Effect of woody debris entrapment on flow resistance

Recent environmental concerns in floodplain management have stimulated research of the effect vegetation and debris have on now conveyance, and their function in a productive riparian ecosystem. Although the effect of stable, in-channel woody debris formations on flow resistance has been noted by several authors, studies concerning entrapment of detrital debris in vegetation are lacking. Logs, limbs, branches, leaves and other debris transported during flooding often become lodged against bridges, hydraulic structures, trees and vegetation, and other obstacles, particularly in and near the overbank areas. Hydraulic measurements obtained in a channel prior to and following the removal of woody debris indicated that the average Manning's n value was 39 percent greater when woody debris was present. An examination of the drag-velocity relation for vegetation indicated that an increase in the frontal area of debris and/or vegetation results in a nearly proportional increase in Manning's n. The influence of debris on flow resistance decreased as flow depth increased.