Floodplain Flow Resistance Greatly Enhanced by Woody Debris Jams created in Riparian Vegetation

Riparian vegetation is a significant component of floodplain flow resistance. We examined channel and floodplain flow resistance for flow events ranging from baseflow to significant floods associated with Hurricane Floyd. The study site is a steep boulder-bed stream with a narrow valley. For flow events up to bankful stage, flow resistance is created by boulders on the channel bed. At flow stages above bankfull, riparian vegetation is the major part of floodplain resistance. Floodplain flow velocities initially increase then drop dramatically during floods. During Hurricane Floyd, small woody debris carried into the floodplain forests developed a series of debris jams, <1 m high, along the edge of the stream. These debris jams created a series of steps in the energy gradient of the floodplain. The velocity of the water on the floodplain was almost zero within the steps and these steps became deposition sites for suspended sand. Sand accumulations on the floodplain was up to 2m in depth for this small channel. During the peak of the flood, flow velocities in the main channel were at or above critical (Fr >1), while adjacent floodplain velocities were almost zero. This interaction between floodplain vegetation and flow hydraulics forces the flood sediment transport zone into a narrow zone in the channel center. This influences both bed material and the long-term development of the valley.