FLOOD10473

• County: Gunnison
• Stream Name: Crystal River
• Basin: Gunnison
• Title: Crystal River Study Correspondence
• Date: 6/25/2004
• Floodplain - Doc Type: Correspondence

1..

..

Mr. Tom Browning
Page 3

5. Hydraulic Analysis: Based on the flows generated in Task 4, a hydraulic
evaluation of the Crystal River will be completed for the 10-, 50-, 100- and 500-
year flood events. The project reach will include the area from approximately
County Road 3 at the U.S. Forest Service Boundary (downstream of the Town of
Marble) to the upstream limit, which is just downstream of the confluence with
Lost Trail Creek. The study reach is approximately 4 to 4.5 miles long. The
mapping that has been obtained by ICON from the CWCB for the study appears
to be approximately 600 feet short on the upstream limit. Therefore, the final
study reach will be determined based on the actual digital mapping available for
the site.

HEC-RAS will be used to complete the flood profiles for the four specified
frequency events. The depth of the river at the time of mapping will be estimated
based on the time of year, stream gage records, and field elevation checks as best
possible. The channel invert will be adjusted accordingly on each cross section
and input into the HEC-RAS model. A I-foot rise floodway will be completed
for the 100-year flood event utilizing methods 1 and 4 encroachment options.

The 100-year floodplain limits and the I-foot rise floodway for the Crystal River
will be delineated on the topography and prepared in GIS format using PEMA's
DPIRM Guidelines and Specifications. Flood profiles will be prepared in FEMA
format for the 10-, 50-, 100-, and 500-year events. Tables will also be prepared in
PEMA format for presentation in the report.

6. Report: A final report will be prepared that meets the minimum requirements for
submittal to FEMA for review and possible inclusion as a future floodplain map
revision. The report cover and layout will be approved by the CWCB prior to
finalizing the document. The report will be 8.5" x 11" comb-bound with 11" x
17" fold-out floodplain maps. Maps for the report will be prepared at a scalle of
1" = 200'. Pinal report text will be in Microsoft Word format.

7. Meetings: Four meetings will be completed for the project. Two meetings will
be held with the CWCB at our office to review the hydrologylhydraulic
evaluation and floodplains. Two other meetings will be held with the project
sponsors. One meeting will be in the Town of Gunnison to discuss the results
with representatives of Gunnison County. The second meeting will be in the town
of Marble to present the floodplain data to the public. It is envisioned that
transportation will be by car to both of the out-of-town meetings.

8. Deliverables: ICON will prepare and submit the following items for this study:

a. Two hard copies of draft report (black and white)
b. Four hard copies of the final report with final floodplain maps
c. Two CD's containing PDF files of final reports and maps in their
entirety, plus final digital text, graphics and maps in their original