The system diverts storm water from a 36-in. city trunk storm sewer that drains 250 acres.
Lake McCarrons is a recreational lake used for boating, fishing and swimming within the urban core of the Twin Cities metropolitan area. Located in Roseville, Minn., it receives drainage from more than 1,000 acres of residential, commercial and industrial land, as well as a mix of state, county and city roads before ultimately discharging to the Mississippi River in St. Paul, Minn. As one of five lakes within the Capitol Region Watershed District (CRWD), a local government unit charged with water resource management unique to Minnesota, protecting the lake is a high priority for both the city of Roseville and CRWD.
After several years of high water quality following an aluminum sulfate application in 2004, water-quality monitoring indicated a recent decline in clarity and secchi depth readings. Many storm water improvement projects have been implemented within the subwatershed and lake, but the 753-acre drainage area running through the Villa Park Wetland System (a series of wetland cells created by constructing permeable weirs that retain sediment) remained the largest source of pollutants, which included total phosphorus (TP) and total suspended solids (TSS).
In addition to removing sediment from these wetland cells (completed through hydraulic dredging in 2013), previous studies clearly identified the need to reduce the volume of storm water runoff being funneled through the Villa Park Wetlands. Past analyses also set a goal to annually capture 45 lb of TP to maintain an in-lake summer average TP concentration of less than 33 ppb.
Teaming Up to Go Underground
Open space and available land within this urban/suburban area is scarce. Combining this challenge with the high costs of physically constructing storm water best management practices (BMPs) makes partnerships critical for project implementation. The $1.2 million (design and construction) Upper Villa Storm Water Reuse Project would not have been possible without the combination of public land ownership, two local government funding mechanisms, one small business and grants from two state of Minnesota agencies. Located beneath a parking lot and popular softball field within Upper Villa Park, the 60,000-cu-ft underground storm water infiltration system, combined with a 13,000-cu-ft modular concrete cistern to harvest and use storm water for irrigation, was the result of three years of planning. Heavy construction began in November 2015, with substantial completion due Dec. 31. An aggressive timeline was needed to maintain fall and spring softball schedules for the city’s parks and recreation department, and maintain access and parking for the small business located adjacent to the project.
The system diverts storm water from a 36-in. city trunk storm sewer that drains 250 acres. Pretreatment is achieved through a proprietary baffle and skimmer, then runoff fills the cistern before overflowing into the underground infiltration system. To maximize the total volume of storm water capture, real-time technology is used to actively manage the level of water in the cistern. Prior to a rain storm, software programmed to communicate with weather forecasts and level sensors within the rainwater cistern open an automated valve to drain the cistern into the underground infiltration pipes. The valve closes after the storm to provide water for irrigation of the softball field. This system is completely automated, and is monitored using a web-based dashboard that can override the programmed logic and switch to manual mode for control of the valve if necessary. The use of this technology is estimated to result in an additional 5 lb of TP removal.
Reviewing the Numbers
To determine the effectiveness of infiltration practice pollutant removal, three custom-built pan-lysimeters were installed at depths of 2, 5, and 7.5 ft below the 10-ft perforated corrugated metal pipes. Samples are extracted from these wells and tested for a suite of pollutants and bacteria to determine the fate of the constituents once they leave the infiltration system. Unfortunately, the sample set from the wells in 2016 was not sufficient to make any substantial conclusions.
Partial-year monitoring data was obtained for May to November 2016 and show that 79% of all storm water flows were diverted into the BMP, resulting in 2.55 million cu ft of volume reduction. Preliminary analysis shows that diversion structure modifications are necessary in order to realize the full benefit of the optimized controls, as a large portion of moderate-sized events were observed to bypass the system. TP removed for the monitoring period fell short of the 45 lb goal by 13 lb. This is likely due to the bypass observed during rainfall, but capturing lower flows. Potable water use reduction estimates (1.3 million gal) also were low, as that system was not fully online for most of the growing season. Total rainwater used was 98,000 gal, while 102,000 gal of potable water were used for irrigation.
CRWD will continue to actively monitor the storm water inflow and bypass to determine the actual functionality of the reuse and the underground infiltration system. With continued monitoring, real-time data will be collected that can be used to better manage and design future storm water projects. Maintenance of the overall system will be turned over to the city of Roseville, where staff will monitor the system monthly to determine maintenance frequency by inspecting the pretreatment structures, reuse vault and underground infiltration gallery.
