Stream‐channel Incision Following Drainage‐basin Urbanization1

ABSTRACT: Urbanization of a drainage basin results in pervasive hydrologic changes that in turn initiate long‐term changes in stream channels. Increases in peak discharges and in durations of high flows result in either quasi‐equilibrium channel expansion, where cross‐section area increases in near‐proportion to the discharge increase, or catastrophic channel incision, where changes occur far out of proportion to the discharge increases that initiated them. Field data and hydrologic modeling of rapidly urbanizing basins in King County, Washington, define conditions of flow, topography, geology, and channel roughness that identify streams susceptible to incision. Channel slope and geologic material are particularly critical; thus simple map overlays, nearly irrespective of contributing drainage area, provide a valuable planning tool for identification of susceptible terrain. Where such conditions exist, basal shear stress provides a quantifiable parameter for predicting likely problems, although knickpoints are typical in such settings and confound simple calculation of sediment‐transport rates. Where urbanization proceeds in such areas, effective mitigation of the incision hazards requires a degree of stormwater control far in excess of standards typically applied to present development activity.