Relative influences of valley geometry and wood on sediment distribution in Biscuit Brook, NY

Abstract

Sediment storage in mountain streams is commonly associated with level of valley confinement. In unconfined channels, sediment is stored in the floodplain and active channel in a network of secondary channels with abundant in-stream large wood. The influence of wood on sediment distribution is potentially significant in alluvial reaches with low valley confinement and continuous wood recruitment. Biscuit Brook, a headwater stream in the Catskill Mountains of New York State, is discontinuously laterally confined: flow paths through wide, hydraulically rough reaches alternate downstream with flow paths through incised bedrock. Alluvial reaches in Biscuit Brook contain substantial large wood and are actively adjusting to the large sediment pulse caused by Hurricane Irene in 2011. This study 1) quantifies relative influences of valley geometry and wood on sediment distribution and storage, and 2) examines whether either wood, valley geometry, or their combination is the primary driver of sediment distribution and storage. I surveyed 57 cross sections, measured bed grain sizes at 19, 100-m longitudinal segments, and mapped 50 wood accumulations during the summer of 2018 to document the relationships between wood, valley geometry, and sediment storage in Biscuit Brook. To address the research objectives, I examine statistical correlations between variables using single and multiple regressions. Because direct measurement of sediment storage in Biscuit Brook was not possible in 2018 given the time and scope of this study, I use median grain size, gradation coefficient, and modeled velocity as proxies for sediment storage. I assess storage via modeled velocity of existing conditions in Biscuit Brook through cross-sectional geometry associated with the presence of wood. I then compare to a model of a no-wood scenario by virtually removing wood in the model. I also compare modeled velocity through differing valley geometries. Results indicate that both wood and valley geometry are significant for sediment distribution and storage, but valley geometry exerts a stronger control. I recommend that valley geometry be considered for more strategic wood placement in stream restoration project design.