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EMAP SEDIMENTATION INDEX: LAND USE AND NATURAL HYDRAULIC CONTROLS ON STREAM SEDIMENTATION
Citation:
Kaufmann, P R., M. Passmore, AND J. Green. EMAP SEDIMENTATION INDEX: LAND USE AND NATURAL HYDRAULIC CONTROLS ON STREAM SEDIMENTATION. Presented at Biological Advisory Committee annual meeting, Athens, GA, May 8-11, 2001.
Description:
Excessive erosion, transport and deposition of sediment in streams and rivers is a major problem in surface waters throughout the United States. It is important to have a reliable measure of stream sedimentation that properly accounts for natural controls on the amount of fine particles in stream beds. Many human activities directly or indirectly alter the size and composition of stream substrates. However, the size distribution of channel substrates also differs naturally in streams of different sizes, slopes, and surficial geology. Based on sediment transport theory, we used EMAP physical habitat measurements including bankfull channel dimensions, slope, channel complexity, and large woody debris to estimate the mean diameter of particles that particular sample streams are capable of transporting during bankfull flows (Kaufmann et al. 1999). We then calculated RBS, an index of Relative Bed Stability (inverse measure of sedimentation) as the ratio of the observed substrate geometric mean diameter divided by the critical diameter at bankfull flow (the mean diameter of particles transported during bankfull flow), modifying this measure to include losses in shear stress due to large woody debris and channel morphology. A high positive value of RBS indicates an extremely stable, immovable stream substrate like that in an armored canal, a "tailwater" reach below a dam, or other situations where the sediment supply is low, relative to the hydraulic competence of the stream to transport bedload sediments downstream. We hypothesized that rates of sediment supply in watersheds not altered by human disturbances (i.e. under "natural" disturbance regime) may be roughly in equilibrium with transport of sediment by the streams draining these watersheds. Under these conditions, RBS should be close to 1. On the other hand, augmented sediment supplies relative to a stream's bedload transport competence is likely to result in "textural fining." Textural fining of the stream bed (RBS<1) typically occurs when sediment supply is augmented due to land use activities that increase hillslope erosion. Very small RBS values (e.g. .01-.001) describe a channel composed of substrates that are frequently moved by even small floods, indicating excessive amounts of fine particles compared with expected values in most relatively undisturbed watersheds.