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<br />51. Vrain Creek Floodplain Update March 5, 1998 <br /> <br />delineation of the floodplain, however,. our work found the following model <br />modifications to be necessary: <br /> <br />. Modeling of ineffective flow areas required adjustment. A number of irrigation <br />laterals ponds, and broad floodplain areas were included in the computation of base <br />flood elevation and floodway width that were blocked out to more accurately reflect <br />conveyance capacities. <br /> <br />. The orientation of cross-sections near the wastewater treatment plant and <br />confluence of the north and south SI. Vrain Creeks needed reconfiguring to simulate <br />streamflow and river current conditions. <br /> <br />The most significant model findings relate to the reach of river above the newly <br />constructed pedestrian bridge between Sta. 152 to Sta. 157. Refinements to the base <br />HEC-2 model resulted in a rise of about 3.0 feet in the general areas. Additionally, the <br />bridge abutments were constructed in the floodway thereby creating an additional rise <br />of about 1.6 ft. The adjacent wastewater treatment plant has historically resided in the <br />floodplain and is protected by earthen berms. The model changes, however, has <br />introduced approximately 10 additional residential structures into the 100-year <br />floodplain. <br /> <br />6.0 RECOMMENDATIONS <br /> <br />The following elements of floodplain management are recommended to be improved or <br />amended in order to best utilize the information obtained by this floodplain study: <br /> <br />. Review the Town of Lyons administrative process to regulate activities in the <br />floodplain and discuss the best manner in which reviews can be conducted to ensure <br />that facilities are not constructed within the floodplain or flood way resulting in <br />excessive impacts within riparian areas. <br /> <br />. Improve building department review and permitting such that Elevation <br />Certificates, Letter of Map Revisions, Letter of Map Amendment applications are <br />properly interpreted and processed. <br /> <br />. Notify homeowners subject to flooding impacts by the St. Vrain Creek to suggest <br />consideration of the purchase of insurance. This would also include distribution of <br />the floodplain maps to local lenders. <br /> <br />. Distribute the accepted HEC-RAS model to the developers of Lyons Valley Park <br />such that it can be updated in accordance with FEMA guidelines and submitted to <br />the Town to demonstrate that base flood elevations will not increase by more than 1 <br />foot by construction of the proposed McConnell Drive Bridge. <br /> <br />. Work with Boulder County and the CWCB for proper incorporation of the model <br />results into an updated flood insurance study. It is assumed that this effort would <br />be sponsored by Boulder and the CWCB for formal acceptance by FEMA during the <br />summer and early fall of 1998. <br /> <br />\\blW _den2\Admin\KIMBERL Y\35600report.doc <br /> <br />4 <br />