INVITED FACULTY TALK - Surface-Layer Armoring And Its Effect On Sediment Transport In High-Gradient Channels
Pitlick, John 1 ; Mueller, Erich 2 ; Segura, Catalina 3
1 Geography Department, CU-Boulder
2 Geography Department, CU-Boulder
3 Geography Department, CU-Boulder
In high-gradient channels the sediment which is present on the bed surface (the armor layer) is typically much coarser than the sediment which moves as bed load during high-flow events. The difference in sediment sizes has lead to the notion that bed load transport in these channels is supply limited, meaning the mass transport rate of sediment is governed by the availability of the different sizes, not the flow properties. Studies we have conducted over the last 10 years challenge this assumption. We have investigated trends in bed load transport in relation to flow and channel properties at more than 100 sites on gravel-bed streams and rivers in Colorado and Utah. In addition we have conducted extensive analyses of flow and transport measurements from streams in Idaho to develop new relations for bed load transport in steep streams with high relative roughness. Slopes at our study sites range from 0.0003 to 0.07; bankfull depths range from 0.2 to 5 m; and bankfull widths range from 2 to 200 m. Our results indicate that as channels get steeper and smaller toward their headwaters, the bed becomes increasingly well-armored, and the material supplied to the channel as bed load tends to be much finer than the sediment that forms the armor layer. These finer sizes would be entrained and transported quite easily were it not for the rate-limiting effects introduced by the armor layer; coarse clasts within the armor layer hinder the movement of finer grains, thus limiting their mobility in relation to the available shear stress. The transport relations we have developed indicate that the shear stress available to move sediment in gravel channels increases in proportion to the threshold shear stress, thus, even in the steepest channels, transport occurs a few days per year, resulting in appreciable sediment transport over a period of years. We propose that in undisturbed gravel-bed streams and rivers there is a tradeoff in the frequency and intensity of bed load transport such that the total load carried in small, steep cobble-bedded channels tends to balance the total load carried in large, low-gradient channels. Our results have implications for water resources management, conceptual theories of ecosystem structure, and models of landscape evolution.