Quantifying and predicting the role of in-stream wood on general bed scour and the implications to s

Year: 2020
Presenter/s: Tim Abbe
Symposium Session: 2020 - 02 Considering sediment dynamics in river restoration design
Topics covered: estuary, fish passage, fish-salmon, floodplain, hydraulics, lessons learned, monitoring, riparian, sediment transport, and wood


ABSTRACT

Wood material can directly influence fluvial processes that control channel planform, substrate characteristics, alluvial topography and floodplain vegetation. The role of wood in trapping sediment and partitioning stress is well established. The removal of wood has also been recognized as a trigger for channel incision. But there is very little work regarding the influence of wood on channel bed stability, particularly with respect to the implications to salmonid redds and egg mortality. Field observations and professional judgement have led some to postulate that in-stream wood improves bed stability as it if would affect scour related to egg incubation survival. For instance, this has been coded into the widely used the Ecosystem Diagnostics and Treatment (EDT) model which increases the probability of egg to fry survival when wood is added to a channel. Yet, to date no one has developed even a basic framework to address the question of whether wood can decrease the general scour depth to which bed material is mobilized and thus impact salmonid eggs. By compiling empirical research into general bed scour and the hydraulic effects of wood, we constructed a framework describing how wood could reduce scour depth. We then proposed that stable wood integrated into the channel bed is effectively an immobile constituent of the substrate affecting the grain size distribution thereby altering dimensionless shear stress that defines the threshold for bed mobility. This provided a means to predict and compare scour depths in channels with different quantities of wood. We applied our reasoning to an incised alluvial channel segment that retains spawning gravels but sees very little salmon spawning relative to upstream and downstream river segments. Our predictions indicate that under existing conditions, general scour during a bed mobilizing event regularly exceeds typical egg burial depths of all Pacific salmon species. When we calculated the effect on bed mobility and general scour depth under a hypothetical scenario where the channel’s wood loading is at a level well within naturally occurring ranges in the PNW, the predictions show a substantial reduction in scour depths which implies an improvement in egg to fry survival. This work is also consistent with the argument that when wood increases bed substrate diversity, as well documented in the literature, it results in a greater distribution of local scour that creates patches of stable gravel that don’t occur in channels lacking wood. Our approach appears to be the first predictive tool which can be field-tested and refined to provide the restoration community with a quantitative tool for designing stable wood placements with the intent of improving spawning habitat. We are looking for additional empirical evidence of scour depths, both from previous studies and scour chains we’ve installed for water year 2020