Collaborative Research: Quantifying Sediment Storage Transit Times For Geomorphically Explicit, Watershed Scale Suspended Sediment Routing

Nontechnical Explanation of the Project's Broader Significance

Suspended sediment and associated contaminants and nutrients are leading causes of environmental degradation in rivers, but predicting the movement of fine-grained material through watersheds remains difficult. Most watershed management planning treats fine-grained suspended sediment as 'washload' that is rapidly transported to receiving waters once particles enter stream channels. However, sediment particles actually move downstream by a series of discrete jumps, with each jump punctuated by a period of resting in floodplains and other alluvial deposits. According for this view, particle travel times from upland sources to estuarine sinks may be very long (up to 1000 years for the Chesapeake Bay watershed, for example). The disparity between these two views is most critical where Best Management Practices are used to reduce the contribution of sediment and other contaminants from upland watershed sources to downstream receiving waters, such as the Chesapeake Bay. Sediment travel times are not currently incorporated into existing watershed modeling schemes used to evaluate and design Best Management Practices, so the time required for them to achieve their maximum effectiveness is unknown. During this project, particle waiting times will be determined for a sub-basin of the Chesapeake Bay watershed, and these results will be incorporated into a new modeling framework for predicting the movement of particles through watersheds that explicitly incorporates measured particle waiting times. These results will help evaluate restoration schemes designed to reduce loading of sediment and other particles from watersheds to ecologically damaged receiving waters.

Technical Description of the Project

Geomorphologists widely acknowledge that suspended sediment routing through large (100-1000 km spatial scales) watersheds must account for the 100-10,000 year timescales associated with alluvial storage, but this knowledge is rarely reflected in watershed models. A few recent modeling schemes include both sediment transport and storage, but the necessary reach averaging introduces a new variable, the sediment-storage (waiting) time distribution that is virtually undocumented. This study will use pollen analyis, optically-stimulated luminescence, radiocarbon dating, and fallout radionuclide concentrations to measure contemporary waiting time distributions for all significant alluvial storage reservoirs in a mid-Atlantic Piedmont watershed where rates of erosion, deposition, and transport are well-constrained. The measured waiting time distributions will be used with a suspended-sediment routing model in a series of scenarios designed to illustrate timescales required for suspended sediment to move from upland sources to basin outlet in the Chesapeake Bay watershed. The results will improve existing theory concerning sediment waiting time distributions and sediment budgets within fluvial systems. The results will also help interpret the complex histories of watershed disturbance amd provide a means for a more accurate prediction of future changes. This study is designed to test the following hypotheses: 1) sediment storage or waiting time distributions can be quantified using measured erosion rates, geomorphic mapping, radiometric dating, and reservoir theory; 2) sediment waiting times for mid-Atlantic upland watersheds range from months to millennia, with characteristic values on the order of 1000 years, implying very long timescales to transport most suspended particles from upland sources to basin outlets; 3) observed waiting time distributions are not exponential, suggesting that valley storage reservoirs are not well-mixed.