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Washtenaw Commons

Ann Arbor,

Stormwater Management


Washtenaw Commons


Nederveld Associates


Diversified Excavating and Site Utilities


October 2015

The Washtenaw Commons is Ann Arbor’s newest and most modern shopping center, combining existing and new retail space reconfigured into one cohesive shopping center.

The construction of the Commons required an underground stormwater detention and treatment system to meet the Washtenaw County WRC stormwater management requirements for stormwater runoff.

The original design called for three separate detention systems constructed from various sized plastic chamber systems. These systems occupied a large footprint adjacent to the building foundation. 

Representatives from Contech worked with the construction manager to redesign the system using corrugated metal pipe (CMP). Three separate CMP detention systems were installed, providing a total of 60,919 CF of storage.

System one used 1,174 LF of 48” diameter, perforated CMP providing 20,214 CF of storage. System two used 425 LF of 72” diameter, perforated CMP providing 15,525 CF of storage. System three used 687 LF of 72” diameter, perforated CMP providing 25,180 CF of storage.

Using larger diameter CMP with increased storage capacity of the system, resulting in a smaller footprint. The CMP layout saved 25% in land space compared to the chamber system. This reduced both excavation and installation costs.

A CDS hydrodynamic separator was used to pretreat the detention systems and to meet the water quality standards for Washtenaw County. CDS uses swirl concentration and continuous deflective separation to screen, separate and trap trash, debris, sediment, and hydrocarbons from stormwater runoff.

The installation of the three systems happened at different times, depending on the construction schedule, with each system being installed in less than one week.  

Technical Description:

  • 1,174 LF of 48" Aluminized Steel Type 2
  • 1,112 LF of 72" Aluminized Steel Type 2
  • CDS® hydrodynamic separator

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