Modeling Workgroup Publications

Carl F. Cerco, Lewis Linker, Jeffrey Sweeney, Gary Shenk, and Arthur J. Butt, 2002. "Nutrient and Solids Controls in Virginia’s Chesapeake Bay Tributaries" Journal of Water Resources Planning and Management, Vol. 128, Issue 3. https://doi.org/10.1061/(ASCE)0733-9496(2002)128:3(179).

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Three models are at the heart of the Chesapeake Bay Environmental Model Package (CBEMP). Distributed flows and loads from the watershed are computed with a highly modified version of the HSPF model. Nutrient and solids loads are computed on a daily basis for 94 sub-watersheds of the Chesapeake Bay watershed. The CH3D-WES hydrodynamic model computes three-dimensional intra-tidal transport on a grid of 13,000 cells. Computed loads and transport are input to the CE-QUAL-ICM eutrophication model which computes algal biomass, nutrient cycling and dissolved oxygen, as well as numerous additional constituents and processes. The eutrophication model incorporates a predictive sediment diagenesis component. Ten years, 1985-1994 are simulated continuously using time steps of 5 minutes (hydrodynamic model) and 15 minutes (eutrophication model). This report comprises the primary documentation of the eutrophication component of the 2002 CBEMP.

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The TSD was developed by the EPA and its watershed partners to be a companion document to the Regional Criteria Guidance. Because it describes the development and geographical extent of the designated uses to which the refined water quality criteria may apply, the TSD serves as a resource to the states to assist them in the development and adoption of refined water quality standards.

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The Benthic Process Model Review Team, assembled by the Modeling Subcommittee during Fall 2000, reviewed the benthic model developed for the Chesapeake Bay Water Quality Model, a component of Chesapeake Bay Estuary Modeling Package. Review of the model presented in the technical, report, Development of a Suspension Feeding and Deposit Feeding Benthos Model for Chesapeake Bay (USCE 0410) was guided by questions provided by the Modeling Subcommittee. The Review Team was further charged with advising the Modeling Subcommittee regarding the future directions in benthic process modeling that will be needed in order to satisfy the goals and objectives stated in the Chesapeake 2000 Agreement.

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In this report, observations of dissolved oxygen concentrations, chlorophyll concentrations, and light attenuation are compared to model estimates taken at the same time and location in model space. The comparison includes scatter plots, cumulative plats, regressions, and summary statistics. The method of adapting the model for application to the proposed water quality criteria through the use of regressions, spatial interpolation, and cumulative frequency plots of criteria exceedences is described.

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The Chesapeake Bay Environmental Model Package is a combination of interactive models. The Community Multi-Scale Air Quality Model and a set of regression models compute daily atmospheric nitrogen and phosphorus loads to the Chesapeake Bay watershed and to the water surface. The Watershed Model (WSM) provides daily computations of flow, solids loads, and nutrient loads at the heads of major tributaries and along the shoreline below the tributary inputs. Flows from the WSM are one set of inputs to the CH3D (Computational Hydrodynamics in Three Dimensions) hydrodynamic model. CH3D computes surface level, three-dimensional velocities, and vertical diffusion on a time scale measured in minutes. Loads from the WSM and transport processes from CH3D drive the CE-QUAL-ICM (Corps of Engineers Integrated Compartment Water Quality Model) eutrophication model. ICM computes, in three dimensions, physical properties, algal production, and elements of the aquatic carbon, nitrogen, phosphorus, silica, and oxygen cycles.

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A series of refinements were added to a previously-completed three-dimensional eutrophication model of Chesapeake Bay. Refinements included increased grid resolution in the western tributaries and in shallow littoral areas, extension of the grid onto the continental shelf, extension of the validation period to 1985-1994, and addition of living resources. Computations of zooplankton, submerged aquatic vegetation, and benthos compared successfully with observations aggregated over annual time scales and at spatial scales on the order of 100 km2.

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The Chesapeake Bay Environmental Model Package (CBEMP) was used to assess the environmental benefits of a ten-fold increase in native oysters in Chesapeake Bay. The CBEMP consists of a coupled system of models including a three-dimensional hydrodynamic model, a three-dimensional eutrophication model, and a sediment diagenesis model. The existing CBEMP benthos submodel was modified to specifically represent the Virginia oyster, Crassostrea virginica. The ten-fold oyster restoration is computed to increase summer-average, bottom, dissolved oxygen in the deep waters of the bay (depth > 12.9 m) by 0.25 g m-3. Summer-average system-wide surface chlorophyll declines by 1 mg m-3. Filtration of phytoplankton from the water column produces net removal of 30,000 kg d-1 nitrogen through sediment denitrification and sediment retention. A significant benefit of oyster restoration is enhancement of submerged aquatic vegetation. Calculated summer-average biomass improves by 25% for a ten-fold increase in oyster biomass. Oyster restoration is most beneficial in shallow regions with limited exchange rather than in regions of great depth, large volume and spatial extent.

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The attenuation of light underwater is an important process in estuaries, directly affecting phytoplankton, submerged aquatic vegetation (SAV), visually orienting predators, and indirectly affecting oxygen depletion and other water quality indicators.

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This report is the third in a series in which the Chesapeake Bay Environment Model Package was used to assess the environmental benefits of oyster restoration in the Chesapeake Bay. Here, the effects of oyster restoration to all potential Virginia oyster habitat were investigated.

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