FLOOD01680

• Designation Number: 126
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: St. Vrain Canyon upstream of Lyons
• Title: Floodplain Information Report - South St. Vrain Canyon upstream of Lyons, Boulder County, CO
• Date: 2/1/1978
• Designation Date: 1/1/1979
• Floodplain - Doc Type: Floodplain Report/Masterplan

II

Exceedence Frequency Per Hundred Years
20 10 5 2 .5 .2 .1
20,000 20,000
A = Sooth St. Vrain Creek at Mouth
8 = South St. Vrain Creek Downstream
! I of Confluence with Middle St. Vrain Creek
C = Middle St. Vrain Creek at Mouth
10,000 D = Middle Sf. Vrain Creek above 10,000
1\ obove Peacefu I Valley
8,000 8,000
6,000 6,000
"
c:
0
II <.>
Q)
U)
.. 4,000 4,000
0-
-
Q)
If
<.>
:a
II ::J
U
.= 2,000
Q) 2,000
'"
~
0
.c:
<.>
UI A
is
-'"
II 0
~ B
1,000 1,000
II 800 800
11 600 C 600
D
II 400 400
11 300 5 300
2 10 50 100 500
Exceedence Interval In Years
II
II
FIGURE 6. DISCHARGE - FREQUENCY CURVES
II
II
-.

stations. Rainfall and snowmelt floods were separated in the frequency

analyses and then statistically combined to obtain the composite flood

flow frequency curves.

For a detailed description of the hydrologic analysis, refer to the
Technical Addendum.

Hydraulic Analysis

Analyses of the hydraulic characteristics of the streams in the study

areas were carried out to provide estimates of the water surface eleva-

tions of floods of the selected recurrence intervals along each stream.

The water surface elevations were computed through use of the Corps of

Engineers' HEC-2 step backwater computer program (reference 3). Cross

sections for the backwater analyses of the south and Middle St. Vrain

Creeks were field-surveyed at close intervals above and below bridges

and culverts in order to compute the significant backwater effects of

these structures. Additional digitized cross sections were secured

when the topographic maps were compiled (reference 1). The locations

of the field surveyed cross sections and the digitized cross sections

from the topographic maps are shown by reference points on the flooded

area maps (Plates 5 through 39) and the flood profiles (Plates 40

through 105). The reference points, 100-year flood elevations and dis-

charges are also listed in Table 3, page 15.

Channel roughness coefficient (Manning's "n") for these computations

were assigned on the basis of field inspection of the floodplain areas.

The ranges of "n" values for channel and floodplain are sunnnarized

below:

Table 5
Roughness Coefficients

IInll

Channel
Floodplain

.030 - .040
.040 - .060

A detailed description of the hydraulic analysis is included in the
Technical Addendum.