If Houston is perceived as physically 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 parking lot problem.
Parking lots also cause a host of expensive and uncomfortable problems for the city, from flooding to impaired water quality to higher ambient temperatures. That’s the bad news.
The good news is that many of these problems can be mitigated 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 as much 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 this matters.
Have you ever wondered why Houston is so large geographically? One important reason is that there are currently four parking spots for every automobile. This creates massive “impervious” areas in the city. (Impervious surfaces are those which water cannot pass through, such as concrete, asphalt and, of course, buildings). More than 40% of the city’s impervious cover is associated with transportation infrastructure, and it is this imperviousness 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 upstream can create flooding downstream even during moderate 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 the system, forcing even more water into streets.
Another issue of importance in Houston’s climate is the urban “heat island” effect. This is a phenomenon in which developed areas with a high density of dark paving, buildings and parking lots experience an increase in outdoor temperatures. The city typically has temperatures 6Ā°-8Ā° F higher than in the surrounding countryside, requiring building systems to work harder in cooling, thus increasing peak electricity demand.
Why so much parking?
Capacity requirements for parking lots in Houston are typically 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 in more detail below:
Trees provide flood control benefits by intercepting and 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 existing 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 significant role in reducing the heat island effect.
Bioretention facilities are engineered versions of rain gardens, which are shallow depressions in the landscape designed to hold water when it rains and then quickly dry-out or drain within one to two days. The vegetation within rain gardens is selected to tolerate and thrive in temporarily submerged as well as dry conditions.
Bioretention is an excellent stormwater management tool that provides detention or storage, reduction in the magnitude of the peak runoff, and improved stormwater quality (or reduction in pollutants in the runoff). This particular strategy is well-suited to roadway and parking lot projects, and is used extensively in various regions of the U.S.
Grassed swales also slow down the peak rate of runoff and provide detention for stormwater runoff. They are most effective around the perimeter of parking lots or between rows of parking.
Soil improvements are an inexpensive way to achieve healthy soil, which leads to healthier vegetation, as well as to reduced stormwater runoff. The ecological function and “sponge capacity” of the soil on-site may be enhanced through soil improvements, which may involve importing engineered soil mixes onto the site. Generally, these mixes have specified ratios of organic materials, sandy soils and minimal clay content.
The low-cost use of a high-quality native mulch material also improves soil health and function, and should be incorporated into all landscaped medians and islands within parking lots.
Permeable pavement is a more expensive yet quite effective stormwater management tool for parking lots. It includes pervious concrete (concrete without fine materials), pervious asphalt, concrete grid pavers, and permeable interlocking concrete pavers. Permeable pavement may function as paving and drainage system all in one by reducing the need for a storm sewer or drainage system or significantly decreasing the size of the storm sewer pipe.
Permeable pavement’s primary stormwater benefit is the reduction of peak runoff rates, as well as providing detention or retention storage. It also provides some stormwater quality benefits. Various permeable pavement products may reduce the urban heat island effect when substituted for traditional asphalt paving primarily due to color and reflectivity.
The overall design and long-term performance of these materials has improved substantially in recent years, provided that recommended maintenance protocols are followed.
Jennifer Walker, PE, Jeff Chapman, AIA, and Julie Hendricks, AIA, are with Kirksey Architecture. They can be reached through www.kirksey.com.
