FLOOD10363 ‎(2)‎

• County: Gunnison
• Community: Gunnison County
• Stream Name: East River, Cement Creek, Alkali Creek, Ohio Creek
• Title: Gunnison County Flood Hydrology, Contract # W99-D-0018 Final Report
• Date: 11/1/2004
• Prepared For: US Army Corps of Engineers, CWCB and Gunnison County
• Prepared By: Tetra Tech, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I 1.0 INTRODUCTION
I Gunnison County (County) is currently preparing floodplain information for use in updating the Federal
Emergency Management Agency (FEMA) Effective Flood Insurance Study (FIS) and associated maps. In
" doing so, the County identified eight river reaches for inclusion in the updated FIS. This report presents
I 'I the analyses used to develop present conditions flow-frequency relationships for the eight stream reaches
" (see Table 1.1, Figure 2.1, and Figure 2.2) in Gunnison County. The study is part of the U.S. Army Corps
,
I of Engineers' Floodplain Management Services Program and is authorized under Section 206 of the
II Flood Control Act of 1960.
I "
II In addition to the work presented herein, a study of the Crystal River (Reach 1) is being completed by the
'I Colorado Water Conservation Board (CWCB). The Crystal River is located to the north of Crested Butte.
II
I It is a tributary to the Roaring Fork River, several miles north of the Gunnison watershed. Due to its
remote and distant location, the Crystal River is not shown on Figure 2.2.
'I
I The Gunnison River, from the confluence with Tomichi Creek downstream to Blue Mesa Reservoir
I Tomichi Creek (Reach 10), is identified as a lower priority creek by the County, the City of Gunnison, and
'I
Ii CWCB, and is therefore not included in this study.
I
I Table 1.1 Study Reaches
Approximate Approximate
Study Reach Drainage
I Reach Length (mi) Area (mi2) Reach Description
I 2 10 70.1 Slate River to confluence with East River
3 1 160.4 East River from the Slate River to Cement Creek
4 6 238 East River from Cement Creek to Alkali Creek
5 7 289 East River from Alkali Creek to the Taylor River at Almont
I 6 6 766 Gunnison River from Almont to Ohio Creek
! 7 5 167 Ohio Creek from Township Line to the Gunnison River
8 2 1,061 Tomichi Creek from the Gunnison River upstream to the Gunnison
I II Airport
9 5 1,012 Gunnison River from Ohio Creek to Tomichi Creek
I
,
,
I This study involves determination of the 10, 2, 1, and 0.2 percent chance exceedence flows for the eight
study reaches. The detailed scope of work is contained in Appendix B. A summary of the steps in the
analysis is presented below.
,I
I " 1. Site reconnaissance.
"
I 2. Develop data sets for the analyses. This task includes reviewing historical flows to assure a
I
! representative annual maxima series is developed by extending the series, as needed, with
I 'I correlations to other gage records.
3. Evaluate drainage area versus flow relationships for the gages on the East River below Cement
Creek and the East River at A1mont to determine whether further review of flows from Alkali Creek is
I , warranted.
I 4. Create data sets of "unregulated" daily discharge for the Taylor River below Taylor Reservoir, Taylor
River at Almont, the Gunnison River at A1mont, and the Gunnison River near Gunnison if creation of
" the data sets is warranted.
I I 5. Complete flood frequency analyses with the data series for each reach.
6. Evaluate analyses results and identify trends. Evaluate whether further analysis that may involve
, snowmelVrainfall runoff modeling or flood wave routing is required.
I I 7. Evaluate the nature of flood events in regards to weather patterns such as snowmelt, rain on snow, or
I cloudburst events.
8. Document the study in a report.
I I
II
I
I
I 1
'I