FLOOD00155

• County: Boulder
• Community: Boulder County
• Stream Name: Upper Boulder Creek, Fourmile Creek
• Title: Floodplain Information Report
• Date: 12/1/1981
• Prepared For: Boulder County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Rainfall input for the hydrologic model was derived from the National
Oceani c and Atmospheri c Admini strati on's Atl as for Colorado (Reference
10). This data was adjusted for expected probability. Values for the
500-year stonn were extrapolated. .After study of the various rainfall
durations contained in the Atlas, a six-hour duration was selected. A
30-minute distribution of the six-hour stonn was developed from a study
of hourly precipitation data recorded for major streams in the South
Platte River basin and refined to 30-minute values. The precipitation
values used in the model are shown in Tables 2 and 3.

Loss rate information for the hydrologic model was based upon

infiltrometer studies by the u.S. Forest Service in forest areas, mixed

. forest and grassland areas, and grassland areas for mountain terrain in

the South Platte River basin over a period of years. The results of

these studi es showed average i nfi ltrati on rates in inches per hour for

pine forest, pine forest and grassland mixed, and grassland of 2.37,

1.94, and 1.5. Infiltration studies conducted for the Missouri River

Basin Framework Study indicated infiltration rates of about 50% of

i nfiltrometer studi es for pl ai ns areas. Based upon these studi es, an

TABLE 2

infiltration rate of one-inch per hour was selected for the Upper Boulder
Creek and Founnile Creek basins.

Founnile Creek

Average Basin Rainfall Depths for Six-Hour Stonn Duration at Various
Frequencies.

1.9
2.8
3.2
4.3

Hydraulic Analysis

Water surface elevations for the 10-, 50-, 100-, and 500-year floods

were computed using the U.S. Anny Corps of Engineers step backwater

computer program HEC-2 (Reference 11).

No high water marks were located from which to calibrate the

hydraulic model on Boulder Creek or Founnile Creek. Stream gauging data

collected at the U.S. Geological Survey stream gauge on Boulder Creek at

Orodell was used to compare results from the hydraulic model. No

adjustment was made to the computer model for the gauge data si nce the

gauge data consisted only of discharges much lower than those analyzed in

Frequency

1 D-year
50-year
1 DO-year
SOD-year

Depth
(Inches)

TABLE 3

Boulder Creek

Average Basin Rainfall Depths for Six-Hour Stonn Duration at Various
Frequencies (inches).

Frequency

10-year
50-yea r
100-year
SOD-year

Depth
(Inches)

this study.

Manning's "n" values ranged from 0.060 to 0.1 in the channel on

1.7
2.5
2.8
3.8

Boulder Creek and from 0.065 to 0.080 in the channel on Founnile Creek.

Overbank "n" val ues of 0.080 were used. Channel "n" val ues were computed
using the fonnula "n" = .22 (S+0.001)0.33 where S is the channel slope
(Reference 12).

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