The influence of vegetation and organic debris on flood-plain sediment dynamics: case study of a low-order stream in the New Forest, England

The presence of large woody debris (LWD) has important implications for the physical and ecological behaviour of rivers, and these aspects have been researched extensively in recent years. However, this research has so far focused primarily on interactions between LWD and in-channel processes, and the role of LWD in flood-plain genesis is still poorly understood. Established conceptual models of flood-plain evolution are, therefore, lacking because they neglect the complex interaction between water, sediment, and vegetation in systems with accumulations of LWD. This study examines the effect of LWD on patterns of sediment deposition within a small area of forest flood plain along the Highland Water, S. England. In-channel debris dams locally increase the frequency and extent of overbank flows, and the impact of such dam on flood-plain sedimentation was observed. Nine separate flood events were monitored through the exceptionally wet winter of 2000–2001. During each of these, water and sediment fluxes were quantified and correlated with general rates of overbank sedimentation. Flood-plain topography, vegetation, and LWD were surveyed and related to micro- and mesoscale patterns of sediment accretion. The amount of overbank sediment deposition was correlated most closely with flood hydrology and sediment input. The amounts (0–28 kg m−2) and patterns of sediment deposition were both greater and more variable than have been observed on nonforest flood plains. The highly variable pattern of accretion can be explained by the combined effects of topography and organic material present on the surface of the flood plain.