Prediction of sediment transport in steep, rough streams

Steep streams compose a large percentage of the total channel length in mountainous drainage basins. Sediment mobilized on hillslopes must first pass through these streams before reaching lower-gradient channels. Little is known about flow and sediment transport processes in steep streams. For example, most sediment transport equations over-predict the sediment flux in steep streams by several orders of magnitude. These conventional sediment transport equations were developed for lower-gradient channels, whose bed configuration differs substantially from that which is found in steep streams. In steep steams, the bed is composed of large, relatively immobile grains and finer, more mobile sediment patches. The immobile grains can bear a large portion of the total boundary shear stress, which reduces the stress available to transport the patches of mobile sediment. Furthermore, the availability of this mobile sediment is limited by the episodic sediment supply from landslides and debris flows.; I hypothesized that most sediment transport equations do not perform well in steep streams because they do not: (1) account for the stress borne by the relatively immobile grains, (2) distinguish between the relatively mobile and immobile sediment, and (3) account for the limited availability of the mobile sediment. I modified a conventional sediment transport equation to account for these three effects. I then tested several sediment transport equations in a small, steep flume. Unlike most of the equations that I tested, my modified sediment transport equation predicted sediment fluxes to generally within an order of magnitude of the measured values.; To understand the sediment transport processes in steep streams, I also made measurements in the Erlenbach torrent (gradient of 10%) in Switzerland. In the Erlenbach, the patches of mobile sediment engaged in size-selective transport. When further modified to account for size-selective transport, my sediment transport equation generally predicted the sediment fluxes in the Erlenbach to within an order of magnitude of the measured values. Thus, predictions of sediment transport in steep channels may be improved by a sediment transport equation that accounts for the stress borne by the immobile grains, as well as the limited availability and size-selective transport of the relatively mobile sediment.