The Influence of Woody Debris Jam Composition on the Local Hydraulics

Numerous studies exist on the hydraulics of woody debris jams and the mechanisms driving their geomorphic influence. While most hydraulic studies treat jams as single, solid objects, jams are clearly not individual cylindrical logs, but rather an accumulation of pieces ranging in size from leaves and twigs to entire trunks. Here we treat debris jams as complex and porous accumulations of heterogeneous material to understand the relative importance of the different size fractions comprising a jam. We systematically dismantled three debris jams in four stages, removing a total of 17,783 individual wood pieces, to experimentally manipulate jam porosity. During consistent bankfull events, surrounding velocity fields and shear stress distributions were measured around each jam for each stage of removal and treatment. We determined the drag force on the debris jam by measuring the momentum extracted through a control volume. Values of the drag force for each jam at each stage of removal captured the effects of size (by way frontal area) and composition of the jam (by way of total jam surface area). Wood piece size in debris jams dictates the surface area to volume relationship. This association in turn determines the rate at which drag force changes with the addition of material so that relatively small additions of material to key members will cause large increases in drag, but that drag on jams with a variety of piece sizes which have accumulated on a key member, is much less responsive to changes in material. Assumptions of non-porosity when modeling woody debris jams can result in a 10-20% over-estimation of drag. Only low porosity jams produce the geomorphic and hydraulic characteristics commonly associated with wood in rivers. Our results on natural debris jams also illustrate the importance of variable wood size fractions when using woody debris jams for river restoration.