Channel evolution of a restored low gradient, sand bed stream .

摘要

Stream restoration design and construction relies on an accurate prediction of stable dimension, pattern, and profile. This study charts the morphologic evolution of a small, restored headwater stream in response to hydrologic, hydraulic, and geologic conditions in a low gradient, coastal plain system. The goal of the project was to define an expected dynamic equilibrium for constructed streams in the geomorphic regime and equate changes in stream form to measured fluvial and geotechnical parameters.;The restored stream reach was near Cove City, NC in the Lower Coastal Plain. The constructed stream was a low gradient, sand bed channel with an equilibrium bed slope of 0.001 m/m, cross sectional area of 1.9 m2 , top width of 4.8 m, maximum depth of 0.7 m, and W/D of 12.;Significant deviations in the resulting stream features were related to structures. Cross sections with root wads had a 12% steeper median side slope than non-structured stream sections. The channel downstream of log sills was 20% deeper than the median cross sectional depth.;Multivariate statistical analysis was used to relate changes in stream dimension to fluvial and geotechnical parameters. Inundation was directly related to increases in stream bank side slopes. Shallow groundwater gradient measured within the stream banks was a significant explanatory variable of direct changes in channel width.;A soil lysimeter experiment was conducted using layered soil profiles to test bank side slope as a function of stream bank stability. Failure of the tested stream banks was due to small, pop-out failures and liquefaction of the underlying sandy soil. Bank failures occurred along linear failure planes that were similar to the initial bank slope.;The maximum depth of scour pools that developed downstream of the log sills at the restored stream was modeled using empirical relationships based on the morphologic jump, headloss over the sill, radius of curvature, bed particle size, and flow turbulence. The predicted and measured scour depths were significantly correlated in a multivariate model that included the morphologic jump, turbulence, and bed material size. The scour pool depth was dependent on downstream conditions and tailwater depth.