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ing lot problem. Parking lots also cause a host of expensive and uncomfort-


able problems for the city, fromflooding to impairedwater quali- ty to higher ambient temperatures.That’s the bad news. The good news is that many of these problems can be miti-

gated using a technique called low-impact development (LID), in which sites are designed to manage stormwater in the same way that natural landscapes do. LID design attempts to minimize imperviousness, preserving and re-creating natural landscape features asmuch as possible.This creates site drainage that treats stormwater as a resource rather than a waste product. This article talks about LID principles that can be applied in some simple and relatively inexpensive parking lot retrofits.

Why thismatters. Have you ever wondered why Houston is so large geograph-

ically?One important reason is that there are currently four park- ing spots for every automobile. This creates massive “impervi- ous” areas in the city. (Impervious surfaces are thosewhichwater cannot pass through, such as concrete, asphalt and, of course, buildings).More than 40%of the city’s impervious cover is asso- ciatedwith transportation infrastructure, and it is this impervious- ness that creates problems. In Houston, stormwater flows from impervious surfaces to

the underground stormwater infrastructure, then into rivers and bayous, eventually making its way to Galveston Bay. Increased development upstreamcan create flooding downstreameven dur- ingmoderate rains. The most crucial water quality issue in the Galveston Bay

estuary is contaminated stormwater runoff.Many Houstonians don’t know that pollutants, debris and sediment in parking lots drain directly into the stormwater system without any treatment. These pollutants impair water quality and reduce the capacity of

F HOUSTON IS PERCEIVEDAS PHYSI- cally ugly by many, its acres and acres of parking lots certainly play a key role in that perception. Beauty is important, but in this case, it’s only one aspect of Houston’s park-

This parking lot located next to a dense inner-city street could bettermanage stormwater using a grassed swale. It would replace the stormdrain at the north end of the lot. In this case, only four parking spaces would be lost. This is an asphalt parking lot, which exacerbates the heat island effect during the long Houston summer. In addition to replacing the asphalt with something cooler, the trees will help shade and reduce the temperature of adjacent parking spaces.

the system, forcing evenmore water into streets. Another issue of importance in Houston’s climate is the

urban “heat island” effect.This is a phenomenon in which devel- oped areas with a high density of dark paving, buildings and parking lots experience an increase in outdoor temperatures. The city typically has temperatures 6°-8° F higher than in the sur- rounding countryside, requiring building systems to work harder in cooling, thus increasing peak electricity demand.

Why somuch parking? Capacity requirements for parking lots in Houston are typi-

cally based on worst case scenarios, such as for shopping during the weeks between Thanksgiving and Christmas. The result is that 50%of parking spots are empty 95%of the time. With about two empty parking spaces per car, and 0.75 cars

in the city per person, including the paving needed to get to those parking spaces, the result is 40 square miles of empty, unused space – all increasing flooding, cooling costs and drive times. (Current codes focus only on the number of parking spaces required, not the problems that result.)

What can be done? Opportunities exist to aid stormwater management and

reduce the heat island effect in parking lots all over the city. Low- impact development practices well-suited to parking lots include trees, native grasses and vegetation, bioretention or rain gardens, grassed swales, soil improvements, and permeable pavement. Most of these practices actually require less maintenance than the typical lawn grasses used within medians. Most of these strategies are discussed inmore detail below: Trees provide flood control benefits by intercepting and

This well-known Houston parking lot could be improved with the addition of an infiltration garden as shown. The parking lot was designed to drain toward the center of the car aisles; in this renovation, stormwater would be infiltrated into the planted aisle, slowing down runoff and cleaning the water. Native trees would serve to shade the parking spaces and reduce the heat island effect. The planted aisle would need to be about 5-foot wide in order to accommodate trees.


evapotranspiring rainwater as it falls. They also reduce erosion and improve stormwater quality by reducing runoff into bayous and streams. Trees are a simple addition or retrofit to most exist- ing parking lots, as well as excellent additions to new parking lots.Most medians or islands 5 foot or wider provide adequate space to add trees.The “urban forest” canopy also plays a signif- icant role in reducing the heat island effect. Bioretention facilities are engineered versions of rain gar-

dens,which are shallowdepressions in the landscape designed to hold water when it rains and then quickly dry-out or drain within Continued on Page 22

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