Structure and hydraulics of natural woody debris jams

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 deflector debris jams in four stages, removing a total of 17,783 individual wood pieces, to experimentally manipulate jam porosity. We measured the surrounding velocity, shear stress, and drag force (F-D). The assumption of nonporosity can result in a 10-20% overestimation of F-D. Back-calculated values of the combined drag coefficient and frontal area term (C(D)A(F))(calc) represented the drag characteristics of natural debris jams, whereas separating frontal area (A(F(emp))) and drag coefficient (C-D) contributions in natural jams is misleading. Values of (C(D)A(F))(calc) for each jam at each stage of removal captured the effects of size and composition of the jam. Wood piece size in debris jams dictates the surface area to volume relationship. This association in turn determines the rate at which F-D and (C(D)A(F))(calc) change with the addition of material. Only low-porosity jams produce the geomorphic and hydraulic characteristics commonly associated with deflector jams. Our results on natural debris jams also illustrate the importance of employing variable wood size fractions when using woody debris jams for river restoration.