Method of Evaluation for Stream Bed Shear Stress and Sediment Transport Capacity in Urbanizing Watershed: Implications for Stream Restoration

In the field of stream restoration, use of a one-dimensional flow model with typical Manning’s n values for an open channel greatly over-predicts bed shear values. This, in turn, incorrectly predicts the size of the mobile fraction on the bed and if used in a bedload transport function over-predicts mass movement of the bed material. This study identified 12 sites for which watershed and reach characteristics were compiled, and bedload sampling was performed. This information was used to produce an empirical relationship between reach pebble count data and an effective Manning’s n value that can be used to produce accurate bed-shear values in a one-dimensional flow model. With this tool, simple field activities can provide sufficient information to allow a stream restoration practitioner to accurately predict bed shear values. Relationships between watershed characteristics and reach scale bed characteristics, and bed depositional patchiness and sediment supply were also explored. It was found that Wolman Pebble Count data can be used to predict an effective Manning’s n value with sufficient accuracy, while watershed characteristics were not adequate to predict reach scale bed characteristics and bed depositional patchiness was valuable as a threshold indicator but not as a predictive variable.