Predictions of bedload transport in vegetated channels: uncertainties and steps forward
Vegetation in river channels and on floodplains alters mean flow conditions, turbulence, sediment transport rates and local sedimentation patterns. Although many advances have been made to predict the impact of vegetation on flow conditions, relatively few studies have investigated how vegetation influences bedload fluxes. We first investigate how known vegetation impacts on flow turbulence can be used to better predict bedload transport and sedimentation within vegetation patches. To elucidate these mechanics we measured 2D velocity fields using PIV and bedload fluxes using high-speed video in simplified flume experiments. We used these laboratory measurements to test and develop bedload transport equations for vegetated conditions. Bedload transport equations did not accurately predict sediment fluxes unless they accounted for the spatial variability in the near-bed Reynolds stress. We then use this patch scale understanding to better predict how vegetation impacts channel morphology. Specifically, we investigate how vegetation influences point bar growth and shape through coupled laboratory experiments and 2D numerical modeling. We measured bedload fluxes, flow conditions and sedimentation rates on a point bar planted with natural vegetation at the Saint Anthony Falls Outdoor Stream Lab. We then calculated the detailed 2D flow field over the point bar throughout imposed flow hydrographs. Our results demonstrate that vegetation caused significant changes in the bar dimensions and depending on the flow level, led to the development of a side channel between the bar and the inner bank of the meander. Such a side channel could precipitate a change in channel morphology to a multi-thread channel. Accurate predictions of sedimentation caused by vegetation patches not only require an estimate of the spatial variation in shear stress (or velocity) within a patch but also how the vegetation alters the adjacent flow field and bedload sediment supply to the patch.