TOWARDS UNDERSTANDING AND PREDICTING SUSPENDED SEDIMENT LAG TIMES IN THE CHESAPEAKE BAY WATERSHED

A “lag time” occurs between initiating a best management practice in a watershed and achieving of its intended benefits in estuarine receiving waters. Lag times for sediment have been a concern for Chesapeake Bay restoration for decades, but little progress has been made towards quantifying them. Lag times occur because sediment is deposited between transport events before reaching long-term storage locations. Once deposited, particles may remain stored for months to millennia before being remobilized and carried farther downstream. The time spent between transport events is termed the sediment “waiting time”, and can be represented by a stochastic probability distribution. Sediment budgets in the Chesapeake Bay watershed indicate that sediment may be stored either within channels or on floodplains, with published estimates ranging from 1.6% to 44% of the annual suspended sediment load stored per kilometer of downstream distance (with many areas of 0 storage). Along the South River downstream of Waynesboro, VA, about 2% of the annual load is stored and exchanged (returned to transport) per km; half of this material is deposited within the channel (mean waiting time - 2 years, 90^th percentile - 50 years), while the other half is deposited on the floodplain (mean waiting time - 3,000 years, 90^th percentile - 15,000 years). Our research group has developed new models relating the fraction of the suspended load stored and exchanged per km to particle travel times (the time required for a particle to move from an upstream source to a basin outlet). As particles are carried sequentially farther downstream, the chances of spending time in storage increase geometrically. Because storage timescales greatly exceed those associated with transport by storm events, travel times for larger watersheds are strongly controlled by sediment storage timescales, while the dynamics of transport have little influence. For example, the time required for an average particle to travel 200 km along the South River approaches 6000 years (though the full distribution of travel times ranges from 1-10,000 years). Our models can provide basin-wide predictions of Chesapeake Bay sediment lag times, but assembling the required sediment budget data will be an enormous undertaking.