Sidestepping stormwater: The newest approach to managing urban stormwater is mitigating it

By John Buck

​Stormwater management, associated flooding and combined sewage overflow (CSO) problems are in the cross-hairs of federal, state and local environmental regulators, environmental organizations, political leaders and the general public. This puts local governments under pressure to stay current with best practices for protecting watercourses from the effects of stormwater, as well as for managing the risk of flooding.

Stormwater management problems are most acute in the downtowns of urban areas where hard surfaces such as roofs, streets and parking lots abound. Many of these areas lack effective, robust combined storm and sewage systems to manage wet weather, resulting in combined sewage in excess of treatment capacity being directly discharged to waterways without treatment as CSOs.

Although urban stormwater cannot be avoided, it can be mitigated by using available practical solutions to reduce the quantity of runoff water during peak rainfall events.

Green roofs and other green infrastructure absorb excess stormwater 

A promising and appealing technology for reducing the size of the stormwater problem is vegetated roofs. Now a mature technology, it boasts a wide range of reliable systems, and an increasing number of contractors, architects and engineers are familiar with the idea. This makes green roof installations practical and affordable. In addition to managing stormwater, green roofs provide evaporative cooling in the summer months, extend the life of roof membranes and provide park-like rooftops that benefit tenants and neighbors.
    
In some cases, cisterns and rain barrels are part of the mix. These elements collect rainwater from the roof (which helps with diversion), and provide no-cost water to keep the green roof or other landscapes healthy during dry spells, as well as other beneficial options for re-use that reduce potable water consumption. 
    
End-of-pipe stormwater infiltration systems, known as rain gardens, bioretention, and/or bioswale systems, act like artificial wetlands to enhance groundwater recharge and support verdant plant communities. They also reduce the quantity and intensity of stormwater runoff and provide treatment to improve water quality.

Retention ponds or invisible structures that leak on purpose

Stormwater bioretention ponds have sprouted up beside many freeways, particularly around cloverleafs and interchanges. Many suburban parking lots have them, too. In the downtown cores of cities, however, such wide-open spaces are scarce. 

As a result, underground retention structures have been growing in popularity. These systems retain the water so it can be released gradually to underlying storm sewers after the storm event is over. They also collect sediment and help filter pollutants such as oil and antifreeze so they do not flow untreated into watercourses.

One of the trends in underground stormwater retention ponds is to design them with a deliberately leaky bottom so that the stormwater slowly infiltrates into the ground, filtered by the soil as it joins the rest of the groundwater flow.

Another trend is to be more imaginative regarding where the underground holding facility is located. If there is no convenient place close to the building from which the stormwater flows, the tank or similar structure can be located some distance away.

Another form of “invisible” stormwater management infrastructure involves tanks that are placed beneath a parking lot. After the parking lot area is excavated deeper than is required, structures much like milk crates are placed beneath where the paved surface will be. This system collects stormwater and diverts it into the holding tanks for gradual release.

Helping it happen, through hard numbers

One of the biggest benefits from these fast-maturing technologies is that there is a wide range of installations whose performances can be quantified. It’s now increasingly possible to tell a property owner, “If you install a green roof, your cooling bills will decline by XX percent, and your water consumption by YY percent.”  

It’s also increasingly possible to put hard-number demands on property owners; to be able to say, “Your building is producing ZZ gallons of stormwater per year. We’ll be adding a surcharge to your taxes to pay for treating that stormwater. We can also help you find ways to reduce that flow.”

Well-qualified external expertise can help local government leaders develop wise policies that utilize the latest ideas about stormwater management to select the most cost-efficient and effective tools to remove stormwater runoff.

John Buck is a Project Manager with Civil & Environmental Consultants, Inc. (CEC), based in Pittsburgh, Pa.  He has 32 years of consulting experience applying biology and soil science to restore disturbed land at substantially reduced cost and with minimal exploitation of natural resources.  Contact him at (800) 365-2324 or [email protected].

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