In the not so distant past, storm water from parking lots, roofs and roads did not require treatment once it left its place of origin. The run off found its way to streams, ponds and ground water without concern or constraint on the quality or quantity of the discharge. This practice contributed to erosion and degradation of the quality of our watersheds.
All levels of government are now involved with storm water management practices. From EPA to local conservation boards, the way storm water is dealt with is highly regulated. The effective design of storm water flow, retention and cleansing facilities is one of the primary jobs of the civil engineers and landscape architects at TFMoran Inc. and it can make or break a project.
The early storm water treatment systems were quite simple and usually consisted of a grassy trench located at the lowest elevation of the site. Run off from parking lots and other paved areas would simply be allowed to flow into these trenches and be “cleansed” by the grass. Although this treatment option is still viable, what if the lowest elevation of a site is up front and painfully visible? Not many property owners, especially retail establishments, want their customers greeted by a murky trench, which can fill up with trash and be difficult to maintain.
In later years underground infiltration systems became popular, but they were very expensive to install and maintain. The idea behind these was to capture the runoff from parking lots (and other impervious areas) in big underground concrete boxes which would allow a slow percolation of the storm water into the groundwater. Although this system encouraged more creative use of a site, the expense was high. Over the years the concrete boxes were replaced by rigid plastic “tubes” that were easier, more flexible and cheaper to build and maintain.
The detention “pond” has been another visible sign of storm water management strategies. At first these ponds could be designed to actually hold water, but over time the pond’s main function was to detain the water before it was allowed to slowly run off the site. The aesthetic appeal of most of these manmade ponds was diminished by having to line them with chunky rip rap and fence them in with 6’ chain link.
All in all, the history of storm water management practices through the 80’s and early 90’s was more of a functional response to an ecological problem rather than an aesthetic response. Then the notion of “environmental design” was born.
Environmental design attempts to harmonize both the functional and aesthetic aspects of storm water management. The modern concept has been around for over 25 years and was initially promulgated by academia, but spread to both the public and private sector as rapid land development threatened water quality.
The new initiatives in storm water runoff include “BMP” (Best Management Practices), “LID” (Low Impact Development), and now “LEED” (Leadership in Environmental Design) certification for site design. They all encourage the use of on-site natural features to protect water quality and promote sustainability.
Some of the small scale hydrologic systems used to replicate the “pre-development” condition of a site are:
· Rain Gardens—these bioretention areas use a combination of soil layers and plants to remove contaminants from the run off before it is absorbed by the ground water.
· Tree filters—small (usually 6’ diameter) concrete planters which collect run off and filter it using the soil and the roots of the trees.
· Pervious Pavement—or porous pavement, are paved areas that absorb water rather than deflect it. The underlying layers of sand and gravel filter out contaminants and reduce, or eliminate, the need for other types of treatment.
· Green Pavement—this is essentially grass underlain with either a plastic or concrete grid system, which keeps the grass and soil from compacting. It is used primarily for overflow parking.
· Green Roofs—the use of plants to collect, filter and reduce the amount of run off from roofs. An additional benefit of green roofs is to help moderate temperatures.
· Bio-swales—vegetated swales which collect, filter and reduce run off from parking lots.
· Underground infiltration systems—manufactured devices, usually recycled plastic, which collect water through a conventional manhole, then pipe the run off to perforated “tubes” where the runoff is detained and filtered through sand and gravel.
Using natural features to manage storm water runoff is an attractive, efficient and economic method of keeping our watersheds healthy. Please visit www.tfmoran.com and click on the link “green technology” for more information on these natural methods of storm water management practices.
Anne Cruess, ASLA is a principal of TFMoran Inc., Bedford, N.H.