Low Impact Development (LID) has quickly emerged as an important stormwater management design strategy.  The core goal of an LID-based design strategy is simple; design sites to have minimal impact on the natural hydrologic balance so that the structural and water quality impacts of development are minimized downstream.  LID implementation however is less straightforward.  In some cases a regulators stated preference for LID, as opposed to a conventional “end-of-pipe” design, is interpreted as a requirement that only landscape based best management practices (BMPs) should be used for stormwater management.

Stormwater management system design is not a one size fits all process.  On urban sites, the most effective and economical solution is likely to be a combination of hydrology-centered planning, educational strategies, landscape-based BMPs, and innovative manufactured solutions.

Manufactured bmps vs. LID – A Misguided Distinction
As developers, it important to ensure your design engineers are including stormwater management systems that are tailored to the individual demands of your site – selecting systems that are truly “low impact” and help you maximize your investment.   

One of the criticisms of manufactured options is that they are too often designed as a “black box” with unknown sizing processes, unproven performance and inadequate maintenance after installation. These criticisms are valid in many cases, but are not so much symptomatic of the type of treatment chosen as they are of hasty or uninformed design.

Proper application of LID principles requires a similarly site-specific approach.  It requires an assessment of predevelopment hydrology and seeks to maintain that hydrology during and after construction through optimum IMP (integrated management practice) selection. However, application of the practices without the principles leads us back to the “black box” method of design. Just as blanket approvals of manufactured BMPs can short circuit the design process, blanket endorsement of IMPs can have the same effect.

ORENCO STATION, HILLSBORO, Ore.
Efficient use of land space, which may be enabled by the use of innovative non-LID techniques, can also shrink the footprint of development and encourage multi-use development typical of new urban designs. A good example is Orenco Station in Hillsboro, OR. This 190 acre site was redeveloped in 1997 with a focus on providing light rail and pedestrian commuting options for its 2,600 residents.  The site incorporates many features familiar to LID practitioners like shared driveways, sidewalks with adjacent planter strips and narrow streets. It also relies on three underground stormwater filtration systems from CONTECH Stormwater Solutions.  The Stormwater Management StormFilter can treat a combined 4.7 cfs of runoff to meet the treatment goals set by the City of Hillsboro.

Exclusive use of land based treatment systems was not feasible on this site due to high land values and the desire to integrate commercial, residential and transportation elements in a density that encourages car-less travel. Use of the manufactured filters also provides a reservoir for pollutants where they are out of human or animal contact and do not affect the aesthetics of the development. An inspection and maintenance contract between the site manager and the filter manufacturer has been in place since installation in 1999. Since that time, the systems have been cleaned every one to two years and approximately 30 yards of material have been recovered not including the spent filter media. The footprint of a landscape-based treatment system that would be able to assimilate or store this load of pollutants in an innocuous way would undoubtedly be massive.  

Choosing the Right Solution
The most suitable stormwater management system designs are often a combination of approaches that take into account local economic, political and environmental concerns. These designs are encouraged by clear performance standards and allowance for flexibility to use innovative approaches to meet them. The LID movement is an important part of the solution for most sites, but it is not a universal remedy. Efforts to require elements of LID on all sites may unnecessarily stifle the other options that are more suitable. SLDT