Compact Models Assess the Impacts of Floodplain Storage on Suspended Sediment Delivery and Restoration Lag Times: A Chesapeake Bay Case Study

Abstract

A lag in time occurs between implementing a watershed restoration design and attaining measurable benefits in impaired waters downstream. Where restoration is intended to reduce the delivery of sediment, lag times can be greatly increased by floodplain deposition. Floodplain processes, however, are rarely included in watershed restoration assessments. Here we present equations that can be applied with limited effort to estimate lag times associated with floodplain deposition. The equations determine if floodplain storage is likely to influence the timing of sediment delivery, and they also provide estimates of sediment travel time distributions when floodplain deposition is important. We use a step reduction in upstream sediment supply to represent restoration from Best Management Practices (BMPs), and use this initial condition to derive an analytical solution for the time needed to deliver restoration benefits downstream. Parameters required by these equations can be extracted from sediment budgets and floodplain deposit age data. Illustrative computations for the Chesapeake Bay watershed predict that only 15% of the sediment load can be transported 200 km without being stored on a floodplain. Once deposited, particles remain in place for ∼300 years before remobilization. For distances of 20–75 km, average travel times range from ∼500 to ∼750 years. These results suggest that BMPs employed in the headwaters of the Chesapeake Bay will not reduce sediment delivery within the decadal timescales assumed by the Chesapeake Bay Program. BMPs should be located close to the Bay to achieve maximum efficacy within reasonable time frames.