FLOOD04795

• Designation Number: 98
• County: Garfield
• Community: Glenwood Springs
• Stream Name: Roaring Fork River & Colorado River
• Basin: Colorado Mainstem
• Title: Floodplain Information Report - Roaring Fork River & Colorado River, Glenwood Springs, CO
• Date: 11/1/1977
• Designation Date: 3/1/1978
• Prepared For: Glenwood Springs, Garfield County, CWCB
• Prepared By: Gingery Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

HYDROLOGIC AND HYDRAULIC DETERMINATIO~S

FLood Characteristics

As noted earlier, most of the annual precipitations in the
Colorado River basin upstream from the areas under study occurs
as snow and a deep snowpack accumulates in the higher regions.
General rainstorms can occur in the area from late spring
through late fall, and convective type cloudburst storms can
be expect~d frequently during the summer.
Major flood flows on the Colorado River and Roaring Fork River
result from rapid melting of the mountain snowpack during the
period from late May through early July. Sno~~elt floods are
characterized by moderate peak flows, large volume and long
duration, and marked diurnel fluctuation in flow. Rainfall
on melting Snow may accelerate the rate of snowmelt and thus
augment flood flows.

Cloudburst flooding is characterized by high peak flows and
short duration. Due to their small aerial extent and other fac-
tors, cloudburst runoff would not significantly affect flooding
along strea~s as large as the Colorado and Roaring Fork Rivers.
Flooding from general rain does not constitute a significant
problem. Studies made for the Garfield County Flood Insurance
Study (reference I) show that the 100 and 500 year floods would
result from sn~~elt on thc Colorado River: and from snowmelt
augmented by rain on the Roaring Fork River.
Hydrologic Analysis
The hydrologic ~nalysis previously completed by the Sacramp.nto
District Corps of Enginee~s for the Garfield County Flood In-
surance Study has been adopted for this study and no separate
hydrologic stuoy ""as perfor:nco. For details of the hydrology
of the streams refer to :hc Corps of Engineers study (reference
1),

~draulic A~alvsis
A~alyses of the hydraulic characteristics of the strea~s in
the study area were' carried out to provide ostirr..ates of the
water surface elevations of floods of the selected recurrence
intervals along each stream. The water surface elevations
were conputed through use of the Corps of Engineers' HEC-l
step backwater computer progr~~ (reference 2). Cross sections
for the backwater analyses of the Roaring Fork River and
Colorado River were field surveyed at close intervalS above
and below bridgp.s in order to compute the significant back-
water effects of these structures. Additional cross sections
were secured from existing 1 tt=200', 5 foot contour interval
maps supplied by the City and adjusted by field profiles of
the river and from U.S. Gcological Survey Quadrangle m~ps.

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Table 2

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Historical Flood Peak Discharges

Colorado River, Glenwood Springs, Colorado
(at U.S.G.S. Gage Station 725, above junction with Roaring Fork
River)

Year

Date

Discharge (cfs)

1909
1914
1917
19l8
1921
1928
1938
1952
1957

June 21
June 3
June 19
June 14
June 15
May 31
June 6
June 8
June 8

27,900
28,100
29,400
30,100
2'>,000
27,400
20,900
20,800
18,900

Colorado River, Glenwood Springs, Colorado
(at U.S.G.S. Gage Station 851. below junction l;lth Roaring Fork
River)

Year

Date

Discharge (cfsl

1973

June 15

20,500

Roaring Fork Hiver, Glenwood Springs, Colorado
(at U.~.G.s. Gage Station 850)

Year Dilte Discha~gc (cfs)
InS June 14 17,600
1921 June 14 17,600
1957 July 1 19,000
1973 June 15 a,2aG

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