Geomorphology at the confluence of stream restoration and flood control

Abstract

This study assesses the geomorphologic response of a stream management project integrating ecological restoration principles with flood control. Damaging floods on the coastal stream San Pedro Creek in Pacifica, California, led to replacement of the straightened and incised natural channel with a constructed meander and vegetated floodplain. The intention was to mitigate flooding and increase habitat for species such as the federally-listed threatened Central California steelhead (Oncorhynchus mykiss). Longitudinal profile data show a degradation and aggradation oscillation. Smaller scale meanders have begun to form on the straight reaches between larger designed meanders. Headcutting backwaters at an inflection point initiated a chute cut-off. Preliminary flood modeling demonstrates less than a 50-year flood protection level. These results suggest that the stream reach is not stabilizing as designed, but is instead adjusting according to geomorphological principles. Specifically, the radius of curvature and the meander wavelength are decreasing. Further research is needed to understand lateral controls and the chute cut-off process in small urban stream restoration and flood control projects.