FLOOD01279

• Designation Number: 215
• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: Flood Hazard Area Delineation - Boulder Creek
• Date: 1/1/1983
• Designation Date: 5/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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- HYDROLOGY AND HYDRAULIC DETERMINATIONS -

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Hydrologic Analysis

A hydrologic analysis was originally performed by the Omaha District, Corps
of Engineers, as part of a Floodplain Information Report for Boulder dated
August, 1969. The hydrology of the Boulder Creek basin was studied during
the preparation of this report and the 100-year discharge was estimated at
7,400 cfs using regional stream gaging records and statistical methods.

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In 1977, the Corps of Engineers prepared a draft report on Boulder Creek for
the Metropolitan Denver and South Platte River Study. The hydrology of the
Boulder Creek basin was restudied in conjunction with this report using the
Environmental Protection Agency's Storm Water Management Model (SWMM). Historical
stream gaging records were used to check the result obtained using SWMM (Figure 2).
The results of the restudy revised the estimated 100-year discharge at Boulder
from 7,400 cfs to 12,000 cfs.

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The final hydrology study accepted by both the Urban Drainage and Flood
Control District and the City of Boulder is contained in the Water and Related
Land Resources Management Study; Metropolitan Denver and South Platte River
and Tributaries; Colorado, Wyoming, and Nebraska; Volume V, Supporting
Technical Reports and Appendices; Appendix H, Hydrolo9Y. The discharge
probability profiles and the 100-year flood hydrographs (Figures 3 and 4)
were extracted from this report.

The hydrologic analysis completed by the COE established peak discharges for
the 10-, 25-, 50-, 100- and 500-year flood events from the west end of Boulder
to the confluence of South Boulder Creek. Rainfall data used in the SWMM
model was obtained from the NOAA Atlas 2, Volume III, Colorado, 1973. (The
NOAA values were adjusted for expected probability). The 500-year values
were estimated by extrapolating on arithmetic-probability paper. A six-hour

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99,9 99.5 99 98 95 90 80 70 6050 40 30 20 10 5

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EXCEEDANCE PROBABILITY IN PERCENT

FIGURE 2 - DISCHARGE PROBABILITY CURVE

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