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Sediment Routing in Channels Draining Disturbed Lands

  • Author(s): Dietrich, William E
  • et al.
Abstract

The poorly understood relationship between specific land uses, consequent altered sediment loads and runoff, and the downstream river channel response to these altered patterns lies at the heart of the problem of management decisions regarding cumulative watershed effects. Most rivers in California have experienced altered sediment supply due to land use. The pristine state of these channels is unknown, and because channels do not respond Immediately to inputs of sediment, it can not be assumed that current channel conditions reflect current sediment loads. This makes stream rehabilitation projects, which are becoming increasingly commonplace. subject to considerable uncertainty as to their outcome. There are many aspects to this relationship between land use, sediment loads, runoff. and channel response that are challenging, including: 1) the prediction of the rate and grain size of altered sediment yield caused by a particular disturbance; 2) the prediction of the changes in runoff amounts, including how these effects influence hillslope erosion rates; and 3) the prediction of stream channel changes that include effects on surface grain size distributions, sediment storage, channel geometry. bank instability, sediment loads, and sediment concentrations. The pressure now to make decisions in advance of complete understanding of this relationship suggests that the problem be turned around: is it possible to examine a channel reach of interest and with some relatively simple observations anticipate how it will respond to altered sediment yields?

The work accomplished here was oriented toward devising methodologies to predict probable channel response to altered runoff and sediment supply due to land use by "reading" rivers. Our approach was based on our recent experimental results that demonstrated a quantitative relationship between sediment supply and the degree and spatial extent of surface armoring in gravel bedded rivers (Dietrich, et al., 1989). The theory we proposed (see Appendix A for complete paper) argues that under an Imposed boundary shear stress, such as that which occurs at bankfull discharge, grain size adjustments modify critical boundary shear stress and, consequently. the excess stress that controls bedload transport rates. When sediment supply is high for the imposed shear stress. the grain size of the bed will be close to that of the load. In contrast, if the supply is reduced, the surface will coarsen, thereby causing reduction in transport rate in concert with the reduced load. Hence the degree of stream surface armoring may be used to define a stream's state of adjustment relative to sediment supply.

Three specific research objectives were proposed in Project W-734. The first was to test in a natural river the validity of recent theories for initial motion of heterogeneous mixtures of gravels and sand size sediment. This objective was accomplished through a combination of tracer studies and direct measurement of friction angles of individual particles. The second objective was to test our theory regarding grain size adjustments (Dietrich, et al., 1989) by measuring vertical and lateral sorting variation in relation to sediment supply. (The theory assumes that the median grain size of the bed is the scale for critical shear stress of the bed and that selective entrainment can occur, causing grain size adjustments to develop from supply variations.) This work was done in conjunction with another project which examined several rivers. The work funded here focussed primarily on one study site and examined it in greater detail. The third objective was to examine whether alternating zones of high and low rates of sediment movement along a particular creek could be identified. This was done through a combination of mapping and tracer studies.

As proposed, this work was accomplished in close collaboration with Laurel Collins, geologist for the East Bay Regional Park District.

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