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Drainage Projects

Posted on: April 13, 2023

Graveline Bayou Watershed Management Plan

In 2022, Cypress Environment & Infrastructure and Tetra Tech (the Team) were contracted by the Jackson County (County) Board of Supervisors to create a countywide Watershed Management Plan (WMP) and comprehensive WMPs for three individual watersheds within the County. The first step in this project was to complete a Watershed Inventory and Assessment, which characterized and prioritized the 12-digit hydrologic unit code (HUC12) watersheds fully or partially located within the County. Using the results from this inventory and assessment, with input from Jackson County staff and Board of Supervisors, three HUC12s were selected for specific WMPs. This is the WMP for Graveline Bayou (HUC12 031700090701).

The Graveline Bayou watershed is located entirely within Jackson County, which allows for investment in projects to best benefit the County citizens. The watershed also includes several of the areas of concern identified by the stakeholders, areas of likely development, and the potential to provide direct improvements to coastal habitats and resources.

The Graveline Bayou HUC12 watershed is approximately 12,400 acres or about 19.4 square miles. It is located in the southwestern portion of Jackson County along the Gulf of Mexico and is bordered by the Davis Bayou-Biloxi Bay watershed to the west and Lower West Pascagoula – Pascagoula Rivers watershed to the east. Portions of the cities of Gautier and Ocean Springs are located within the watershed. Graveline Bay and Graveline Bayou span from the center of the watershed down to the coastline.

Currently, the largest land use type in the watershed is Cities/Unknown at 49.6%. The next largest is Forestry/Agricultural at 22.3%. Other land uses include Residential (10.0%), Undeveloped/To Be Classified (9.6%), Conservation (6.6%), Civic/Institutional (1.7%), and Utilities and Infrastructure (0.1%). Based on the Jackson County future (2040) zoning layer, the greatest land use change is expected to be an increase in Conservation areas by 12.0%. The next largest change is a 10.6% increase in Residential. Most of the future increases in these land uses come from a 14.7% reduction in Forest/Agricultural land uses with the remaining coming from a 9.2% reduction in Undeveloped/To Be Classified lands.

There are 17 National Pollutant Discharge Elimination System (NPDES) permits within the Graveline Bayou watershed of which 13 are still effective. These NPDES permits represent point sources within the watershed and can contribute a variety of pollutants, depending on the permit activities. In addition, nonpoint source pollution is a potential source within the watershed. Urban stormwater runoff picks up and transports nutrient loading from fertilizers, grass clippings, and pet waste, as well as other pollutants including sediments, pesticides, oil, and grease. These land uses can also contribute pollutants through septic systems, both functioning and failing, as well as leaking sewer infrastructure. Agricultural stormwater runoff carries nutrients from fertilizers, as well as livestock waste, pesticides, and herbicides.

Through interviews with local stakeholders, 12 areas of concern were identified, which were primarily drainage and stormwater runoff management issues. Based on feedback from the stakeholders, the Team determined that flood control and runoff reduction management measures are needed to address the areas of flooding and drainage concerns. While there are no currently listed impaired waterbodies, with the planned growth in the watershed, measures to improve and maintain water quality were identified. The Board of Supervisors also requested that consideration be given to the loss of land from erosion along the coast. In addition, the Graveline Bay Coastal Preserve includes critical habitat that should be preserved. Therefore, the Team focused on these concerns when evaluating potential project locations and best management practice (BMP) options for this WMP.

The Team identified a set of geospatial metrics to evaluate and prioritize the more than 2,500 parcels throughout the Graveline Bayou watershed to determine feasible locations for BMPs. Through the parcel screening process, 20 parcels were identified as the best potential locations. Field visits were then conducted for these 20 parcels. The purpose of these field assessments was to gather more information about the conditions on and near each of the parcels such as existing stormwater management infrastructure, drainage features, nearby areas of concern, and potential management measure opportunities.

Following the field assessment, a final analysis was conducted to evaluate and prioritize management measure options for each parcel based on potential performance and applicability to watershed management needs. The Team evaluated BMPs including:

  • Conventional stormwater infrastructure, such as upgrades to existing infrastructure and pipe upsizing.
  • Stormwater BMPs, such as wet detention ponds and stormwater treatment wetlands.
  • Green infrastructure BMPs, such as bioretention and enhanced swales.
  • Stream restoration practices. Performance criteria metrics were developed to evaluate and prioritize the conceptual projects.

Projects have been identified for nine of the 12 areas of concern. The County is already planning projects in the other three areas of concern. Additional projects on the prioritized parcels were also identified to help address the areas of concern and/or other water management needs in the watershed (see Table ES-1). For each potential project, the Team prepared high-level concept designs, estimated costs, and estimated nutrient and sediment reductions. The number of people served by each project was also estimated based on the preliminary project watershed, number of homes within the watershed, and an average of 2.62 people per household from United States Census Bureau data (2022). Additional field verification, data collection, and/or engineering will be required to verify feasibility, refine the BMPs, and update load reductions and estimated costs. The County will likely need to pursue outside funding to implement the projects and options for funding sources are provided.

This WMP also includes an implementation schedule for the recommended projects. As changes occur in the watershed and additional data become available, watershed management needs and opportunities may change. Therefore, this WMP should be revisited regularly and revised as needed to ensure that the watershed continues to be managed effectively into the future.

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