Methods for controlling land development stormwater runoff are evolving from large flood control structures in isolated site locations toward smaller de-centralized facilities integrated throughout a site. This evolution affects land yields, timing and technical elements of site design, and the tools that designers use.


Background
Traditional approaches to treating stormwater runoff have often resulted in stormwater management (SWM) facilities located at storm drain outfalls.

Known as "end-of-pipe" facilities, economy of scale theory was usually applied, assuming that fewer, larger facilities would incur less design and construction cost. Preliminary design of these facilities was generally a simple task as the site layouts merely had to devote an adequate area for SWM in the downstream portion of the site. The facilities were designed to primarily control peak discharge rates (detention), with a secondary goal of pollutant removal - often via permanent pools or extended detention.

Unless expensive underground systems are employed, this approach usually results in developable land being used for a SWM infrastructure. With larger drainage areas, the facilities take up more land, impound more water, are often an eyesore, and could represent a potential hazard in the event of a failure. Further, maintenance of these facilities is expensive, whether it is repairs to pipe and structures or the cleanout of accumulated sediments and other pollutants. If a large facility is not properly maintained, it can even become a point-source for pollution when large storm events re-suspend accumulated sediments and pollutant.

In recent years, alternative methods of designing stormwater management have been gaining visibility and acceptance. Over time, researchers have determined that the larger detention facilities do not avoid streambank erosion, do not effectively remove pollutants, and can represent a significant maintenance burden. Design guidelines promulgated by the Center for Watershed Protection (CWP) in several states and methods such as Low Impact Development Design (LID) pioneered by Prince Georgeís County, Maryland focus on the design of many small Best Management Practices (BMPs) distributed throughout a site. These approaches hold the potential of gaining buildable land by integrating the SWM throughout the site while utilizing available green space to also provide runoff treatment.

Recently, the conventional focus on peak discharge management has been evolving toward water quality treatment (the removal of urban pollutants in runoff) and recharge (replenishing groundwater that is no longer available because of impervious cover). This evolution of practice further supports the application of small BMPs integrated throughout a site in lieu of larger end-of-pipe facilities.


Change is Good - But Challenging
When applied wisely, this change in the way stormwater runoff is captured and treated makes great sense. Land yields can be increased when the land previously used for large end-of-pipe facilities is converted to buildable area. The costs of site and SWM infrastructure is often combined and thus reduced by designing multi-use BMPs. For example, a bioretention facility can be used for both SWM and landscaping. It treats stormwater using plants and soil media for pollutant removal with a maximum impoundment depth of one foot. Strategic locations of bioswales, which are low velocity conveyance in vegetated channels, can replace or reduce storm drain infrastructure. And minor changes in grading techniques can disconnect and treat impervious areas.

Although this all sounds great, in practice it represents a significant change in how drainage and SWM elements are designed, who is involved in the design process, and when the design is applied.

Further, the ìmenuî of stormwater management practices is dramatically expanded, which demands creativity, experience, common sense and higher levels of professional judgment from multi-disciplinary design team members.


Front-Loaded Design
To successfully apply integrated SWM design, thought and time MUST be applied to the design elements of site grading, drainage, and stormwater treatment in the early stages of design. Known as "front-loaded design" in green building, this means that the "quick" layout that clients typically want in the feasibility phase may not be as quick - or as inexpensive. On a typical project, the preliminary site layouts are commonly designed by land planners or landscape architects, and stormwater treatment is often left to be addressed in later design stages by engineers. To embrace a front loaded design process, professionals from these differing disciplines must jointly educate themselves on emerging techniques and increase their collaboration on site and stormwater design in the early stages of design.

The designer who wishes to integrate SWM into the site design - to realize the benefits discussed above - must either convince their client that more fee is needed, take an up-front "hit"on fee, or maximize their efficiency by using new tools and methodologies to design stormwater practices.

Fortunately, new tools and methodologies are emerging that can allow efficient sizing of integrated stormwater practices - tools that cross disciplines to be applied by planners, landscape architects, and engineers.

Based on a familiar Microsoft Excel spreadsheet platform, with Microsoft Visual Basic for Applications features to enhance the user interface, T.E. Scott and Associates developed TSA TOOLS. These software tools were developed specifically for integrated SWM design and are intended for use by various disciplines including civil engineers, surveyors, architects, landscape architects, and land planners.

Going beyond the typical in-house spreadsheets, all TSA TOOLS software products have been developed with the understanding that efficient and successful designs follow a defined design process. Whether used by a seasoned designer or a recent graduate, the software interface presents a step-bystep approach to accomplishing design tasks.



Conclusion
Integrated SWM design requires a designer to embrace out-of-the-box thinking, evaluate alternative design approaches, and then effectively communicate the ideas to the developer. The costs - and benefits - of front loaded design processes must be advocated and communicated at the appropriate time.

Whether it is the CWP approach, LID, or other methods, integrated SWM design represents a significant change in the way sites are designed. As with most advances in technology, those who delay educating themselves and applying the right tools will find themselves facing a challenging game of catch-up in the years ahead. SLDT