Stormwater management design trends

November 2013 » Features » PROGRESSIVE ENGINEERING
MS4 regulations drive retrofitting of stormwater facilities.
Jennifer Rauhofer, P.E.

Stemming from the U.S. Environmental Protection Agency's (EPA) Clean Water Act (CWA), the Municipal Separate Storm Sewer Systems (MS4s) regulations are prompting a new design trend: retrofitting. These regulations are impacting municipalities and property owners around the country, as funding for implementation is now required.

A submerged gravel wetland retrofit was incorporated into a retaining wall repair in Takoma Park, Md.
In Anne Arundel County, Md., a dormant bioretention facility treats runoff from an office building parking lot.

While industry experts are familiar with stormwater legislation, there are a number of moving parts to the larger stormwater discussion. To provide some history and background on the regulations, the CWA was the primary federal law in the United States regulating water pollution. Originally, it was established with the purpose of removing toxic substances in the waterways by 1985. A portion of that law was the introduction of a permitting system – the National Pollutant Discharge Elimination System (NPDES) – designed to regulate sources of pollution.

Sources of pollution, including government and industrial facilities, are not allowed to discharge pollutants into waterways without a permit from the NPDES.

The maximum amount of a pollutant that a water body can receive and still safely meet water quality standards, commonly referred to as the Total Maximum Daily Load (TMDL), is established in the NPDES permitting process. A Watershed Implementation Plan (WIP) is the next step in restoring waterways.

A significant aspect of a WIP can be the restoration of existing impervious areas. Restoration in this case means either the removal of impervious areas or providing stormwater facilities for areas that have no treatment. Local authorities are on the hook to create a WIP that spells out how it works with federal and state governments to achieve and maintain water quality standards while meeting TMDL goals. Frequently, the WIP leads to the need to perform retrofits of stormwater facilities.

Interest in CWA implementation has increased recently, which has sometimes been an inherent hot-button issue within the political arena. However, with greater public emphasis and awareness on environmental stewardship, elected officials have begun to take greater interest in issues impacting the environment, including this one.

A turf grass bioswale treats sheet flow from a commercial parking lot in Ownings Mills, Md.
Urban pollutants collect in the control structure of an underground District of Columbia detention facility.

Retrofitting trends
As a result of the regulations now being implemented, many municipalities are finding themselves in a position of retrofitting properties with a stormwater facility of some sort or bringing the older, outdated facilities up to contemporary standards. The following steps need to take place in the retrofitting process:

Step 1: Identify potential retrofit sites. One of most difficult aspects is finding a location to provide stormwater retrofits, particularly in urban areas. A significant amount of space is required to make a real impact. Due to utility conflicts and a lack of green space in most cities, it becomes difficult to find an adequate amount of land to meet requirements. In the suburbs and rural areas, less development and more open space equates to an easier stormwater retrofitting process. In urban settings, buy-in from the community is important – and often hard to get. Frequently, a facility removes already-scarce parking areas. Following are other options:

  • Modify and update existing stormwater facilities that were designed to only provide flood control. To qualify as a retrofit, the facility should be upgraded to provide water quality by removing pollutants.
  • Add features such as catch basin inserts or tree box filters to existing storm drains.
  • Reconstruct parking lot islands as bioretention/rain gardens or other green infrastructure techniques to filter runoff before discharging into traditional storm drain systems.
  • Integrate water quality treatment into capital improvement projects such as streetscapes and repaving. Green infrastructure such as permeable pavement can be used instead of traditional impervious pavement. Retrofit costs decrease dramatically when incorporated into other planned construction activity.

Step 2: Selectively reduce the number of potential sites. This is a multi-step process. The first phase is to identify as many sites as possible through rapid field assessment. From that point, establish parameters to identify constraints and narrow candidate sites. Through desktop analysis, develop concepts based on sites that make the most sense from a financial standpoint. Typically, determine the return on investment of the project to ensure the greatest "bang for buck." Part of the cost-benefit analysis focuses on privately owned land versus publicly owned land.

Step 3: Develop concepts and preliminary cost estimates. This step will provide decision makers with necessary information they need to conduct Step 4.

Step 4: Select final locations to design and take to construction.

Steps 5 and 6: Design, permit, and construct. An emerging trend is to use a design-build process to reduce costs otherwise devoted to the design and bid process. This differs from the traditional approach where specifications are developed that can go to a public bidding process. The design-build process increases the engineer's control of and involvement in the construction process.

Step 7: Understand operations and maintenance (O&M) issues. The cost to maintain most stormwater systems can be significant when compared with the original construction cost. While design and construction costs are a one-time expenditure, O&M costs are perpetual.

Make regulations affordable
Another major trend is public-private partnerships (P3s). A true P3 is a separate entity where a public agency and private-sector organization are partners. The skills, assets, and motivation of each are shared in delivering a service for the benefit of the public. Increasingly, P3s are being considered as an implementation mechanism. This approach enables both the public agency and private entity to share in the risks and rewards potential in delivering the service.

The P3 model has been used in other situations such as highways and military housing. While proven to be a viable way to implement other infrastructure projects, it is just now beginning to be applied to stormwater management retrofit programs.

Conclusion
In the world of stormwater today, most trends are driven by the MS4 regulations being implemented by the EPA. From retrofitting to new and creative ways to fund infrastructure, the overarching topic of stormwater management will continue to take center stage for quite some time to come as the government, property owners, and stormwater industry professionals all do their part to clean up the nation's waterways.

Jennifer Rauhofer, P.E., is president, and Theodore Scott, P.E., CPESC, LEED AP, is executive vice president and founder of Stormwater Maintenance and Consulting (www.mdswm.com), which specializes in civil engineering, landscape architecture, environmental analysis, maintenance, and construction as it relates to existing and planned stormwater infrastructure.


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