The St. Mary’s County Department of Public Works maintains stormwater management facilities across the County, and contracted EA to provide survey, design, and permitting for 15 public stormwater management pond retrofits, in order to maximize water quality treatment provided at each site. The project obtained Chesapeake Bay grant funding from the Maryland Department of Natural Resources, and allowed the County to work toward its Watershed Implementation Plan goals for reduction of total nitrogen, total phosphorus, and total suspended solids from stormwater.
A pre-design investigation was conducted at each site, and included a desktop analysis, field investigation, topographic survey, geotechnical investigation, and delineation of wetland and stream features classified as Jurisdictional Waters of the United States. Concept retrofit design considered physical feasibility, existing site conditions, standard stormwater management practices, and regulatory requirements, including pathway to permit approval. Stormwater management needs were evaluated in accordance with the 2000 Maryland Stormwater Design Manual Volumes I and II and the St. Mary’s County Stormwater Management, Grading, and Erosion and Sediment Control Ordinance. The existing pollutant loading at each site was determined based on Chesapeake Bay Water Model loading rates; the proposed facility pollutant removal efficiency was based on Chesapeake Bay Program documents; and the load removal calculations were based on MDE (2014) guidance, “Accounting for Stormwater Waste Load Allocations.” Chesapeake Bay Water Model Version 5.2.1 (2014) loading rates for total nitrogen, total phosphorus, and total suspended solids were utilized to determine existing pollutant loading, in order to maintain consistency with MDE methodologies.
Retrofit designs were determined based on uniform stormwater sizing criteria for the state of Maryland for treatment and storage of stormwater runoff, maximizing the sediment and nutrient load reductions for each site, and minimizing construction costs to meet the available grant funding ($3.9M). Due to the limited guidance for retrofit designs, EA worked closely with the St. Mary’s County Soil Conservation District and NRCS to develop technically acceptable retrofit designs that ensured impacts identified were avoided or minimized. Proposed retrofits included rehabbing existing pond embankments, and integrating micro-scale facilities (i.e., submerged gravel wetlands) into existing facilities where possible. During design development, review of sites for Small Pond Code 378 compliance identified four sites requiring dam breach analysis. For these sites, cross-section locations were determined near features of concern (structures and driveways) and peak flows for a sunny day breach (no rain, all low flow orifices and weirs clogged, pool stage set at crest of riser) were developed using the National Weather Service dam break equation, and run in HEC-RAS to develop stages and velocities. The analysis of the four sites determined that no identified structures would be at risk, no breach zones would result in a high danger zone within the channel area, and all breach zones in the overbank area would have a very low danger zone.
Final design documents developed included construction plan details, notes, quantities, costs estimates, and specifications. In addition, EA obtained permit approvals for the proposed work, after working closely with USACE, MDE, and NRCS throughout design development. In 2017, construction of 13 existing stormwater management facilities throughout St. Mary’s County having minimal water quality features were retrofitted to provide additional treatment using modern wet ponds and submerged gravel/wetlands. A total of 207 impervious acres were treated, resulting in the reduction of 1,170 pounds of nitrogen reduction per year, and 129 pounds of phosphorous reduction per year. This project provided 90% of the total Phase II Watershed Implementation Plan nitrogen removal goal of 1,300 pounds of nitrogen per year.