Hydraulic Roughness

Summary

Hydraulic roughness must either be inferred from some map of land use/cover or be remotely sensed. In either case, a surrogate measure or classification must be trsnformed into representative hydraulic parameters. It is rarely measured directly due to the difficulty of obtaining depth and velocity measurements in overland flow except on research-type runoff plots. Assigning hyudraulic roughness to general land use/cover classifications is difficult, because these classifications are rarely made according to hydrolic characteristics. The hydrologist must exercise some judgment in assigning hydraulic roughness parameters to these classification schemes. Further, the temporal natural of the coefficients and within-class variations are important sources of variance in predicted hydrolic response. While the values shown in the map may be used to simulate runoff in the overland flow areas of the basin, channel hydraulic roughness must be supplied from other sources. Field observation or experience is usually required to assign appropriate values of hydrulic roughness to channels. This may be said of the overland flow areas as well. In both cases, we can assign initial values and then adjust by calibration. The hydraulic roughness parameter maps are a key factor in controlling the velocity at which runoff travels through the drainage network and reaches the channel. This idea is addressed in subsequent chapters.