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USGS Science Summary — Predicted Sediment Scour from Conowingo Reservoir on the Susquehanna River as a Result of the September 2011 Flooding

Prepared by the U.S. Geological Survey (USGS) on September 13, 2011

The purpose of this summary is to present the predicted amount of sediment that was scoured from the Conowingo Reservoir on the lower Susquehanna River for the period September 7-11, 2011. The estimates are compared to sediment scour estimates during Tropical Storm Agnes, June 22-27, 1972. Based on USGS estimates, about five times more sediment was scoured from the Conowingo Reservoir during Agnes than during the September 2011 flood event. Overall, more than twice as much river flow entered the Bay from the Susquehanna River during Agnes than during the flooding from Tropical Storm Lee.

River flow during the flood of September 2011 exceeded the predicted threshold for sediment scouring, which is 400,000 cubic feet per second (cfs) for the Conowingo Reservoir. The river flow exceeded the threshold for about 3 days (figure 1), with a peak flow of 778,000 cfs. The USGS developed a sediment-scour prediction equation for the Conowingo Reservoir and two other reservoirs on the lower Susquehanna River (Langland and Hainly, 1997). The equation is updated routinely by appending the maximum discharge and sediment load after each flood event and mostly reflects sediment scour from the Conowingo Reservoir.

Chart showing mean daily streamflow for the Susquehanna River at Conowingo.

Figure 1. Mean daily streamflow for the Susquehanna River at Conowingo, Maryland, resulting from Tropical Storms Agnes and Lee.

The amount of sediment scoured from the lower Susquehanna River reservoirs in September 2011 is estimated to be about 4 million tons. The range of sediment scour (based on 95th percentile confidence intervals of the equations) is between 2.5 to 7 million tons.

In contrast, the historic flood of June 1972 exceeded the sediment scour threshold for about 5 days (figure 1), with a peak river flow of 1,130,000 cfs, delivering more than twice as much water to the Bay from the Susquehanna River during Agnes than during Tropical Storm Lee (figure 1). The estimated amount of sediment scoured from the lower Susquehanna River reservoirs to the Chesapeake Bay during this period was 20 million tons (with a range between 14 million to 27 million tons based on the 95th percentile confidence interval).

Previous USGS studies have estimated that the Conowingo Reservoir will reach its sediment storage capacity in 20 to 25 years (Langland, 2009). Based on the estimated 4 million tons of scour, approximately 2 years of additional sediment-storage capacity may have been added to Conowingo Reservoir from the September 2011 flood.

In addition to the amount of sediment scoured from the reservoirs, a large amount of sediment runoff was also carried into the Chesapeake Bay. The USGS is continuing to monitor streamflow and sediment concentrations in the Susquehanna River to determine the total sediment load (scour plus riverine) transported to Chesapeake Bay. The USGS collected additional samples for nutrients, pesticides, and metals to document other pollutants that were delivered into the Bay.

References:

Langland, M.J., and Hainly, R.A., 1997, Changes in bottom-surface elevations in three reservoirs in the lower Susquehanna River, Pennsylvania and Maryland, following the January 1996 flood – Implications for nutrient and sediment loads to Chesapeake Bay: U.S. Geological Survey Water-Resources Investigations 97-4138, 34 p., 3 pls.

Langland, M.J., 2009, Bathymetry and sediment-storage capacity change in three reservoirs on the lower Susquehanna River: U.S. Geological Survey Scientific Investigations Report 2009-5110, 21 p.

Contacts:

Mike Langland, langland@usgs.gov

Joel Blomquist, jdblomqu@usgs.gov




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