Allianz Field is a soccer-specific stadium in Saint Paul, Minnesota for Minnesota United FC of Major League Soccer. The “Loons” have been a MLS franchise since 2017, where they began playing at the TCF Bank Stadium located at the University of Minnesota. The team privately financed construction of the $250M stadium that will seat 19,400, with the ability to expand to 24,474 seats.
The stadium stands on a 35-acre site that was formally a bus barn used by Metro Transit until it was demolished in 2002. A masterplan has been drawn up for the redevelopment of the broad area, including the stadium site and adjacent properties. This may include building a new hotel, office space, and the redevelopment of the existing shopping center.
The stadium is intended to be a state of the art, best in class soccer stadium, and as such incorporated innovative and forward thinking stormwater planning. The City of St Paul and the Capitol Region Watershed District partnered together to ambitiously provide an example of not only meeting stormwater regulations, but also providing stormwater as a resource.
The stadium is ringed by three grassy plazas, and a fourth green space will be placed along University Ave., near the Snelling Avenue Metro Green Line Station. The largest of these green spaces, called the Great Lawn, is a .63-acre green space directly in front of the main stadium entrance. All of the stormwater from Allianz Field and the surrounding new developments will be directed underneath the Great Lawn, where it is captured in two DuroMaxx Rainwater Harvesting (RWH) Cisterns. One cistern uses 1,114 LF of 120” diameter DuroMaxx while the other uses 440 LF of 96” diameter DuroMaxx. Together they provide 850,000 gallons of storage. Both systems utilized 72” manifolds to reduce costs and provide full access to the system. The RWH system had a watertight specification that needed to meet the City of St Paul requirements.
Site plans originally called for a concrete RWH solution, however, the specified system could not meet the watertightness requirements of the City for stormwater re-use applications. Contech’s RWH design team evaluated the watertight specification and was confident the DuroMaxx cisterns with welded couplers at the pipe joints would meet project requirements. In addition to meeting project requirements, the system was highly customized to provide a more cost-effective solution than concrete storage modules.
The DuroMaxx cisterns are pretreated using two CDS hydrodynamic separators. The CDS is a swirl concentrator hybrid technology that uses continuous deflective separation – a combination of swirl concentration and indirect screening to screen, separate and trap debris, sediment, and hydrocarbons from stormwater runoff. In addition to TSS removal, the CDS systems provide 100% trash capture from the design storm events, protecting the downstream RWH systems from gross pollutants, and ultimately extending the service life of the systems. CDS pretreatment upstream of the RWH system also reduces post-processing treatment of the stormwater runoff for re-use.
Captured runoff is treated with ozone, a series of filters, and further purified with ultraviolet light prior to re-use. A water main that surrounds the stadium on the north side will distribute the clean stormwater to other properties for irrigation and non-drinking purposes. Indicative of the environmental enhancements that are part of the design of Allianz Field, 192 mature trees planted around the stadium use the stormwater system for irrigation, and are placed amidst nearly three acres of new public green space and plantings that have been added to the area.
The stormwater system also provides additional storage for rate control, in order to minimize stormwater discharge from the site and meet project detention requirements. An 8’ x 20’ volume based Peak Diversion StormFilter system with two CDS systems used for pretreatment were also provided to meet project water quality requirements and will treat runoff from impervious parking lots. The StormFilters utilize 42 cartridges filled with ZPG™ media, a proprietary blend of zeolite, perlite, and granularly activated carbon to improve the performance of perlite and target organics, soluble metals, and other pollutants. The StormFilter was chosen over an underground sand filter due to ease of maintenance and a proven track record of pollutant removal performance.
The Contech design team provided a value engineering proposal to meet project requirements including watertight specification, and a highly custom layout, meeting and exceeding the performance and cost of the existing solution.
Additionally, the DuroMaxx cisterns were buried approximately 20’ below grade. Contech provided custom structural and buoyancy calculations for the engineer of record to review, saving consulting billable hours and ensuring the structural integrity of the solution. Several iterations of system layout, diameter options, and maintenance access were completed to assist the engineer in meeting all project requirements.
Contech had to provide a solution that fit in the same footprint as the concrete solution, at a lower price, and provide higher performance. Contech’s nationwide team of experts rose to the challenge and provided the solution quickly, keeping construction of the project on time for the April kick-off match.