FLOOD07708

• Designation Number: 317
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: Bullhead Gulch and Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Bullhead Gulch and Tributaries
• Date: 7/1/1991
• Designation Date: 11/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

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At the downstream limits of this study, the peak lOO-year runoff is 5,830 cfs for future
conditions. This results in contributing runoff volumes of 1.02 cfs/ac for the future basin
conditions.

4.0 HYDRAULIC ANALYSIS

4,1 PREVIOUS STUDIES

There are no previous studies which defIne the l00-year floodplains of Bullhead Gulch or its
tributaries. There is a Flood Hazard Area Delineation (FHAD) (Muller, 1983) study for Boulder
Creek located at the downstream limits of this study.

4,2 FLOODPLAINS

A hydraulic analysis was performed for all the major drainageways to identify flooding problems
and hazards. Cross-sections were digitized from aerial topography (Landmark, 1990) at 300- to
500-foot intervals. Water surface profIles were calculated using the V,S. Army Corps of
Engineers HEC-2 water surface computer model (USACOE, 1988). The fInal floodplain
delineations and water surface profIles are included in the Appendix of this report. Copies of
this FHAD report will be available for examination at the Engineering Departments of Louisville
and Lafayette, and at the offIces of the UDFCD and CWCR.

All major culverts and bridges within the study reach were fIeld inspected and measured, All
water surface elevations for stream crossings modeled using HEC-2 were based on hand
calculations and were assumed to stay in place during the lOO-year storm event, Small culverts,
36-inch diameter or less, were assumed ineffective due to debris blockage. Flow separation was
assumed for all irrigation ditches and no flow diversions were considered,

Mannings roughness values ranged from 0.03 to 0.04 in channel areas and 0.04 to 0.45 in
overbank areas (SCS, 1963). Photographs of the channel and overbanks were taken to document
the conditions for which the roughness values were assigned. These photographs are available
for review at the offices of UDFCD and the ewCR

FLOOD HAZARD AREA DELINEA nON
BUWlEAD GULCH AND lRIBUTARIES

FINAL
7131191

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