MONITORING SEDIMENT STORAGE ON A RESTORED FLOODPLAIN: CLIMATE VARIABILITY AS A CONSIDERATION IN ASSESSING LONG-TERM EFFECTIVENESS

Plum Run, a tributary to Brandywine Creek, has impaired water quality resulting from urban runoff and erosion of legacy sediment from deeply incised channels. Recently, strongly eroding stream banks at Strode’s Mill, near West Chester, PA, threatened the historic mill building and led to a project designed to reconnect the stream to its floodplain by removing about 1000 cubic yards of sediment along 500 feet of stream opposite the mill. Immediately, flows able to enter the floodplain began to raise its elevation by depositing sediment. Floodplain deposition is consequential from two perspectives: 1) it tends to limit the effectiveness of the planned outcome by inhibiting flows from accessing the floodplain; but, 2) it also removes sediment that causes impaired water quality from Plum Run.

Previous work (Gerlach & Lutz, NEGSA, 2014) used total station measurements made over one year to model spatial and temporal changes in deposition and to estimate deposition rates. This study continued total station measurements, aiming to quantify the effects of remediation, to determine the mechanisms of change, and to make projections of future stream-floodplain connectivity and functionality. A smart phone app, ArcCollector, was used to construct a GIS map composition of the floodplain, including the watershed, streams, and points of interest in this and related studies. We also use aerial photos dating from the 1930’s to present to establish the historical framework of succession from agricultural development to urbanization.

Our results show that sediment loading over the last year has been much less than in the previous year. We hypothesize that the loss of effectiveness may be a result of decreased precipitation rates this year compared to the previous year. An analysis of local precipitation data supports this idea: during the last year overall precipitation has been lower and there have been fewer intense precipitation events. Our research highlights climate variability (e.g., decadal, centennial) as a factor in assessing long-term outcomes of floodplain and other remediation. We explore long-term precipitation data to place the two years of deposition data from Strode’s Mill into context and to qualitatively predict how the floodplain and stream environments will interact in the future.