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Colorado Water Conservation Board Department of Natural Resources 721 Centennial Building 1313 Sherman Street Denver, Colorado 80203 Phone: (303) 866.3441 FAX: (3D3) 866.4474 STATE OF COLOMDO - / .'-. April 13, 1998 Roy Romer Governor lames S. Lochhead Executive Director, DNR Darie$ C. Lile, RE. Director, ewes Mr. Rick Higgins, Deputy Director Town of Frisco Public Works Department P.O. Box 4100 TownHal1 Frisco, CO 80443 Dear Rick: I have reviewed the CLOMR report entitled "Request for Conditional Letter of Map Revision, Tenmile Creek, Frisco, Colorado" dated March 1998 as prepared by McLaughlin Water Engineers, Ltd (MWE) , As you know, the report summarizes the FEMA floodplain mapping history for Tenmile Creek, and recommends hydraulic model improvements and channel improvements in order to remove the River Pines Subdivision from the regulatory 1 DO-year floodplain. The proposed hydraulic model improvements consist of additional channel cross- sections and revisions to the Manning's "n" roughness values. The proposed channel improvements consist of widening Tenmile Creek along the left bank below 4th Avenue for a distance of about 320 feet. I would like to offer some comments on the CLOMR request. MWE created a revised existing conditions HEC-2 model based on additional surveyed cross- sections (CWCB 1996) and updated Manning's "n" values which were calibrated to known flows and high water marks that were measured in the spring of 1997. This new model computed 100- year water surface elevations from Dillon Reservoir upstream to 4th Avenue that were different (up to 1 ,5 feet lower) from the elevations shown in Frisco's effective Flood Insurance Study (FIS). This finding is consistent with model results from a preliminary revised existing conditions run prepared by the CWCB in 1996, The CWCB model was not calibrated to known flows and high water marks, but rather the "n" values were adjusted from the effective FrS model to reflect more realistic channel conditions. MWE's calibration effort is commendable, out I would urge that their revised model reflect a more consistent channel "n" value from station 1+25 to station 17+37. In other words, the physical stream characteristics may not warrant having the "n" value bounce around from as low as 0,030 to as high as 0,076. There are a number of variables that could adversely affect the calibration effort for anyone point along the reach, Therefore, it would seem reasonable to select a representative or average channel "n" value for the whole reach that is based on the calibration results. Once the revised existing conditions HEC-2 model was established, MWE prepared a proposed conditions model based on proposed improvements to the Tenmile Creek channel. "-