Washtenaw County, Michigan Water Resources Commissioner Rules and Guidelines: Procedures and Design Criteria for Stormwater Management Systems

Michigan's Washtenaw County Water Resource Commissioner guidelines promote stream channel protection and stormwater management for flood control, and have been updated to address improved water quality and availability.  The guidelines outline the best management practices (BMPs) that should guide the design criteria for green infrastructure installations. The rules address when different BMPs are appropriate, how to design and build them (including calculations for runoff), how to test their effectiveness, and how to maintain them over time.

The report explains how natural hydrologic cycles are disrupted by human and natural changes to watersheds, including the removal of vegetation and topsoil, urbanization, and the diversion of water through pipes and catch basins. The guidelines suggest that stormwater management techniques should be more environmentally sensitive and strive to keep the volume, rate, timing, and pollutant load of runoff the same as it was during pre-development conditions.

To achieve these goals, the guidelines outline a hierarchy of management techniques, placing infiltration techniques that reduce runoff at the top. Retention and detention of stormwater was identified as the second most effective stormwater control, with the guidelines favoring wet techniques over dry techniques. If none of those techniques are possible, conveyance and discharge are suggested as the final option. In general, green and vegetated techniques are encouraged over structural changes.

The 2016 revisions promote using stormwater as an on-site resource, when possible, with strategies designed to better mimic natural hydrology that promoting infiltration.   

The report also provides equations and methods that the Water Resources Commissioner will use to review proposed stormwater management systems. This includes instructions on how to calculate runoff and a description of how various BMPs can reduce the amount of detention or retention required in the project. Section V details the design requirements for a range of stormwater management systems. 

Publication Date: 2014, Revised 2016

Author or Affiliated User:

  • Evan N. Pratt

Related Organizations:

  • Washtenaw County, Michigan Water Resources Commissioner

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  • Agency guidance/policy
  • Best practice

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