Welcome back to CSN’s Stormwater Spotlight Series! Over the next few months, we will be showcasing some of the top BUBBAs projects from this past year. Up next we head to the District of Columbia to learn more about the winner of our Best Retrofit category: The Carter Barron Stormwater Retrofit Project! Here are a few excerpts from their project narrative:
The Carter Barron Stormwater Retrofit Project Area is a 30‐acre site located in northwest Washington, D.C. Nestled within Rock Creek Park, the site is home to the Carter Barron Amphitheatre and the Rock Creek Tennis Center and sits at the headwaters of the Blagden Run watershed, a sub‐watershed of Rock Creek.
The Project Area was identified as a priority restoration area by U.S. Fish and Wildlife Service (FWS), National Park Service (NPS) and the District Department of Energy & Environment (DOEE) due to its impact on existing habitat along Rock Creek.
The targeted 11‐acre impervious area has no stormwater controls because it was developed prior to the promulgation of the District’s stormwater regulations. During rain events, stormwater swiftly leaves the Project Area from drainage outfalls, concentrating flows into erodible gullies, lowering localized infiltration and the groundwater table, and therefore impacting and reducing native habitat along Rock Creek. Five distinct gullies have been created by stormwater from outfalls draining the Project Area.
The goal of the Project was to fully retrofit the targeted 11‐acre impervious area with green infrastructure to restore natural hydrology, prevent erosion, reduce stormwater pollution, and protect and restore existing natural habitat for a federally listed endangered species.
The beauty of this project is that it achieves multiple goals, including stabilizing eroding gullies and zero-order streams; promoting stormwater infiltration and improving groundwater recharge; protecting the federally listed endangered species’ habitat (and habitat for other species); reducing the risk of downstream flooding; and improving water quality. The project accomplishes these objectives in the following ways:
- Protects amphipod habitat by reducing erosive stormwater flows from project site outfalls near the amphipod’s known habitat while improving habitat for other water-dependent species by raising the groundwater table, increasing base flow in existing seeps necessary for the amphipod, and potentially creating new ones (especially this year!).
- Reduces the risk of downstream flooding – particularly in the Blagden Run watershed where the project area makes up approximately 15% of the watershed’s impervious surface – by reducing peak flows and increasing the time of concentration for stormwater leaving the project area.
- Improves water quality by treating up to the 1.2-inch storm event through the use of green infrastructure or Low Impact Development (LID) techniques. LID has been shown to remove stormwater pollutants including nitrogen, phosphorous, thermal pollution, sediment, and pollutants that bind to sediment, such as metals and organic compounds, which are particularly associated with runoff from parking lots.
Throughout the design process, several new and innovative techniques were used to meet the objectives of the project:
Turf shelves: Several of the bioretention retrofits included a turf “ponding shelf” as part of the design. The intent of the ponding shelf is to allow a smaller footprint for the bioretention media by providing a large, shallow area that will be accessed by ponding during larger storm events. Because the ponding shelf is not intended to take credit for infiltration that may occur within its area, it can be sodded and receive heavy foot traffic. This has the added benefit of allowing park visitors and activities to utilize the ponding shelf area outside of larger storm events.
Subsoiling: Soils in green spaces with heavy pedestrian and/or vehicle traffic are considered “compacted cover” and are assumed to “generate 25 percent stormwater runoff for a design rain event”. Typical aeration practices can reduce compaction and improve the hydraulic characteristics of the topsoil, but only penetrate a few inches and do not address compaction of soil layers that can develop at greater depths (typically within the upper 24”) below the surface. As a part of this project, subsoiling will be applied to the northern athletic field to decrease compaction, improve infiltration and reduce runoff from the site.
Turret Pretreatment Chambers: The Rain Guardian Turret Pretreatment Chamber is a fully reinforced concrete structure that provides a simple, easy to maintain pretreatment facility. Often, the most maintenance associated with bioretention facilities can be the continuous cleaning and repair of the inlets. Debris (floatables, sand, leaves, etc.) can enter the bioretention facility and collect at the bottom of the inlet or be carried deep into the bioretention basin. Additionally, heavy rains can often create scour holes at the inlet. The Rain Guardian Turret provides a simple screening and collection area at the inlet for large debris (plastic bottles, leaves, etc.) that can be easily cleaned from the curb line without requiring maintenance staff to enter the bioretention facility. Additionally, a lower screen removes finer sands and debris from entering the facility. By removing the top grate, the lower chamber can be cleaned. The screen can also be removed and washed with a hose. The internal drop also provides energy dissipation that will reduce scour potential from incoming velocities. In the event that the entire system becomes clogged, a weir at the top of the turret continues to allow flows to enter the facility.
Sand Seepage Berms: A series of 3 sand seepage berms, constructed perpendicular to flows, provide additional storage, treatment, and velocity reductions of overflows from the bioretention basin. The sand seepage berms pond six inches of water behind each structure, promoting infiltration and seepage through the 50/50 mix of sand and green wood chips (a carbon source). Treatment of runoff occurs through combination of filtration through the sand and nutrient removal from beneficial bacteria that utilize the wood chips as a carbon source. The slow release of runoff through the berms provides additional habitats improvement by raising the local water surface elevations and allowing water to form pools on the upstream side of the berms.
Post and Wattle Structures, Log Jams, and Brush Mattress Weirs: Within the forested areas that surround the parking lot, eroded gullies have formed due to concentrated flows originating at storm drain outfalls. The restoration approach was to provide a low impact approach to minimize impacts to existing riparian forests and the groundwater seeps that are habitat for native species. Within these gullies a series of post and wattle structures, log jams, or brush mattress weirs (depending on the degree of erosion) were placed to slow concentrated flows and spread the flow out of the eroded channels, promoting additional shallow subsurface interactions to benefit existing habitat.