Define DWF, WWF; estimating WWF

Dry weather flow (DWF)

DWF is the flow in a sewer when there is no storm runoff. It consists of domestic sewage, industrial wastewater and groundwater infiltration. It is taken as the average flow during a dry period, usually seven consecutive days without rain.

Wet weather flow (WWF)

WWF is the flow in a sewer during and after rain. It is the sum of the DWF and the storm water that enters the sewer:

$$\text{WWF} = \text{DWF} + \text{storm runoff}$$

Estimation of WWF

1. Find the DWF from the design population, the water supply rate (sewage = 75-80% of supply) and a peak factor, plus infiltration.
2. Find the storm runoff by the rational method:

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

where $C$ is the weighted runoff coefficient, $i$ the rainfall intensity (mm/hr) for a duration $t_c = t_e + t_f$, and $A$ the area in hectares. 3. Add them: $Q_{WWF} = Q_{DWF}+Q_s$.

For a combined system the whole storm runoff is taken. In a partially separate system only the runoff from roofs and courtyards (say one-third to one-half of the area) is used. As a rough rule, a sewer may be designed for about 6 x DWF, but the rational method is preferred.