Status Nearly 30 scenarios completed for NAB and CBP over a year’s effort. Report on application of CBEMP in preparation. October time frame for draft. Targeted management scenarios in progress: Dredging, remove 3 mcy. Completed. Dredging, remove 3 mcy with sediment bypass. Mid-September. Dredging, remove 31 mcy, equivalent to 1996 bathymetry. Completed.
Scenario Procedure The CBEMP is run for 1991 – 2001. Today’s runs are based on Chesapeake Bay TMDL loadings. Loads from a major scour event in January 1996 are added to the WSM loads. Scour is computed by ADH applied to 2008 – 2011 hydrology including TS Lee. We obtain 1996 scour by a scaling procedure. Nutrient composition of solids is based on observations during TS Lee.
Conceptual Model of Sediment Movement through Conowingo Reservoir Sediment and nutrient releases are event-oriented. Upstream Reservoirs and Watershed Water Column Erosion event increases depth, diminishes subsequent erosion events. Conowingo Dam WIPS decrease sediment loads. Also decrease deposition. Bed Sediments Sedimentation rate is largely independent of bathymetry. Scour is strongly dependent on bathymetry.
1996 Scour Loads for Three Bathymetries
Model Results We’re going to concentrate on difference plots. Dredging 3 mcy (LSWRA28) – TMDL with existing bathymetry (LSRWA21). Dredging 31 mcy (LSRWA13) – TMDL with equilibrium bathymetry (LSWRA19).
Dredge 3 mcy Decrease Increase Dredge 31 mcy Decrease Increase
Dredge 3 mcy Decrease Increase Dredge 31 mcy Decrease Increase
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Decrease Increase Dredge 3 mcy Decrease Increase Dredge 31 mcy
Conclusions Dredging 3 mcy will improve summer-average bottom DO in the deep trench of the bay, Potomac River, and Baltimore Harbor by 0.02 to 0.04 mg/L based on a 1996 scour event. Dredging 31 mcy will improve summer-average bottom DO in the deep trench of the bay, Potomac River, and Baltimore Harbor by 0.04 to 0.06 mg/L based on a 1996 scour event.
Conclusions Dredging 3 mcy will reduce SAV growing-season chlorophyll by 0.02 to 0.05 in a large expanse of the bay, extending from Baltimore harbor past the mouth of the Potomac River, based on a 1996 scour event. The magnitude of chlorophyll reduction from dredging 31 mcy is comparable to dredging 3 mcy, based on a 1996 scour event. The improvement is more extensive and prolonged, however.
Conclusions Improvements in SAV growing-season light attenuation obtained by dredging are limited, generally less than 0.01 / m. These results are influenced by the timing of the scour event, January 1996. Most solids have settled out by the subsequent SAV growing season.