FLOOD-INUNDATION AND FLOOD-MITIGATION MODELING OF THE WEST BRANCH WAPSINONOC CREEK WATERSHED IN WEST BRANCH, IOWA

The U.S. Geological Survey (USGS) in cooperation with the city of West Branch and the Herbert Hoover National Historic Site (HEHO) of the National Park Service assessed flood-mitigation scenarios within the West Branch Wapsinonoc Creek watershed. The scenarios are intended to demonstrate several means of decreasing peak streamflows and improving the conveyance of overbank flows from the West Branch Wapsinonoc Creek and its tributary Hoover Creek where they flow through the city and the HEHO located within the city.

Hydrologic (HEC–HMS) and hydraulic (HEC–RAS) models of the watershed were constructed to assess the flood-mitigation scenarios. Both models were calibrated to three historic rainfall events that produced peak streamflows ranging between the 2-year and 10-year flood-frequency recurrence intervals at the USGS streamgage (05464942) on Hoover Creek. The historic rainfall events were calibrated by using data from two USGS streamgages along with surveyed high-water marks from one of the events.

The calibrated HEC–HMS model was then used to simulate streamflows from design rainfall events of 24-hour duration ranging from a 20-percent to a 1-percent annual exceedance probability. The unsteady-state HEC–RAS model was calibrated to represent existing conditions within the watershed. Simulations were ran in the HEC–RAS model with the existing conditions and streamflows from the design rainfall events to serve as a baseline for evaluating flood-mitigation scenarios. Then three different flood-mitigation scenarios were developed with HEC–RAS: a detention-storage scenario, a conveyance improvement scenario, and a combination of both. In the detention-storage scenario, four in-channel detention structures were placed upstream from the city to attenuate peak streamflows. To investigate possible improvements to conveying floodwaters through the city, a section of abandoned railroad embankment and an old truss bridge were removed in the model, because these structures were producing backwater areas during flooding events. The third scenario combines the detention and conveyance scenarios so their joint efficiency could be evaluated. The scenarios with the design rainfall events were run in the HEC–RAS model so their flood-mitigation effects could be analyzed across a wide range of flood magnitudes.