Super Size My Stormwater Treatment System, Please!

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Courtesy of BaySaver Technologies, Inc.

Many municipalities have vast areas of developed impervious acreage from which stormwater drains without any form of treatment. These drainage systems were designed decades ago, predating regulatory requirement for nonpoint source pollution control. Every year pollutants from these sites discharge high levels of sediment, nutrients, trash, and heavy metals into local waters, impairing usage, harming wildlife, and ruining aesthetics. Redevelopment projects within the municipality, particularly waterfront projects, represent opportunities to rectify stormwater problems of the past. Large redevelopment projects are generally undertaken by developers with the support of local municipalities as part of revitalization projects. In effect, there is synergy between the public and private sectors to improve the overall quality of these sites. A major part of this improvement relates to reducing the impact of stormwater runoff. Developers are called upon to mitigate stormwater pollution to the maximum extent practicable (MEP) for these redevelopment projects. However, there are typically a number of issues when dealing with stormwater in these 'end of pipe' applications. Generally, there is limited area to build a stormwater treatment structure and the developer must contend with upstream flow and associated pollutants entering the site.

These design constraints often require innovative designs. With respect to system design, off-the-shelf solutions can be a challenge to use because redevelopment projects are typically constrained by physical design parameters, such as footprint, available head, tight hydraulics grade lines, and high flow rates. Therefore, many projects require engineered solutions to meet the design objectives. These engineered solutions represent site-specific designs and are typically adaptations of proven technologies. An excellent example of this type of technology is the XK BaySaver Separation System. The patented XK BaySaver Separation System was conceived to satisfy stormwater applications with treatment flow rates of over 22 cfs (cubic feet per second) and peak design flow rates of over 100 cfs. The system is designed to effectively and economically remove sediment, petroleum hydrocarbons, and gross debris. This is done by gravity separation, flotation, and flow control.

Using the same technology and principles that are incorporated in the standard BaySaver Stormwater Separation System, the XK design is hydraulically scaled to meet the needs of high flow stormwater treatment applications. The XK can be designed either inline or offline. This feature is important for several reasons. An offline design should be considered when the peak design flow is significantly higher that the maximum treatment rate (peak design flow > 400% of the treatment flow). In this case, it may be more economical to bypass those high flows around the treatment system as opposed to designing the system to prevent resuspension. An inline design offers the flexibility to fit the BaySaver BMP into a tight footprint, which often is the case for retrofit and redevelopment applications. The XK BaySaver Separation System has three levels of treatment that vary accordingly to the rate of flow through the system (see Figures 1 and 2). Under low flow conditions, polluted stormwater is treated in series through both compartments. Influent flows enter the primary compartment where, due to loss of velocity, coarse sediment and debris falls out. Flow then passes from the primary compartment to the storage compartment by way of the trapezoidal weir, which is cast in the center baffle wall. This weir acts a flow control and limits the amount of flow that can enter the storage compartment. After going through the weir, the flow is further slowed in the storage compartment, which in turn, allows for greater separation of fine sediment and floatables. Flow exits the storage compartment by way of the return pipe, which is attached to the outlet control box. Once the flows reach the outlet control box, it passes to the outlet.

As flow rate increases, the water surface elevation rises in the primary compartment and flow begins to pass up the hood in the outlet control box and through the hood weir. This hood weir acts the same way as the T-pipes in the standard BaySaver Separation System by conveying flow from the center of the water column in the primary compartment, thereby ensuring that floatables and some of the suspended sediments are still being transported to the storage compartment under this moderate rate.

During very large storm events and when the XK BaySaver Separation System is installed online, the water surface elevation in the primary compartment will rise to the point that it will begin to crest the top of the outlet control box. Although some treatment of sediment and debris is still occurring at this stage, the design goal is to convey the flow through the system as quickly as possible without creating a backwater on the influent pipe. Through the use of three cases studies, this paper will highlight redevelopment projects from around the country where site specific engineered stormwater treatment solutions were design to meet difficult site conditions. The XK BaySaver Separation Systems described are some of the largest in the nation.

Asbury Park Redevelopment, Asbury Park, New Jersey

The City Council of Asbury Park decided in 2002, by a vote of 5-0, to approve one of the largest waterfront redevelopment projects in New Jersey. The redevelopment of Asbury Park will cost an estimated $1.2 billion and will take place over the next 10 years. Plans include the construction of as many as 3,000 residential units and 450,000 square feet of commercial space. Schoor DePalma was selected as the civil engineer firm and charged with designing effective stormwater treatment systems for the site. Given that this redevelopment was to take place on the coast, in a highly urbanized area, there were several unique challenges presented. High ground water required extensive use of shoring as well as pumping, copious utility conflicts, and tight hydraulic conditions all combined to require custom structural solutions. Due to the coastal discharge requirement, the project also needed Coastal Area Facilities Review Act (CAFRA) approval. To satisfy the CAFRA permit requirements, the selected stormwater treatment technology had to be approved by the New Jersey Corporation for Advanced Technology (NJCAT); a quasi-public/private firm that performs performance verification of promising environmental technologies. XK BaySaver Separation Systems were selected for this project as the result of a timely verification by NJCAT, as well as for the site-specific adaptability of the design. Eleven separate treatment systems were needed to process the total drainage area of the redevelopment. Three systems had treatment rates between 27 and 48 cfs of flow. By using high flow rate systems; the design engineer was able to reduce the total number of treatment structures by six, which was extremely advantageous given the previously described site conditions.

Branson Landing, Branson, Missouri

Branson Landing is anticipated to draw 4.5 million visitors annually. This $300 million master-planned project is one of the largest redevelopments in the Midwest, with nearly 450,000 square feet of retail space as well as luxury condos. Built on 95 acres, with 1.5 miles of waterfront, the Branson Landing will revitalize the Citys waterfront community. HCW Development Company and the City of Branson moved forward on this project with the goals of increasing tourism activity, stimulating growth, and adding appeal to the city of Branson as a leisure destination. As part of the development requirements, public and private stakeholders agreed to provide stormwater treatment before discharge into Lake Taneycomo. The proposed treatment was a first for the City of Branson. Three separate storm drain lines with waterfront discharge required treatment. The proposed storm drain system for the site had to be designed to convey flow coming from the new development as well the additional flow generated upstream in Branson. With two-year flow rates in the range of 90 to 267 cfs, the required treatment capacity significantly exceeded the range of standard separator systems. BaySaver Technologies designed three custom XKs with treatment rates of 26, 33, and 90 cfs. These treatment units were sized to provide 80% annualized removal of suspended solids. The installation of these systems in 2005 garnered significant private and public interest. Representatives of the Missouri Department of Natural Resources and the City of Branson, as well as members of the local press were on site during installation. Not only were these systems some first in Branson, they were also the largest systems to be installed within Missouri.

Village at Shirlington, Arlington, Virginia

The Village at Shirlington is an excellent example of a joint public-private revitalization project located in Northern Virginia. Boasting 135,000 square feet of retail space, over 400 new condo and apartment units, and parking for 1,500 cars, this project was the realization of a vision shared by Arlington County and Federal Realty Investment Trust. The goal was to increase the density of the development through the addition of housing, while attracting South Arlington residents during the weekend by offering a new cinema, high-end shops, and award winning restaurants. BaySaver Technologies was contacted by the firm Dewberry and asked to provide a solution to help mitigate the non-point pollution impact of the future project. The design goal was to provide treatment of 65 cfs of flow and to retrofit the stormwater treatment system into an existing 42 drainage line. Site conditions further required the system to fit within a very tight footprint, confined on one side by a busy road, and on the other by a new building. To accommodate these conditions, the system would have to conform to the available footprint and allow the drainage system to stay live during construction. The XK BaySaver designed for the project, the first of its kind by BaySaver Technologies, required a concrete vault 22 long and 14 wide. This structure represented the largest piece ever fabricated by Americast, an East Coast precast provider, and allowed for quick installation. This saved the site contractor valuable time over a traditional cast-in-place solution and by using a single large structure solution as opposed to several smaller systems.

Summary

Ultra-urban redevelopment projects represent environmental opportunities to incorporate progressive development solutions for the betterment of all. In many cases these projects are the product of collaborative public/private endeavors with the intent of accomplishing a greater good. However, given the nature of redevelopments, innovative site solutions are often required due to existing conditions. Stormwater treatment is no exception. It takes careful planning, smart engineering, and adaptability to translate the potential of an opportunity into reality. Through the use of technologies such as the XK BaySaver Separation System non-point pollution sources can be economically mitigated, extending revitalization not just to the human element, but also to the environment.

 

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