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Last modified
11/23/2009 2:02:06 PM
Creation date
6/11/2007 1:12:42 PM
Metadata
Fields
Template:
Floodplain Documents
County
Larimer
Community
Larimer County and Incorporated Areas
Title
FIS - Larimer County and Incorporated Areas - Vol 1
Date
12/19/2006
Prepared For
Larimer County and Incorporated Areas
Prepared By
FEMA
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />The hydrology for the Big Thompson River near Loveland was prepared by Ayres <br />Associates. In 1971, the USACE presented flood flow frequencies for the lower portion of <br />the Big Thompson River near Loveland based on statistical analysis of USGS gage data <br />(Reference 13). Those flood frequencies were verified and used for the hydraulic study by <br />Resource Consultants, Inc., which became effective in 1981. For the purposes of the latest <br />study revision, the flood flow frequencies presented in the USACE study were further <br />verified by augmenting the stream flow data with entries from the intervening period of <br />record. An updated flood frequency relationship was developed in accordance with criteria <br />outlined in Bulletin 17B, Guidelines for Determining Flood Flow Frequencies with the aid <br />of the flood-frequency analysis program HEC-FFA, developed by the Hydrologic <br />Engineering Center of the USACE. The updated flood frequency analysis used a <br />systematic record of 80 years. Comparison showed that the effective flood discharges were <br />higher than those from the updated flood frequency but typically plotted within the 90% <br />confidence interval. The effective flood discharges were therefore adopted for this <br />revision. <br /> <br />The hydrology for Dry Creek - BTR near Loveland was prepared by Anderson Consulting <br />Engineers, Inc., utilizing the HEC's Hydrologic Modeling System program, HEC-HMS. <br /> <br />The hydrology for the Big Thompson River near the Town of Johnstown was prepared by <br />Anderson Consulting Engineers, Inc. A thorough review of discharges from the 1970 <br />hydrologic study of the Big Thompson River and the 1974 Floodplain Information Report <br />for the Big Thompson River both completed by the USACE was performed. These <br />discharges were then utilized in the hydraulic analysis of the Big Thompson River in the <br />Johnstown area. <br /> <br />The discharges for Buckhorn Creek, Redstone Creek, and Black Canyon Creek were <br />calculated based upon the regression equations found in Technical Manual No.1 <br />(Reference 10). Parameters needed for the regression equations were taken from U.S. <br />Geological Survey topographic maps at a scale of 1:250,000 (Reference 14), SCS County <br />Soils Maps (Reference 15) and County Land Use Maps Reference 16). For Buckhorn <br />Creek and Redstone Creek, discharges for each design point were calculated for the <br />portions of the basin above 7,500 feet and below 7,500 feet, and the largest discharge at <br />each point was used. <br /> <br />The hydrology for the Boxelder Creek/Cooper Slough area was prepared by Anderson <br />Consulting Engineers, Inc. (Reference 75). The peak discharges for the 10-, 2-, and 1- <br />percent annual chance floods were determined using the MODSWMM model. <br /> <br />The USACE hydrology for the Cache La Poudre River was developed using HEC-l, a <br />general rainfall runoff computer model developed by the US ACE's Hydrologic <br />Engineering Center (Reference 39). The appropriateness of this routing method was <br />verified using historical data for the 1976 and 1983 floods on the Cache La Poudre River. <br />The HEC-l routing method is considered valid to use for the routing of the hypothetical 1- <br />percent annual chance flood. The results of the USACE hydrologic analysis were reviewed <br />and accepted by Larimer County, FEMA, and the Colorado Water Control Board (CWCB). <br /> <br />20 <br />
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