FLOOD01898

• County: Larimer
• Community: Estes Park
• Stream Name: Big Thompson, Fall Rivers
• Title: Hydrology Report for Estes Park and Larimer County FIS
• Date: 2/1/1984
• Prepared For: FEMA
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. DATA REVIEW AND ANALYSIS

To verify the results of the previous studies, peak flow data were

collected and analyzed. The annual peak flows for both rainfall and snowmelt

floods for gaging stations in the study region were obtained. This infor-

mation has been published by the USGS for most of the stations used in this

analysis (Reference 4). Additional annual peak flows for rainfall and

snowmelt floods were determined by requesting and searching the original USGS

strip charts for gaging stations in the area. Several stations in the study

region were eliminated due to the limited number of years of record or the low

elevation of the gaged watershed as compared to the study streams.

For the hydrologic analysis, seven stations were used. The station

descriptions are listed in Table 3.1 and their locations are shown on

Figure 3.1. The peak flow data for each station is tabulated in Appendix A.

3.1 Data Analyses

The data collected from the USGS gaging station records were analyzed

according to standard procedure. The procedure is based on the Water

Resources Council Guidelines (Reference 5) and the method described below.

The strip charts for gaging stations in the study area were obtained and

reviewed in accordance with the guidelines presented in Reference 4 to deter-

mine the date of the maximum annual peak attributable to rainfall and the

maximum annual peak attributable to snowmelt. Generally, the snowmelt peak

occurred in Mayor June and had a daily rise and fall over a period of several

days. The rainfall peak was very sharp, with a time to peak of a few hours

for medium- to large-sized basins and less for small basins.

Stages were obtained for the snowmelt and rainfall annual peaks. Station

rating curves were then used to translate the stage to flow rate. The peak

snowmelt and rainfall flow data for each station are presented in Appendix A.

A frequency analysis was conducted on the two sets of peak flow data

collected for the seven gaging stations listed in Table 3.1. The frequency

analysis (Appendix A) were conducted according to U. S. Water Resources

Council Guidelines (Reference 5). For stations with broken records, condi-

tional probability adjustments were used. Similarly, for the North Fork of

the Big Thompson River gage (STA 06736000), the 1976 rainfall peak (8,710 cfs)

was excluded from the frequency analysis for that station. This was done as

the 1976 rainfall peak was determined to be a high outlier by Reference 5