Routing (attenuation)
Hydrological routing is the process of converting a hydrograph that passes through some part of a flow system to allow for the changes that occur during its passage. There are three main types of routing:
- catchment routing, which converts a rainfall excess (rainfall - losses) hyetograph into a hydrograph at the catchment outlet, allowing for the distribution of rainfalls over the catchment surface, and various lags or delays along flow paths;
- river routing, which allows for the changes in hydrographs as they flow along river reaches, caused by variations in the river channel geometry causing storage effects, reservoir routing, which allows for storage effects in a concentrated, 'level pool' reservoir.
Routing includes changes such as translation (the movement of the hydrograph in time) and attenuation (the spreading out of the hydrograph, and consequent reduction in its peak).
This is caused by a number of effects. On a catchment is mainly due to time effects, with rainwater falling on all parts of a catchment taking different times to flow to the catchment outlet, and so spreading out the runoff hydrograph. Storage and roughness effects also play a minor part.
In a stream, routing effects are mainly due to changes in the stream cross-section creating a number of storages, with roughness playing a minor part, unless there are significant overbank flows. In a reservoir, storage effects are the sole cause of attenuation.