Using Stormwater BMPs for Retrofits
Conventional sewer systems were built on the concept of collecting runoff from impervious areas at concentrated points of entry and conveying large volumes of often polluted and fast-flowing runoff. There has been a predominant focus on capacity that has made dealing with increasing volumes of urban runoff problematic. The capacity limitations on many urban sewer systems will require significant capital investment. U.S. EPA's latest Clean Watersheds Needs Survey estimates that $9 billion is needed for stormwater management and nearly $55 billion is needed for CSO correction (2004 dollars) (U.S. EPA, 2008). This presents a significant economic burden, while the lack of adequate infrastructure capacity presents a significant public health and environmental burden.
Stormwater BMPs can be used to address capacity concerns by managing the demand on the existing infrastructure. Because stormwater BMPs treat stormwater at the source, they can prevent it from ever reaching the collection system. Reducing the inflow into the collection system "creates" capacity by lowering system demands. Limiting demand on stormwater or wastewater infrastructure is the opposite side of the spectrum from increasing pipe capacity or building storage tunnels. Reducing upstream loads and the conveyance demand on the collection and treatment system is a more sustainable approach because it begins to address the source of the problem (excess runoff) rather than responding to it in a reflexive manner. This approach, however, requires a significant shift in the common approach applied to water infrastructure and altering how sufficient capacity is assessed.
Moving from Centralized to Decentralized
Applying stormwater BMPs requires assessing urban management opportunities differently. Conventional controls are largely buried, unseen, and given little thought by anyone outside of the utility responsible for their operation and maintenance. Stormwater BMPs are distributed and woven throughout the urban landscape and necessitate a different implementation approach. Managing stormwater in a decentralized fashion requires assessing the urban landscape comprehensively. Retrofitting BMPs into a space-limited urban area is complicated by existing utilities, compacted and contaminated soils, right-of-ways, and maintenance access. One of the strengths of stormwater BMPs is the multitude of available BMPs that may be used and the manner in which treatment trains may be constituted.
The inherent flexibility of stormwater BMPs makes them a critical component of new urban design focused on functional and sustainable design and landscapes. They are typically integrated in one of the following manners:
- Stand-alone systems: In urban retrofit applications, BMPs can be applied as discrete, stand-alone systems to meet specific localized water quality needs.
- Hybrid systems: An application that creates a treatment train of stormwater BMPs or BMPs and conventional controls. This type of approach is often used for separate sewer systems to provide enhanced water quality prior to discharge.
- Complement to existing grey infrastructure: Often employed in combined sewer areas to relieve the hydraulic burden on grey infrastructure and enhance its operational performance.
Conventional stormwater practices are most often installed as inlet or end-of-pipe units that provide settling or filtration. Stormwater BMPs provide a range of options before stormwater enters the collection system. A main difference in implementation is the need to understand land use and function to appropriately employ this type of system. Conventional controls typically do not require this level of understanding, as the focus of implementation is suitability with the configurations and hydraulic function of the collection system. Since stormwater BMPs work before the stormwater enters the collection system, program managers must consider land use, pollutant type and load, building construction, and basin type for selection of the optimal decentralize treatment practice and exclusion of others.
For additional information on siting BMPs, see Using Green Stormwater BMPs in Urban Areas.
A concept plan for integrating stormwater BMPs into urban residential design demonstrates the multiple options available for managing runoff.
A great amount of attention has been given to managing stormwater from new development. Development patterns that contributed to increases in impervious cover and land use changes that proceeded at approximately twice the rate of population growth focused regulations and control efforts on mitigating the effects of new sources of stormwater runoff (Beach, 2002). However, urban areas represent the "past sins" of stormwater management and are a primary cause of stormwater pollution—meaning retrofits are a critical component of water quality improvement.
Integrating stormwater BMPs in a decentralized concept allows stormwater to serve multiple functions in the landscape and save money at the same time. Rather than whisk the water away to a detention pond, a deep pipe, or convey it to a treatment facility that is far away and out of site, BMPs allow stormwater to be visible and integrated into the development pattern. Improved stormwater quality goals are met and other planning goals such as provision of recreation amenities, creation of habitat areas, or establishment of flood control buffer zones can be met with the same land use strategy. While the initial impetus for applying BMPs is water quality, a number of other planning goals can be met, saving money, energy, and time.
Stormwater BMPs can improve conditions both as retrofit applications on existing development and as part of infill development projects. Urban infill development will likely, in the absence of a stormwater management plan at the site level, add to the volume and water quality problems in a community. Given that many retrofit opportunities exist on small parcels (less than one acre), they may fall outside of the normal "limits to growth" approach of regulating runoff on existing stormwater infrastructure. Rather than place a moratorium on infill development, development can proceed in conjunction with the use of a distributed approach that focuses on performance of controls and impact on the existing systems of development.
Stormwater BMPs can be used in all green approaches to community development to improve their stormwater control results. Specific BMPs or control approaches may or may not be applicable depending on site conditions, but the strategy of integrating stormwater BMPs is feasible in nearly every situation. Although the eventual goal of widespread implementation of stormwater BMPs is a significant reduction in stormwater runoff across a basin or catchment area, the essential nature of these controls allows for incremental discrete application. In this way, larger water management goals can be accomplished in a stepwise fashion that allows localized issues to be dealt with quickly.
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