Sierra Club views the RFK site along the shore of Kingman Lake – fed by the Anacostia River, as a lynchpin in the environmental recovery of the Anacostia and its water quality. The Anacostia floodplain must play a key part. The floodplain includes all the low-lying areas adjacent to the river at the RFK site that are subject to periodic flooding. An examination of the current 500 yr. floodplain map shows that most of the 177 acre site – approximately 117 acres are in the floodplain. Only about 60 acres rise above the floodplain; basically, where the stadium is currently located, street frontage along Oklahoma Ave. Independence Ave. and C st., and the area adjacent to Eastern High School.
The Anacostia floodplain is a dynamic natural system that plays a crucial role in both the ecological health and hydrological processes of the Anacostia. Ecologically, the Anacostia floodplain supports diverse ecosystems, including wetlands, riparian forests, and unique wildlife habitats. Hydrologically, it acts as a natural sponge, temporarily storing excess water during flood events and slowly releasing it, thereby attenuating flood peaks and reducing downstream flood impacts. It also filters pollutants, improving water quality and contributes to groundwater recharge. Parts of DC already experience inland flooding from heavy storms. The RFK campus can be designed, at minimum, to not contribute to flooding potential, but could ideally contribute to effective flood prevention along the Anacostia River.
However, the Anacostia floodplain is also attractive for development because it is flat and in proximity to attractive water features. There is a long history of human settlement and infrastructure development in the Anacostia floodplain, and much of the river’s floodplain has been modified and developed with structures. Climate Change is already resulting in heavier rainfalls, increased stormwater volumes and more flooding. This modern reality needs to be a priority for site design. Here we examine two contrasting approaches to managing the Anacostia floodplain:
- Protecting the floodplain – This approach prioritizes the preservation and restoration of the floodplain's natural functions to enhance its capacity for flood attenuation, thereby reducing flood risks to communities located further downstream.
- Developing in the floodplain using Best Management Practices (BMPs) – This approach acknowledges the pressure for development but seeks to mitigate flood risks and environmental impacts through engineered solutions and careful planning. Understanding the implications, benefits, and drawbacks of each approach is critical for sustainable land use planning and effective disaster risk reduction.
Protecting the floodplain
The key considerations and strategies for floodplain protection rely on zoning and land-use regulations or conservation easements, and are largely driven by the District’s goals for environmental and water quality protection and sustainable development. The District’s goals for wetland restoration and creating new ones within the floodplain will enhance water storage capacity and filtration to help manage stormwater and reduce flooding. The primary benefit is the significant reduction in flood peaks and volumes downstream, leading to less damage and fewer disruptions from flooding. The restoration of riparian buffer by replanting native vegetation along the banks of Kingman Lake will stabilize soils reducing erosion and sedimentation into the Anacostia, and improve water quality. Floodplains act as natural filters, removing pollutants and improving the quality of surface and groundwater. Riparian areas also safeguard critical habitats for a wide array of plant and animal species, contributing to biodiversity and ecological health. This, in turn, creates unique recreational opportunities with the Anacostia Riverwalk trail running through the floodplain, opening up easy access for hiking, fishing, and birdwatching. Importantly, floodplain protection reduces long-term costs, leading to substantial long-term savings in flood disaster relief, infrastructure repair, and insurance claims.
Finally, floodplain protection is a critical component of climate change resilience. Natural floodplains are adaptable to changing precipitation patterns and sea-level rise.
Developing in a Floodplain with Best Management Practices (BMPs)
Developing within a floodplains has been undertaken with Best Management Practices (BMPs) designed to minimize flood damage and environmental impact. BMPs in this context refer to a suite of structural and non-structural measures that aim to make development more resilient to flooding and less disruptive to the natural floodplain functions.
Key Best Management Practices include:
- Elevated Structures: Building homes and businesses on stilts or elevated foundations to keep the lowest floor above anticipated floodwaters.
- Compensatory Storage: When fill is used to elevate structures, an equivalent volume of flood storage capacity is created elsewhere on the site or within the watershed to ensure no net loss of floodplain storage.
- Permeable Surfaces and Low-Impact Development (LID): Incorporating green infrastructure such as permeable pavements, rain gardens, and bioswales to reduce stormwater runoff, allowing it to infiltrate the ground rather than contributing to flood volumes.
- Setbacks and Buffers: Maintaining vegetated buffer zones along watercourses to allow for natural floodplain processes, filter runoff, and provide space for floodwaters.
There are arguments in favor of eveloping with BMPs, including utilization of land for economic development and options for development in areas where developable land is scarce. However, the downside of developing with BMPs include a residual risk of flooding, especially with extreme events or climate change impacts – both on-site, but also exacerbating downstream flooding. If BMPs are not perfectly implemented or fail, they can exacerbate downstream flooding by displacing floodwaters. There will also be continuous BMP maintenance as well as their installations that increases initial construction costs. The expense of insuring buildings in a floodplain must also be considered, as it can be prohibitive. While BMPs aim to make development safer in floodplains, they are often a compromise that attempts to balance human needs with natural processes, and they do not fully replicate the natural flood attenuation capacity of an undisturbed floodplain.
Conclusion
In contrast to developing within floodplains, the approach of protecting and restoring floodplains for their natural flood management capabilities prioritizes the ecosystem services they provide. Healthy floodplains are a cost-effective and resilient form of natural infrastructure for reducing flood risk, particularly for communities located further downstream. The choice between developing in a floodplain with BMPs and protecting it for natural flood
management involves complex trade-offs. While BMPs can make development safer, they do not fully replicate the benefits of a naturally functioning floodplain. Protecting floodplains offers a more sustainable and resilient long-term solution for flood risk reduction, coupled with significant ecological and social benefits. A balanced approach often involves a combination of strategies: strictly protecting the most critical floodplain areas, restoring degraded sections, and applying robust BMPs only in areas where development is unavoidable and can be done with minimal impact. Ultimately, prioritizing the natural functions of floodplains is widely recognized among planning professionals as an essential component of comprehensive and sustainable flood management strategies in the face of a changing climate.
Written by: Larry Martin
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the-anacostia-floodplain.docx