Status

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.

WIPS decrease sediment

loads. Also decrease

deposition.Sedimentation rate is largely independent

of bathymetry.

Scour is strongly dependent on bathymetry.

Erosion event increases depth, diminishes subsequent erosion

events.

Water Column

Bed Sediments

Conowingo Dam

Upstream Reservoirs

and Watershed

1996 Scour Loads for Three Bathymetries

Clay (ton) Silt (ton) Sand (ton)0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

existing bathymetryafter dredgingafter scour event

N (ton) P (ton)0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

existing bathymetryafter dredgingafter scour event

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

Dredge 31 mcy

Decr

ease

In

crea

se

Decr

ease

In

crea

se

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.

Status

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