Achieving Development Density through Sound Stormwater Design

Pentagon Village is a 12-acre mixed use redevelopment located in Edina, MN that was once the home of the Pentagon Park office complex. The approved development includes a 5-story office building, 2 hotels, 3 restaurant/retail pads, a 3-story parking garage, open space including a plaza, and park areas with hardscaping, fountains, and public art amenities.

Although the site appears to be free of any floodplain or stormwater concerns, the development and surrounding area is actually located within the 100-year floodplain of the Nine Mile Creek which can become severely inundated during large storm events.

In addition to flooding concerns and design considerations, other stormwater-related issues needed to be addressed with the civil engineering design, such as rate control, water quality, and onsite infiltration. Previous concept plans for the site included large ponding areas, but these significantly decreased the potential for development density. Our team designed a site that kept buildings above the floodplain and utilized a 362,000 gallon below-ground vault system that addressed the stringent stormwater regulations of the Watershed District and City. Jay Scott, who led the project development, shares, “The Westwood team worked closely with the regulators in preparing a creative site and stormwater design that allowed us to develop the property to its maximum potential.”

For the below-ground stormvault detention system, the team depended upon the structural capacity of the open-bottomed StormTech chambers to support parking lot loadings while also allowing infiltration into the underlying soils. By utilizing a variety of vault sizes and heights, the design accommodates the fluctuating groundwater elevations and provides vertical separation to protect the underlying groundwater resource.

During design, our experts prepared multiple hydrologic models and scenarios to demonstrate that the project would not adversely impact floodplain levels on adjacent properties. The modeling also showed how the proposed design met or exceeded the stormwater runoff and water quality requirements established by local, state, and federal agencies.

As part of the project approval, the Watershed and City also required an ongoing inspection and maintenance program for the constructed stormwater system. With experience providing these services for a multitude of other sites, our team supplied the Pentagon Village project with an owner operations manual, annual inspections of the below-ground vaults, recording of sediment depths and system conditions, recommendations for cleaning and repairs, and an annual report documenting the inspection. Annual inspections and maintenance will ultimately extend the life expectancy of this system and continue to serve the local water resource it was designed to protect.

Westwood’s land development team manages and solves complex drainage problems for our clients while, at the same time, protecting stormwater quality and the infrastructure serving the public. We’re invested in developing sound stormwater management and design solutions, so we’d love to help on your next project! Chat with our experts.