Concrete and Stormwater Management Info Sheet

In Brief

New stormwater management approaches are emerging around the country under the banner of Low Impact Development (LID). The basic idea is to maintain or create the pre-development hydrology of a site. The twin goals are to reduce the demand for infrastructure and the maintenance it requires, and to protect water quality by mimicking natural patterns of flow and infiltration. Concrete influences several aspects of LID, among them infrastructure design and the choice of surfacing products. Other strategies, like rainwater harvesting, help reduce the need to use treated drinking water to flush toilets, water vegetation, and wash clothes. Concrete cisterns allow rainwater storage to take place under buildings, parking areas, and patios. 

The Issue

As our cities and towns have grown, so have our challenges in managing stormwater. The result has contaminated our rivers, streams, and coastlines, and hurt the fish and other creatures that live there, including ones we like to eat (or the things they like to eat). It has also increased the risk of flooding, from simply a clogged drain at a neighboring development to rivers topping their banks because they get too much flow too fast. It has also diverted water from recharging the aquifers, creating saltwater intrusion in coastal areas and water shortages in those that depend on groundwater for drinking.

In areas served by sewers, storm and sanitary sewers have often been combined, and when the rain overwhelms the capacity of the treatment plant, the combination of all the sewage, along with the pollution from our pets, cars, litter, and lawn chemicals picked up by the rain, goes untreated into the nearest body of water. In many places this is still happening. Other areas have separated sewers (or invested in separating them) which eliminates the raw sewage overflow, but still carries the polluted runoff directly to the nearest water. The storm water is concentrated so that it scours stream beds and it's often warmer than the receiving water.

To avoid overwhelming storm drain systems, larger developments are often required to create holding areas, ponds to settle out some of the pollution and sediment and slowly release the water over time rather than all at once. Storm ponds are also required in areas not served by sewers. Ponds can work fine, and if soils are good they can be designed to infiltrate water back into the soil. But they take up space and don't necessarily return the water to the ground.

In addition to managing runoff, we are concerned about drinking water availability in most places. Yet we use Class A drinking water to irrigate gardens and lawns, wash cars, flush toilets. Rain water can be collected and routed through designated pipes to provide these services, where clean water is desirable but drinking water quality is not necessary.

What You Can Do

Low impact Development guidelines suggest ways to increase rain infiltration on site. Roofs, roads, sidewalks, and parking lots all lead to runoff that has to be managed. The first step is to reduce these surfaces, designing narrower roads and only as much parking area as needed. This can lead to struggles with development engineering or fire district requirements, but more jurisdictions are working out mutually agreeable solutions: for example requiring sprinklers in a subdivision where fire truck turnarounds are narrower than current code. 

The second step is to make impervious surfaces, pervious. Concrete products can contribute to this is a variety of ways. Concrete roof decks can support green roofs, perhaps with plants, providing aesthetic appeal as well as absorbing most rainfall. Pervious concrete has reduced fine aggregate (sand) so there are voids left between the bigger gravel, allowing water to pass through like a sieve. A gravel base holds the water and allows it to percolate into the soil or collect slowly in a perforated pipe to another infiltration feature. The film that builds up on the gravel surfaces cleans the water. Permeable pavers serve a similar purpose but are laid in blocks and can have varying openings filled with small gravel or even grass for low intensity areas. More variation in color and pattern might be achieved this way.

Pervious paving has other advantages, these two parking lots in Denver were photographed close in time the morning after snow fall and plowing. The asphalt is puddling and can refreeze, whereas the air circulation and drainage of the pervious concrete creates a drier, cleaner, safer surface. Source: perviouspavement.org

 

The third step is to increase the capacity of vegetated or landscaped areas to absorb and hold rain and connect these to the impervious surfaces. Rain gardens, bio-swales, and grassy slopes are all intended to facilitate absorption. The first two combine an excavated area that is filled with layers of compost and soil, sometimes with underlying rock base, and plantings that can tolerate 'wet feet'. The grassy areas also need a good soil amendment like compost to ensure they can hold water; these aren't hard pack with sod on top, but they are used more for dispersion. Pervious pavements may not be appropriate in all applications, but combining them with this type of infiltration feature still assists in achieving the overall strategy. Curb cuts, basically gaps in a standard curb, release water into the infiltration features. Edge treatments are also used to both delineate the drive aisle and help direct water to the appropriate area.

 

Street-side swale and adjacent pervious concrete sidewalk located in the High Point neighborhood of Seattle, Washington.

On some sites, traditional methods of stormwater management make more sense, and concrete will continue to provide critical infrastructure like catch basins. 

Rainwater harvesting simply means collecting and storing rainwater for use when needed. Plumbing codes and water rights law may still present problems in some jurisdictions, but many are working on ways to recognize this important water conservation strategy. Concrete cisterns provide strong storage and can support the weight of a patio, parking lot or building on top. 

A qualified engineer is needed to determine the best overall site design including the integration of various strategies and facilities, holding capacity, and placement. Check with local jurisdictions for their experience in reviewing LID approaches and be prepared to educate them if they are new to the idea.