FLOOD02815

• Designation Number: 280
• County: El Paso
• Community: Monument
• Stream Name: Crystal Creek & Dirty Woman Creek
• Basin: Arkansas
• Title: Flood Hazard Identification Report - Crystal Creek and Dirty Woman Creek, Monument, Colorado
• Date: 2/1/1987
• Designation Date: 5/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGIC AND HYDRAULIC DETERMINATIONS

Flood History

No information exists documenting flood history of either Crystal
or Dirty Woman Creeks. Documentation of flooding on nearby Monument
Creek does exist, however (Ref, 2),

Flood Characteristics

"Typical" flooding along Crystal and Dirty Woman Creeks is caused
by rainstorm runoff.

Hydrologic Analysis

Hydrology for both Crystal and Dirty Woman Creeks was obtained
through the Colorado Urban Hydrograph Procedure (CUHP), developed by the
Urban Drainage and Flood Control District, This is the method specified
for ungaged drainage areas greater than 160 acres in the (pre 1 imi nary)
Storm Drainage Criteria Manual for the Town of Monument, Major
parameters used in the hydrologic calculations are rainfall values of
1,90", 2,65", and 3,02" for the 10-, 50-, and 100-year 1 hour rainfalls,
respectively, Soil Conservation Service soil type for each basin is 50%
Band 50% C. Each basin was presumed to have about 15% impervious area,
The 500-year peak flow for each basin was derived by an extrapolation of
a plotting of the 10-, 50-, and 100-year peak flows for each creek on
log-probability paper (see Figure 1).

Results of the CUHP analysis and the extrapolation of the 500-year
peak flow lead to the following results:

Crystal Creek Dirty Woman Creek
Recurrence Interval Peak Flow Peak Flow
(years) (cfs) (cfs)
10 240 1230
50 510 2620
100 640 3360
500 1050 5800

Hydrology for Crystal Creek was consistent with a hydrologic
analysis of the 100-year peak flow performed by Tri-Consu1tants for
Casey' s Subdivision located along Crystal Creek between 1-25 and Beacon
Light Road, Tri-Consu1tants used the Soil Conservation Service method to
determine the 100-year peak flow.

Two of the road crossings examined will back flood waters up
significantly due to the undersized culverts running under them, Beacon
Light Road on Crystal Creek and the Denver Road on Dirty Woman Creek will
both serve as temporary storage reservoirs for the flood waters, A
standard reservoir routing was performed on each of these two
"reservoirs". Through this analysis, it was determined that even at the
10-year peak flow, flood waters would overtop the roads and the peak
flows downstream from the roads would be virtually the same as that
upstream from the roads. Like-wise the 100-year peak flows would not be
reduced downstream from the roads.

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