October 2, 2003
Parking lots designed to suck up storm water
By LEN ZICKLER and DUANE DIETZ
First, it is the environmentally responsible thing to do. The state, various cities and counties have mandated the U.S. Green Building Council’s LEED certification, or at least achieving LEED standards, on new building projects. LID relates directly to LEED certification in the categories of “sustainable sites” and “water efficiency.”
The second reason involves project economy. Adoption of the new state Department of Ecology Storm Manual for Western Washington will result in storm facilities 50 percent to 300 percent larger than existing regulations. For a residential project, this translates to fewer units per acre. For a commercial project, the increased size of storm facilities may result in a site’s inability to accommodate both desired building size and the amount of parking/buffering necessary to meet zoning requirements.
LID employs a more natural approach to stormwater management. With conventional site development, about 70 percent of the natural drainage leaves the site. The goal of LID is to maintain the natural hydrology of a site. Open space is increased, storm water is managed closer to the source, and water quality is improved.
While LID principles have been applied to residential and public construction, few standout commercial projects exist in Washington state. Projects such as malls, warehouses and big box stores offer an opportunity to re-examine stormwater issues through the lens of LID. This article will focus on a new type of parking lot design and suggest other LID techniques which have application in commercial development.
LID for parking lots
Conventional parking lot construction typically begins by stripping the land of its vegetation and upper layers of organic soil. The inorganic subsoil is then graded, compacted to 95 percent, and curbs and paving are added. These features drain to catch basins, underground conveyance systems and large storm facilities at the low elevation of the site. Irrigation and plantings are installed in the remaining inorganic soil, usually with minimal soil supplements.
Parking lot trees are plunked down every 10 to 12 stalls and elevated in 5- to 8-foot-wide curbed islands. The landscape contractor uses soil from the tree holes to mound up the planting islands. This creates a quick drainage situation.
Look at these islands next time you go shopping. You’ll see that people have walked all over them, destroying the groundcover and further compacting the soil. The irrigation barely wets the ground before it runs off to a catch basin. Trees in this environment rarely live more than 7 to 10 years.
A conventional 10-acre commercial site will have about 3 acres of buildings, 5 acres of parking (around 650 stalls) and 1 acre of interior and perimeter landscaping. This area is graded to drain to on-site storm facilities requiring about 150,000 cubic feet of storm storage and additional water quality treatment features. For a site of this size, storm facilities typically consume 1 or more additional acres of land.
The typical 10-acre LID commercial site will have about 3 acres of buildings, 5 acres of parking and nearly 2 acres of perimeter planting and parking lot planting strips. In this example, LID storm drainage features are about one-third the size of conventional storm facilities. LID essentially doubles the amount of green space on a site, manages storm water closer to where it falls, and provides a way to preserve rentable building square footage and parking area.
After careful site analysis and design, a LID design for parking improvements begins by providing 8- to 10-foot-wide depressed strips of landscaping between parking bays. The landscape strips, serving as alternatives to piped conveyances, are designed as planted filter strips or rain gardens by adding organic soil to upper layers of the planting bed.
Under-drains, utilizing gravel trenching and perforated pipes, can further enhance drainage in the filter strips. All impervious parking areas are then graded to drain towards these strips. Parking lot islands within bays are removed. Islands at the ends of bays are still utilized as aesthetic elements to visually interrupt the parking area.
The trees and shrubs that would normally be planted in small islands are moved to the strip, creating a greater area for root development, water intake and a healthier medium for plantings. Because the strip collects runoff, irrigation requirements are minimized or eliminated. Check dams are added within the strip to slow storm water flows and help to hold water in place for infiltration.
Because these strips are sunken, people are less inclined to walk through them, thinking their feet will get wet. Inexpensive bridges over the strips can be added to direct public circulation. Many curbs are eliminated through the use of wheel stops.
The significant advantage of LID is in the elimination of a majority of catch basins and piped conveyances plus reduction of required on-site storm drainage infrastructure by as much as two-thirds. The result is more real estate for building and parking.
Other LID techniques which could be used on commercial projects include:
AHBL recently worked with the Washington State Department of Transportation in applying LID techniques to the design of temporary office facilities supporting the construction of the second Tacoma Narrows Bridge. These facilities are located on a highly visible 6-acre site which has tremendous potential for future public use.
Neighbors were concerned with objectionable views into the site and the potential destruction of its natural environment. By applying LID techniques, AHBL was able to eliminate the need for expensive storm drainage infrastructure, including catch basins, pipe conveyances and a storm-water detention pond. The resultant design saved an additional 2 acres of existing mature trees and included more aesthetic and extensive landscaping.
LID appears to be a viable approach to meeting new environmental regulations while insuring the feasibility of commercial projects.
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