2006-06-12_REVISION - M1982015 (23)

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CONDITIONAL LETTER OF MAP REVISION (CLOMR) FOR • ST. VRAIN LAKES Page 3 The existing topography for the Pelican Shores development, located just upstream of WCR 13, was provided by JLB Companies, NGVD 29. This topography was 4.5 feet lower than the St. Vrain Lakes datum as determined from survey shots on monuments within the Pelican Shores development. This information is reflected in the HEC-RAS model. All elevations listed in this CLOMR request are on NAVD 1988 datum unless otherwise noted. IV. FLOODPLAIN INFORMATION FEMA has adopted the FIRM maps 080266 0855C and 080266 0850C, both dated September 28, 1982. The floodplain is defined as Zone A for the area under investigation. Refer to the maps section in Appendix D for these maps and work map sheets. V. DRAINAGE DESIGN CRITERIA • The regulations, guidelines, and drainage design criteria used are those contained within the Weld County Design Standards and the Urban Drainage and Flood Control District. VI. BASIN HYDROLOGY This reach of the St. Vrain River has had no formal Flood Hazard Area Delineation (FHAD) studies performed to date. The hydrologic data used for the St. Vrain River were developed in an ACOE study in 1977. The hydrologic data uses are also consistent with the peak discharge data from the Longmont Flood Insurance Study published in 1987 by the Federal Emergency Management Agency. A copy of the table from this FIS is located in Section XV. The first cross-section of the "Existing Lakes" reach is located upstream of the first lateral weir, before any flow has entered the lake, however HEC-RAS requires that a flow rate be specified at the top of every reach and therefore a flow rate of 1 cfs was input for the "Existing Lakes" reach to meet this requirement. The following table lists the flows used at the top of each reach with the respective cross-sections. CROSS- SECTION 10 YR. FLOW CFS 50 YR. FLOW CFS 100 YR. FLOW CFS 500 YR. FLOW CFS I-25 TO WCR 13 71770 5,950 12,850 16,700 41,960 EXISTING LAKES 71599 1 1 1 1 FULL RIVER 65735 5,950 12,850 16,700 41,960