FLOOD02581

• Designation Number: 557
• County: Gunnison
• Community: Unincorporated Gunnison County
• Basin: Gunnison
• Title: Hydrology Addendum
• Date: 2/1/2002
• Prepared For: Gunnison County and CWCB
• Prepared By: Montgomery Watson Harza
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrololdc Analvsis
The Gunnison County PIS presents both detailed and approximate floodplain delineations.
Detailed delineations were developed using standard hydrology methods and are mostly well
documented. Hydrologic information used for the FEMA approximate floodplain delineations
are not available or were not documented by FEMA. However, the Colorado Water
Conservation Board (CWCB), using regional regression methodologies for computing l00-year
flood-flow values, has developed acceptable hydrologic information for undeveloped basins.
The CWCB regional regression equation methodology is fully described in a publication by the
CWCB entitled "Guidelines for Determining 100-year Flood Flows for Approximate Floodplains
in Colorado". Due to regional differences in flood flow approximations, three separate equations
were used in the county. The CWCB regression equations are shown in Table 2.

T bl 2 R' I R

E

t: U d 1 dG

C

a e . el!iona eereSSIOn ~auatJons or n eve ope unDlson ountv
Sub- Discharge Equation Drainage Basin
Re2ion ID Sub-Re2ion Name (cfs) Limitations (sq.mi.)
COL-3 Lower Gunnison River Ql00 = 180.8(A)o.578 2 to 8000
COL-4 Upper Gunnison River Ql00 = 50.1(A)o.838 20 to 300
COL-5 Tomichi Creek Ql00 = 84.7(A)o.487 100 to 1070

For the equations in Table 2, the Ql00 is the loo-year peak flow in cubic feet per second, and A is
the upstream (contributing) drainage area in square miles. The regression equations are only valid
for the range of drainage areas as shown. Calculation worksheets are shown in the appendix.
The 100-year peak discharge for each of the undeveloped approximate floodplains is shown in
Table 3.

a e . u 0 cu at ISC arees
Drainage IOO-yr
Flooding Source Drainage Area Description Area (mi2) Discharge (cfs)
Alder Gulch at mouth 18.51 Outside Bounds
Alkali Creek at mouth 13.62 Outside Bounds
Anthratice Creek at mouth 49.37 1,700
Beaver Creek (Gun. basin) at mouth 28.34 1,200
Beaver Creek (Cebolla basin) at mouth 15.19 900
Big Blue Creek Upstream of T 47 N R 5 W, 58.39 1,900
Secti on 23
Cabin Creek at mouth 16.34 Outside Bounds
Carbon Creek at mouth 25.52 800
Castle Creek at mouth 22.37 700
Cebolla Creek Upstream of Beaver Creek 256.76 4,500
Upstream of Goose Creek 342.08 5,300
at mouth 380.48 5,600

T bl 3 S

f CaI I ed D' h

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