In August of 2014, Washtenaw County Water Resources (WCWRC) adopted new rules requiring infiltration of the 90th percentile design storm for all new site plans. The rules permit an alternative of traditional detention storage at 120% of calculated 100-year storm capacity only if WCWRC determines that soils testing was adequate and found inadequate infiltration capacity. In 7 years, nearly 250 sites have been reviewed, and this presentation will describe the review requirements, stakeholder engagement process, outcomes, and win-win benefits to water resources AND the development community. Nearly 25% of sites have been able to eliminate detention and infiltrate the entire 100-year storm. By definition, using the 90th percentile storm (1″ in the study area) returns 90% of annual rainfall into the groundwater, a tremendous benefit to urban and suburban rivers, lakes and streams. Third party information will be provided to demonstrate that using the 90th percentile storm also provides more reliable TSS treatment than detention or mechanical separators. In most urbanized areas, communities require stormwater management with one major goal being reduction of impacts due to development. It is well known that detention basins can simulate pre-development runoff rates but they do not simulate pre-development conditions in five (5) areas. First, detention does not address added volume and thus contribute to downstream water quantity issues. Second, reduced infiltration from natural conditions is beneficial to water quality through groundwater recharge for cooler, stream inputs, steadier base flow, and TSS removal. Third, detention basins have fixed capacity and therefore very limited resiliency. Fourth, detention basins sized for 100-year storms tend to require 5-10% of the contributing land area. Fifth, think about every basin you have seen at an apartment complex and how many geese and ‘tootsie rolls’ you see surrounding them. Ewww.

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Contributor(s)

Evan Pratt