   Answer: A. Saturated soil cannot absorb more rain. (Key point: Soil wetness)

## Related in Sanitary Engineering

- [Define DWF, WWF; estimating WWF](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-quantity-wwf) — Sanitary Engineering, chapter 2
- [DWF and WWF in sewer design](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-importance) — Sanitary Engineering, chapter 2
- [Differentiate DWF and WWF; sources](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-sources-sewage) — Sanitary Engineering, chapter 2
- [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
- [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
- [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×

## Related in other subjects

- [Factors affecting water demand; variation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-demand) — Water Supply Engineering, chapter 3, asked 5×
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×
- [Canal discharge for dry and wet crops](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-20000-30000) — Irrigation and Drainage Engineering, chapter 2, asked 2×
- [Canal discharge for dry and wet crops (30000/40000 ha)](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akor-discharge-dry-wet-30000-40000) — Irrigation and Drainage Engineering, chapter 2, asked 2×