FLOOD05348

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek, Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 6/1/1975
• Prepared For: Boulder
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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derived by the Corps of Engineers for the portion of Boulder
Creek i~~ediately upstream of the current study reach is considered
applicable all the way to the Boulder-Weld County Line. Figure 1
is a hydrograph of the design flood used in this study of Boulder
Creek.

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Hydraulics

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Bridges and culverts form obstructions to flood flows. The

usual effect of construction is to raise the water surface eleva-

tion upstream. Bridges are susceptible to structural damage from

the force of flood water and floating debris. Culverts are also

susceptible to flood damage and are more likely to be plugged by

debris. It is probable that if one of these structures were over-

topped by the rapidly moving flood waters erosion and washout

would occur.

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The five structures crossing Boulder Creek in the study reacn
were individually examined to determine what degree of potential
blockage by debris, if any, would be assumed in the flood hazard
area analysis. The factors affecting blockage include the water-
way area of the bridge or culvert, amount of floatable debris in
the reach susceptible to being moved by flood water and relative
location of the structures in respect to each other.

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The culverts at the furthest upstream structure and the bridge

at Mineral Road both experience pressure flow. The other three

structures have clearances sufficient to allow free flow to occur.

In no instances are any of these structures overtopped. However,

all except the Mineral Road bridge have side flow over their

approaches due to overbank flow and lower elevations. Details of

the bridges are shown on the drawings at the end of this report.

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Flood water elevations were determined by using the Corps of

Engineers' HEC-2 computer backwater program. These elevations

are tabulated in Table 1.

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Hazard Area Delineation

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Boulder Creek flood hazard area has an average width of about
3,000 feet throughout the 3.1 nile study reach. Overbank

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LennillG RIG' Cmhl:lll1ln v'v'iJlcr t[1~lneers, inc

---- - - ._- -,----_.~~.---~~------~~-~--------_._---~-~~~-~--~--- - - - - ---~-- -----~ ~ ---- _.- ---

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IOO-YEAR FLOOD HYDROGRAPH
BOULDER CREEK

12

"

32

10

9

Hydrograph from Carps of
Engin..rs'study

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o

" 8
"'
'"
'"
~ 5
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o

4

3

2

o
o

4

6

8

10

12

14 16 18
TIME IN HOURS

20

22

24

26

28

30

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FIGURE!