Runoff

Runoff is the part of rainfall that flows over the land and through the upper soil into streams. Engineers need it to size drains, culverts, bridges, spillways and storage.

Not all rain becomes runoff. Some is intercepted by leaves, some fills hollows, some soaks in, and some evaporates. How much runs off depends on the intensity and duration of rain, the soil, slope, land use and how wet the catchment already is. Paving a city raises runoff sharply, which is why urban flooding has grown.

For small catchments, the rational method estimates the peak flow from a runoff coefficient, the rainfall intensity for a duration equal to the time of concentration, and the area. The time of concentration is how long water takes to travel from the farthest point to the outlet.

Larger catchments need hydrograph methods such as the unit hydrograph, which spread runoff over time and capture the shape of the flood.

Runoff estimates feed straight into design. A culvert sized for too small a flow overtops and washes out the road; one sized far too large wastes money. Designers therefore choose a return period suited to the importance of the structure: a minor drain may be designed for a storm expected every few years on average, a highway bridge or dam spillway for far rarer events. Local rainfall intensity-duration-frequency data are essential, and they are being revisited as rainfall patterns change.