THE EFFECT OF SAND CONTENT ON SEDIMENT TRANSPORT, DEPOSITION, AND BAR MORPHOLOGY

Studies of stream adjustment to urbanization have focused primarily on the geomorphic consequences of increases in discharge, such as channel widening (e.g. Hammer, 1972, Pizutto et al., 2000). Urbanization can also affect sediment supply, grain size, or sediment transport mechanics, which can alter channel stability and morphology. Sediment deposition can initiate feedbacks among geomorphic and hydraulic variables leading to significant alterations of stream morphology. In summer 2012, simple alternate bars in the channel at Cherry Hill Road were replaced by large diagonal bars with very sharp bar fronts. These elevations of these bars were reduced during floods associated with Hurricane Sandy. These changes occurred over short timescales (< 6 months) and coincide with an increase in upstream sand supply associated with major road construction (of the Inter-County Connector) in the watershed. This research is designed to test the hypothesis that changes in bar morphology were caused by varying amounts of sand in the bedload, which reduces the critical dimensionless shear stress of the gravel, thus increasing sediment transport and deposition rates.

Field data show that dimensionless shear stress for Hurricane Sandy was well below the τ^*_crit for homogenous gravel, indicating that sand content is responsible for lowering the critical dimensionless shear stress, leading to the substantial sediment transport observed. Using time series of important flow parameters, a model for sediment transport in lower Little Paint Branch Creek was created. Suspended sediment profiles indicate that 500 µm and larger sand is not suspended at significant heights above the bed. Rouse number calculations indicate that sand is accumulated during small events and winnowed during large events. The model shows that a substantial number of small events occurred in the months prior to Hurricane Sandy, leading to the lowering of the critical dimensionless shear stress. This suggests that sand content, and the small flow events that lead to the accumulation of sand, have a significant impact on bar morphology and channel morphology for systems experiencing a sudden influx of sand sized sediment.