Low impact development retrofits: Addressing site constraints
Evaluating various design alternatives of a neighborhood-scale low impact development retrofit project requires understanding and addressing site-specific constraints. On a recent project, we had to consider some of the risks I discussed in Cautious Implementation of LID. Notably the need for setbacks to steep slopes and wells, and the potential for adverse impacts to waterways and wetlands from stormwater runoff and infiltration. Impacts, costs and maintenance of centralized versus decentralized systems also influenced selection of LID best management practices.
The project area is primarily residential with traditional storm drainage collection systems consisting of catch basins directly connected to underground storm pipes with an occasional drywell. An arterial roadway with medians is adjacent to the residential area. The drywells are generally located near steep slopes with pipes discharging to a steep ravine. Downstream of these pipe outfalls is a municipal well and surface wetlands and waterways. There is little or no water quantity control and no water quality treatment provided within the project area prior to the outfalls.
The primary objectives were to reduce pollutant discharges within a wellhead protection area and downstream surface waters and wetlands. Additional considerations included:
- Minimizing infiltration within potential landslide prone areas
- Minimizing impacts to the residential properties
- Providing for long-term maintenance
Where a centralized facility might have a lower construction cost and simplified maintenance, it also presented greater impacts to select residential properties and public utilities. Equally important was consideration of the uses within the public right-of-way such as driveway access, on-street parking, trash collection zones and the occasional portable basketball hoop. Dispersed facilities will lessen the impact on these existing conditions while still providing the water quality and flow control benefits to the watershed. Larger centralized facilities were selected in the arterial roadway medians where impacts to residential properties were less of a concern to reduce overall construction costs and long-term maintenance.
The project design and analysis included quantifying the water quality benefit associated with the retrofit project for funding assistance eligibility through the Washington State Department of Ecology’s Water Quality Program.
Through implementation of a variety of BMPs – both centralized and decentralized – the retrofit project is expected to provide equivalent flow control and water quality benefit of a new or redevelopment project.
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