The geomorphological dynamics of a restored forested floodplain
PublisherUniversity of Southampton
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This thesis investigates geomorphological processes within the forested floodplain of the Highland Water, a small, lowland river in the New Forest, southern England. Geomorphological processes were monitored (a) before restoration, in order to define reference conditions, and (b) after restoration, in order to monitor the performance of the restoration against the reference conditions. The results demonstrate that the restoration was successful at moving the restored system towards target reference conditions by re-connecting the channel and floodplain, and consequently floodplain geomorphological dynamics were increased after restoration. However, the restored floodplain was considerably more connected and more dynamic than an upstream semi-natural reference reach, indicating that the restored channel was perhaps undersized. Floodplain channels were an important geomorphological feature observed on semi-natural floodplains, particularly in association with hydraulically effective wood jams. Experiments into sedimentation and erosion showed that overbank flow scoured the surface and distributed sediment, and rates of erosion and deposition were higher within floodplain channels than elsewhere on the floodplain surface. These channels were therefore a major control over the spatial distribution of energy and materials on the floodplain at the patch, feature and reach scale (10-1 to 102 m). The formation of in-channel wood jams, which force flow overbank, relies on the accumulation of wood. Experiments to investigate transport of small wood recorded travel distances ranging from 0 to over 1000 m. Shorter travel distances were associated with higher in-channel geomorphological diversity, particularly the presence of in-channel wood jams. This thesis therefore provides a greater understanding of the geomorphological processes operating on a forested floodplain in conjunction with monitoring the performance of a river restoration project that incorporated a forested floodplain.