Following a collapsed culvert where the Distillery Branch passes beneath Copeley Road near the Copeley Apartments, Nitsch Engineering performed a two-phased study at the University of Virginia’s Distillery Branch sub-watershed. The study resulted in the immediate repair at the collapsed culvert, as well as a larger series of watershed scale improvements to target the true root of the problem – urban stormwater runoff. By addressing the water quality and volume issues in the larger watershed to alleviate the stressed stream conditions, Nitsch Engineering’s approach provided an opportunity for UVA to contribute to their Municipal Separate Storm Sewer System (MS4) stormwater compliance requirements by reducing sediment and nutrient discharges.
The feasibility phase of the study was intended to assist UVA in determining the appropriate corrective action for the collapsed culvert. Working closely with UVA staff, Nitsch Engineering studied three options including re-lining the culvert, upgrading the culvert, and a combination of daylighting the stream and re-lining the culvert. During the study, we observed and confirmed through hydrologic modeling that the entire stream channel was suffering from significant erosion and flooding issues due to decades of upstream development from the 70-acre watershed. We confirmed the (collapsed) culvert was under capacity for accepting the current flows.
As a result, Nitsch Engineering recommended that UVA focus on upstream green infrastructure retrofits to help relieve the rapid, urban storm flow impairing the stream and to take a pro-active approach to preventing issues such as the culvert collapse. Based on the recommendations of our Phase I report, UVA determined they would re-line the culvert and invest in further study and retrofits upstream to address the erosion, flooding, and capacity issues.
The second phase of the study identified upstream projects to address rapid, high-volume flow in Distillery Branch (and, in turn, reduce the surcharge condition at the Copeley Road culvert). During this phase, Nitsch Engineering performed hydrologic modeling using Stormwater Management Model (SWMM) and water quality calculations to evaluate the effectiveness of various green infrastructure retrofits to reduce direct runoff and sediment and nutrient discharges to the stream.
Working closely with UVA’s Landscape Architects, Nitsch Engineering explored retrofits that could be tied to other planning projects in the watershed, such as pedestrian safety improvements along Massie Road. We explored opportunities to integrate stormwater management projects along the roadway corridor including stormwater planters, bioretention areas, and infiltration trenches. Nitsch Engineering also explored retrofits that could be tied to localized drainage issues and expansions of existing stormwater BMPs, all in an effort to help UVA get “two projects for the price of one” for as many strategies as possible.
Nitsch Engineering prepared conceptual designs for 10 green infrastructure retrofit projects, such as infiltration and bioretention systems, to slow, cool, filter, evaporate, infiltrate stormwater. In addition to the hydrologic and water quality modeling, we also performed order of magnitude cost estimates for each strategy. The combined strategies have the ability to bring the watershed’s “effective” impervious cover to a more balanced condition, meaning they have the ability to reduce the impervious coverage that directly discharges to the stream below 25%, a nearly 10% improvement compared to current conditions.
Finally, Nitsch Engineering prepared a comprehensive matrix and ranking system to help the University prioritize the most effective strategies for alleviating existing issues and supporting compliance with long-term nutrient reduction requirements under the University’s MS4 permit. For each green infrastructure retrofit, the matrix compares impervious area treated, volume reductions, nutrient reductions, and cost. The matrix is currently being used by the University as part of their decision-making process for near-term implementation.