Abstracts with Programs
1994 Annual Meeting
Geological Society of America
Seattle, Washington - October 24-27, 1994
APPLICATION OF THE UNIVERSAL SOIL LOSS EQUATION
IN ESTIMATING RELATIVE SEDIMENT YIELDS
ASSOCIATED WITH URBANIZATION
By
Janet H. Curran, Michael D. Watkins and Donald W. Tubbs
Relative erosion and sedimentation impacts of various land uses were
evaluated for several forested upland areas in King County,
Washington by incorporating predicted relative changes to physical
conditions into a consistently applied analytical procedure that
includes the Universal Soil Loss Equation (USLE) for sheet wash and
rill erosion. Geomorphic data were collected during field
reconnaissance of several partial drainage basins to characterize
physical conditions and geomorphic processes within the study areas.
The results of our field study and observations of similar sites
under construction indicate that sheetwash and rill erosion rates are
among the geomorphic process rates most impacted by construction.
USLE was used in conjunction with field observations and project
specifications to estimate to estimate sheetwash and rill erosion, as
one element of sediment budgets for existing (baseline) conditions,
and for construction and post-construction conditions under several
development alternatives.
The USLE length factor represents the length of uninterrupted surface water flow developed on the slope. This factor is sometimes taken as slope length of an entire site, resulting in exaggerated sediment yields. A more appropriate method consists of setting the length factor equal to the estimated length of uninterrupted flow. Slope lengths of 5 to 200 feet were observed within the study areas under existing conditions, between interruptions consisting of topographic benches and vegetative debris. For the construction phase, slope length was set equal to the spacing (100 to 300 feet) between erosion control measures. Estimated slope lengths for the fully developed condition (50 to 125 feet) were based on proposed lot sizes and spacing of interceptors such as curbs and ditches.
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