The Allen Creek is a 19-kilometer (12-mile) urban drainage way that conveys runoff from approximately 14.6 square kilometers (5.7 square miles) of the City of Ann Arbor, Michigan. The downstream reach of Allen Creek is enclosed, which constrains the flow and results in severe flooding (with 100-year flood depths up to 2.5-2.7 meters (8-9 feet)) in commercial and residential areas near its discharge to the Huron River. Much of this flooding is due to the 90-year old enclosure at the creek’s outlet having a full pipe capacity that is only capable of conveying flows from the 50% (2-year recurrence interval) storm. Floodwaters from the Allen Creek are constrained, as a railroad berm prevents surface flooding from flowing directly to the Huron River. To alleviate flooding, the City of Ann Arbor has proposed hydraulic improvements that feature a flood control culvert through the railroad berm that will provide a secondary outlet to the Huron River. These improvements will result in a floodplain reduction of up to 1.6 meters (5.4 feet).
The existing floodplain was officially updated in 2012 using a HEC-RAS model. However, the HEC-RAS model did not adequately account for dynamic routing, increased flow through the Allen Creek enclosure, or surface (flood) storage. Furthermore, the proposed reduction in the floodplain requires a FEMA Letter of Map Revision; using the HEC-RAS model was not appropriate because it failed to adequately reflect the real physical characteristics of existing and proposed conditions. As such, the City’s existing City-wide SWMM-based model of their stormwater collection system was used in the FEMA application to define the revised floodplain and floodway boundaries. This effort required careful documentation, given the significant differences in the modeling framework between HEC-RAS and EPA SWMM.