Impact of large wood jams on channel hydraulics and backwater rise

Abstract:

Installation of engineered logjams and encouragement of natural large wood jam formation through land management practices are commonly used for river restoration and natural flood management projects in order to reestablish the hydraulic and ecological benefits associated with logjam presence in riparian systems. Channel-spanning logjams create an upstream backwater of slower, deepened water, increasing flow heterogeneity, sediment deposition, and ecological productivity. The accumulation of wood pieces acts as a porous obstruction, so that flow progressing through the structure experiences frictional losses due to drag on wood elements. Prediction of logjam-induced backwater rise is necessary to compare logjam effects across river systems, improve design of river restoration projects and evaluate flood hazards in hydraulic models. We present recent results demonstrating that a logjam can be modeled as a porous obstruction generating momentum loss proportional to the number, size and packing density of the logs and the length of the logjam. Energy and momentum constraints are combined to predict backwater rise from unit discharge and a dimensionless structural parameter. This novel approach allows description of preexisting logjams with a common metric. The new model is used to demonstrate how logjam design and spacing impact backwater rise across varying catchment characteristics. Finally, field measurements of backwater rise due to engineered logjams are used to explore extension to partial jams.