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 ef fect of stable, in-channel woody debris formations on flow resistance has been noted by several authors, studies concerning entrapment of de trital debris in vegetation are lacking. Logs, limbs, branches, leaves and other debris transported during flooding often become lodged agai nst bridges, hydraulic structures, trees and vegetation, and other obs tacles, particularly in and near the overbank areas. Hydraulic measure ments obtained in a channel prior to and following the removal of wood y debris indicated that the average Manning's n value was 39 percent g reater when woody debris was present. An examination of the drag-veloc ity relation for vegetation indicated that an increase in the frontal area of debris and/or vegetation results in a nearly proportional incr ease in Manning's n. The influence of debris on flow resistance decrea sed as flow depth increased.