20230830 Environmental Board Packet Combined

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Amelia Ditch Study August 10, 2023 Page 6 Another component of the long-term work would be floodplain management in the area. With the known flooding concerns and FEMA floodplain designation, it is evident that the area surrounding the ditch would be well served by increased flood storage capacity. This capacity would come in the form of large basins adjacent to the ditch. In major storm events, the ch annel would overtop its banks and flow into these basins. This increased storage will reduce flooding concerns for the existing properties downstream, as well as slightly ease flood levels within Lake Amelia. These basins would also provide ecological value and water quality treatment. Depending on the proposed planting and hydrologic regimes, the basins may be restored to different wetland types. Based on how much of the area is delineated as wetland, these restored wetlands would offset any impacts from development or increase their footprint. Similar to the proposed basins on the Shenandoah Improvement Project, these would be proposed to draw down via evapotranspiration or very slow groundwater infiltration. This abstracted runoff would result in a signi ficant degree of pollutant removals from Lake Amelia as well as provide dynamic habitats for plant and animal life. For more direct water quality treatment, biofiltration BMPs are proposed on further upstream areas where there is sufficient elevation difference. Soil information indicates in situ soils are conducive to filtration when provided an underdrain. As such, biofiltration basins could be constructed offline from the ditch alignment at minimal cost. These basins would not serve as significant floodplain or ecological value but would further increase pollutant removals. Significant drainage improvements are seen in the modeling as a result of this proposed work, as summarized in Appendix B. Due to impounded flows in the proposed biofiltration basins, HWLs are increased slightly in the drainage areas upstream of them. However, these are not areas with drainage concerns, increased HWLs will not cause major flood area increases. There are HWL decreases of 1.1’ in DA 5 and DA 9 because of the increased storage proposed in the wetlands alongside the ditch channel. Further downstream, HWLs drop more, up to 2.7’. This is both an effect of the ditch cleanout in the short-term and an anticipated replacement of Culvert E in the medium- to long-term. Additionally, flow rates increase significantly across the site, especially around the culverts that would be replaced. Some flow rates were modeled to decrease near the biofiltration basins and wetlands due to changes in flood storage, but overall, the system will allow for greater flows. This long-term work includes many more benefits to the tributary area to the lake but comes at an increased cost compared to the short-term improvements. The total estimated cost for the work is $1.71 million, plus an additional $150,000 in wetland maintenance and monitoring over the 5 years following the project. The majority of these costs are anticipated from the large volumes of earthwork required to create the wetland and filtration basins. Additional costs come from storm sewer required for the BMPs and the required seeding over the large area of disturbance. This cost includes a 30% contingency factor to account for potential changes between this concept and future design implementation. It should be noted that this estimate is unlikely to be accurate by the time major development is occurring in the area and will need to be revised to reflect updated costs in the future The estimate does not account for any potential costs for easement or property acquisition for the stormwater features. A detailed breakdown of this estimate can be found in Appendix C. WATER QUALITY MONITORING Water quality samples were collected monthly within Lake Amelia and the ditch to Lake Amelia from May through August 2022. See the Water Quality Sampling Location map found in Appendix A. Data was collected to understand the existing water quality and how the water from the ditch may affect Lake Amelia’s water quality. Samples were analyzed for total phosphorus, orthophosphate, and total suspended solids. Results can be seen in Figures 1-3 below.