Author(s): R. F. Leclerc; B. R. Hall; I. Haltas; B. W. Cruey

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Abstract: An approach for conducting a screening-level hydromodification analysis in small, ungaged watersheds is proposed using a case study on Alder Creek, California. The28.5 km2watershed consists largely of rangeland and will be developed into residential and commercial land uses over the next several years. Topographic stream surveys, hydrologic and hydraulic information were developed at eight index points in the stream network and used to compute a dimensionless shear stress index for existing and proposed hydrologic conditions. The index is based on average shear stress computations for a full range of flows and identifies the excess shear that exceeds the critical shear stress for the D50 bed material size. The approach uses a shear stress index ratio developed from existing and proposed condition indexes and provides a method to quantify the potential for increased erosion and channel instability following changes in hydrologic regime. Results show the shear stress index will increase by 1% to 61% from existing to proposed conditions at the measured index points. Comparison with regional empirical studies indicates a likelihood of future channel instability in the stream network of between 10% and50%. Channel conditions observed in an Alder Creek subwatershed which has experienced development in the last decade show little to no channel instability, confirming the resilience of the watershed to resist morphologic change.

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Year: 2009