Representation of Packed Log Jams in a Two-Dimensional Hydraulic Model

Packed log jams (PWJs) can form naturally in streams and engineered log jams have been strategically placed in streams in river restoration projects. Their presence impacts stream hydraulics, flood inundation, morphology and ecology. Proper representation of large woods in two-dimensional hydraulic models is important, but proper guidelines are needed for any models, considering that such models have been widely used for assisting river restoration design and fish habitat evaluation. Existing large wood representation methods are inadequate. In this study, the porous-media method, widely used in groundwater modeling, is adapted and extended to represent large wood in streams. A modified formulation is proposed, which adopts only one calibration parameter to compute the drag force due to large wood presence. Two sets of experimental data with PWJs are used to assess the performance of the method. The porous-media method is found to produce good results when compared with the measured data of backwater rise as well as water depth and velocity variations along the flow. A general usage guideline is proposed on the proper way to apply the method and verified against the PWJ experimental data. Further, a regression equation is developed to estimate the large wood calibration parameter; it can be useful when no measured data are available for calibration. The proposed method, the developed guidelines, and the regression equation are found to produce satisfactory results in comparison with the measured PWJ data.