Board of Adjustment Agenda and Packet 2016 11 16

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Coal Creek Business Park Floodplain Study B. Floodplain 1. Effective Mapping The effective SFHA is shown on the Federal Emergency Management Agency (FEMA), Flood Insurance Rate Map (FIRM), Map Numbers 08013C0583J and 08013C0584J, effective date December 8, 2012. The floodplain is designated Zone AE. A Zone AE floodplain is defined as the base floodplain (1.0%-annual-chance/100-year) where base flood elevations (BFEs) are provided. A regulatory 1.0 -foot rise floodway has been delineated for this reach of Coal Creek. The floodway is defined as the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 1.0% -annual -chance (100 -year) flood can be conveyed without substantial increases in flood depths. See Appendix A for FIRM maps. 2. Previous Studies The effective hydrology and hydraulics for Coal Creek are documented in the Flood Insurance Study Boulder County, Colorado and Incorporated Areas, Federal Emergency Management Agency, Flood Insurance Study Number 08013CV001B, revised December 18, 2012. The FIS includes Summary of Discharges, Floodway Data Tables, and Flood Profiles for the reach of Coal Creek adjacent to the PROJECT. Hydrology and hydraulics for the study reach of Coal Creek were updated by Taggart Engineering Associates, Inc. in 1996. The hydraulic analysis was prepared using HEC -2 and was obtained through a FEMA Engineering Library FIS request in August 2016. It should be noted that the effective modeling was prepared prior to the development of Coal Creek Business Park and does not appear to accurately reflect the existing 1.0% -annual - chance floodplain limits. Martin/Martin, Inc. was not able to determine if any modeling was prepared as part of the original Coal Creek Business Park development. Subsequent hydrologic and hydraulic analysis of the study reach was prepared with the Flood Hazard Area Delineation (FHAD) Coal Creek and Rock Creek, prepared by RESPEC Consulting & Services, dated November 2014. The FHAD study includes hydrologic and hydraulic analysis for Coal Creek and Rock Creek from Boulder Creek to Highway 128 (West 120th Avenue). The FHAD study shows revisions to both the SFHA and BFEs, which are not reflected in the effective mapping. It is Martin/Martin's understanding that FEMA is currently adopting the FHAD study as the effective mapping for Coal Creek through the Physical Map Revision (PMR) process. According to the Urban Drainage and Flood Control District, both the State of Colorado and October 26, 2016 PAARTINirviARTIN Page 3