BEDLOAD ENTRAINMENT IN A LONG-TERM FLOODPLAIN WETLAND RESTORATION EXPERIMENT, BIG SPRING RUN, PA
Based on our bio- and lithostratigraphic work we determined the site was an herbaceous wet meadow environment with waterlogged soil near the surface throughout the mid- to late-Holocene, prior to 1) valley-wide deposition upstream of a low head dam built in the 18th c, and 2) post-dam breach channel incision that occurred during the early 20th c. The dam trapped sediment that buried a pre-existing anastomosing channel-floodplain system (ACFS) and toe-of-slope colluvial deposits.
The pre-restoration phase of this study examined bedload entrainment and critical shear stresses during intermediate to high stage conditions at BSR. Unlike the pre-settlement wet meadow environment, the modern incised BSR channel is capable of frequent bedload transport of gravel-sized clasts. A USGS gage station at the downstream end of the restoration reach documented stream discharge of 0.1 (base flow) to ~6 cms from September 2009 to July 2011. In the restoration reach we recorded bedload transport of painted tracer gravel (b-axis up to 6.6 cm) at basal shear stresses of 54–110 Pa for water depths of 0.8–1.6 m during moderate to large flow events. Post-restoration water depths and water surface slope will be substantially decreased, with basal shear stresses predicted to be <1.5 Pa for water depths up to 0.2 m. The restoration design includes removing 21,000 m3 of historic sediment, which will significantly lower water depth and establish small stream channels similar to the pre-settlement wet meadow environment. Future post-restoration monitoring of gravel mobility will test our predictions of decreased shear stress due to the restoration process.