Storage and residence time of suspended sediment in gravel bars of Difficult Run, VA

Date
2017
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Water quality in the Chesapeake Bay has been negatively affected by increased turbidity and nutrient delivery associated with excessive suspended sediment supplied by tributary streams. Reducing sediment eroded from upland sources in the rapidly urbanizing Mid-Atlantic Piedmont province is the basis for best management practices (BMPs) that seek to reduce Chesapeake Bay sediment and nutrient loads. Models used to evaluate these proposed reductions in Chesapeake Bay sediment loads, however, do not account for the affect of sediment storage in tributary streams. The impact of storage on watershed signals, like sediment load reductions, has been addressed for some deposits (e.g. the floodplain), but little is known about the quantities or timescales associated with in-channel storage deposits in Piedmont streams. This study assesses the contribution of gravel bars to sediment storage in Difficult Run, an urbanized Piedmont stream in northern Virginia. Gravel bars (n=14) and other geomorphic features were mapped along two study reaches of Difficult Run. Sediment samples collected from two gravel bars at each study reach (four bars total) indicate 50% of sediment stored in gravel bars is suspendable. Suspended sediment in gravel bars has a residence time of 24 years, as computed from measurements of scour chains (n=50) installed in the four sampled bars. Suspended sediment stored in gravel bars at one study reach accounts for 3.2% of the annual suspended sediment load of that reach. While gravel bars do not store a significant amount of the annual suspended sediment load of Difficult Run, the suspended sediment in bars remains in storage for decades. These findings reinforce the theory that sediment in streams of the Chesapeake Bay watershed spends more time in storage than in transport, disrupting sediment signals, such as Chesapeake Bay sediment and nutrient load reductions, as they move downstream.
Description
Keywords
Earth sciences, Gravel bars, Piedmont, Residence time, Scour chains, Sediment storage, Suspended sediment
Citation