DWF, WWF and storm water factors

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.