MEASURING SEDIMENT ACCUMULATION ON A RECONSTRUCTED FLOODPLAIN, BRANDYWINE CREEK WATERSHED, PA

Brandywine Creek is a key drinking water resource for southeastern PA and northern Delaware. The Brandywine Valley Association’s “Red Streams Blue” initiative seeks to improve water quality in impaired streams (“Red Streams”) to meet state standards (“Blue Streams”). Plum Run, a tributary originating near West Chester, PA, was one of the first targeted for remediation. At Strode’s Mill, Plum Run’s channel was disconnected from its former floodplain by re-deposition of legacy sediment originating behind an upstream mill dam. Erosion from heavy flows threatened the historic mill and art gallery. Remediation in 2011 used rip-rap to provide immediate protection for the mill, but the key element was removing approximately 1,000 cubic yards of soil and lowering the stream bank by 3 feet over a 500 foot reach to create an easily floodable surface on which trees were planted to initiate a riparian buffer.

By 2012, it was clear that sediment was being deposited on the re-established floodplain. The monitoring reported here is the first step toward determining the net rate of sediment accumulation over time. In early 2013, stations were surveyed to allow repeat measurements of the floodplain elevation with mm-scale reproducibility on a 1-m grid using a Sokkia total station. We report differences in elevation from two surveys completed five months apart. These initial measurements indicate that sediment accumulated near the stream channel at an average rate of 7 mm/month, forming a small levee. Average deposition over a 150 square meter grid was about 3.4 mm/month. Farther from the channel, deposition occurred in geographically coherent regions of lower elevation. Field observations suggest that ecological factors (deer trails and areas of marsh grasses) and human factors (deer fencing) had a significant influence on deposition rates and patterns. We will present results from coring to show the stratigraphic development of the new floodplain. Our results show that monitoring deposition can provide important insight into how reconstructed floodplains can influence sediment transport and siltation.