Sediment storage and transport in coarse bed streams: scaleconsiderations

摘要

Bedforms in gravel-bed rivers range in size from 10~(-2)to 10~3m and have a wide rangeof affects on sediment transport and channel stability. To study the affect of thesebedforms on sediment transport a classification scheme is proposed that breaks thebedforms into micro, meso, macro and megaforms. Then using two creeks in BritishColumbia and one in California, the trend of sediment transport rates is related tothe bed state, sediment and wood storage and the associated bedforms are discussed. At small spatial and temporal scales (micro/mesoform scales), variability in sed-iment transport rate can be ascribed to the changing state of the bed, which largelydepends on the sediment supply regime. Stabilizing bedforms develop when sedimentsupply is low, and reduces the depth of the bed active layer and the mobility of thegrains, thereby decreasing the sediment transport rate. Sediment rating relations in alow sediment supply channel are steep and are believed to be extremely sensitive tosmall changes in sediment supply and flow regime when compared to higher sedimentsupply systems. High sediment supply suppresses the development of stabilizingbedforms and increases the mobility of grains and the depth of the active layer. Atthe reach scale (macroform scale), high sediment mobility is shown to producecomplex cycles of aggradation and degradation that can persist for decades. In-channel woody debris can strongly influence the timing and magnitude of theseaggradation–degradation cycles and also has an important effect on the developmentof megaforms.