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Little Topashaw Creek - Effects of Wood Structures - High Flow Stages
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High Flow Stages

The effect of adding large woody debris structures on channel conveyance was evaluated using a one-dimensional numerical model of the channel (HEC-RAS) and surveys of some 39 cross sections along the 2 km study reach. Since the watershed is ungaged, a regional regression equation was used to compute the peak discharge for the 2, 5, and 10-year events, and these were used as input to the HEC-RAS model. Two models were constructed: one using the channel geometry that was present immediately prior to construction, and another one using the same geometry, but with elevated roughness values (n = 0.15) for panels covered by large woody debris structures. Manning n values for regions of the model not under debris were set a 0.28 for straight reaches and 0.036 for sinuous reaches. Twenty-eight of the 39 cross sections were modified to allow for debris structures. The figure here shows that the debris structures resulted in stages about 1 m higher for a flow equal to 0.4 times the estimated two-year discharge, and that effects were greatest in the upstream reaches, where the channel is narrower. Effects on cross-section mean velocity mirrored the effects on stage, with greatest reduction in velocity in the upstream reaches. The selected n-values were used to predict water surface elevations for cross section 27 for an observed flow of 10.9 m3/s on November 24, 2000, and the predicted water surface elevation was 0.3 m lower than the observed elevation.