BEDLOAD ENTRAINMENT IN A LONG-TERM FLOODPLAIN WETLAND RESTORATION EXPERIMENT, BIG SPRING RUN, PA

Big Spring Run (BSR), PA, a low-relief 2^nd-order Piedmont stream (drainage area 15 km²) in the Chesapeake Bay watershed, is the focus of a state and federally funded investigation to test a new approach to stream, floodplain, and wetland restoration. BSR is a single-thread meandering channel incised into ~1.1 m of fine-grained (silt, clay, and fine sand) historic sediment. Active migration of bars that consist dominantly of sand and gravel (D₅₀ ~3 to 13 mm) occurs during moderate to high flow events. The primary source of gravel is quartz-rich Pleistocene (periglacial) colluvial deposits exposed by channel incision and bank erosion along valley margins.

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 18^th 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.