Irregular bed forms in steep, rough channels: 2. Field observations

Abstract

A linear stability analysis using a flow model appropriate for steep, rough channels suggests that initial wavelength selection of alternate bars is strong for extant flow conditions in North Boulder Creek, Colorado. This is due in part to effects of velocity correlation terms in the streamwise momentum balance that involve deviations in velocity components about depth‐averaged values. The analysis also suggests that alternate bars and midchannel bars can “compete” during initial bed form growth at low sediment transport rates. These points are consistent with the observation that alternate and midchannel bar forms in North Boulder Creek have comparable topographic relief, as revealed by spectral analysis of the bed topography. Moreover, the dominant wavelengths of these bar forms match the fastest growing wavelengths predicted by the stability analysis. However, whereas the alternate bar topography can be attributed to free‐bar growth, much of the midchannel bar topography probably is a forced‐bar topography that is related to flow accelerations associated with variations in channel width. The complex bed topography of North Boulder Creek thus consists of a decipherable mixture of bed forms that have evolved as free alternate bars together with forced bars whose positions and geometries are fixed in relation to variations in channel width or to large immobile boulders.