Due to the patchwork of flood studies in and around the City of Rochelle, mapping of the floodplain and floodway in the Rochelle areas was non-contiguous and inconsistent. To remedy this situation, and expedite future development, the City proactively initiated re-mapping of the floodplain and floodway in the Greater Rochelle Area. The City of Rochelle retained WBK to prepare the supporting documentation to IDNR-OWR and FEMA for a Physical Map Revision for the FIRM maps in and around Rochelle.
As part of this effort, WBK hydrologically modeled the 116 square mile drainage area and hydraulically modeled the 48 miles of streams to calculate floodplain elevations for development of the flood maps and profiles. To account for large expanses of overbank storage and intrabasin flow, the system was modeled using UnSteady HEC-RAS. GIS extensions of HEC-RAS and HEC-HMS were used to effectively input model parameters, and also map the floodplain. Since this was the first mapping for an un-steady floodway in the State of Illinois, significant coordination with IDNR-OWR was required. WBK also worked with FEMA and their mapping coordinator to provide the supporting documentation for production of the flood maps.
The study resulted in a Physical Map Revision for the Rochelle Area resulting in 13 new FEMA FIRM maps for the Greater Rochelle Area. The City plans on using the unsteady hydraulic model as a tool to evaluate future flood reduction alternatives.