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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storm duration and corresponding time-distribution was selected for the
SWMM model based on a study of hourly precipitation data recorded for major
storms in the South Platte River Basin.

values are as follows:

Overbank
Channe 1

.04 to. 20
.035 to .065

A constant loss rate was assumed for all events modelled regardless of
frequency. For the mountainous basin area a loss rate of one inch per hour
was used. The losses assumed for the urban area were based upon the impervious-
ness of each subbasin modelled. The values of percent impervious (Ia) for
each type of land use were taken from the Urban Storm Drainage Criteria Manual
prepared for the Denver Regional Council of Governments by Wright-McLaughlin
Engineers in 1969. For undeveloped high plains areas the loss rate was assumed
to be 0.5 inches per hour.

In certain areas the higher "n" values were used to account for obstructions
caused by existing floodplain development. Some of the larger building
obstructions were coded out of the cross sections.

Starting water surface elevations were obtained from the Omaha District COE,
as the COE is currently preparing an adjoining flood study of Boulder Creek
through the unincorporated area of Boulder County.

Hydraulic Analysis

A hydraulic analysis was completed along the channel of Boulder Creek to
determine water surface elevations for the 10-, 50-, 100- and 500-year flood
events. This hydraulic analysis was completed using the HEC-2 Water Surface
Profile Computer Program developed by the Army Corps of Engineers (Reference 3).

In certain locations, flood flows diverge from the main flow path of the
stream and become hydraulically disconnected from the flows in the channel.
These flows generally spread over large sheet flow areas and are depicted as
shallow flooding areas on the floodplain mapping. The extent of such flood-
ing was estimated from upstream energy relationships and judgement of the
engineer.

A diffusion routing technique developed by the COE, Missouri River Division,
was used for flood routing calculations.

Cross section data for Boulder Creek was obtained photogrammetrically by
digitizing sections marked on the aerial photography of the stream and flood-
plain. These sections were taken from the same photography flown for the
topographic mapping used in this study (Reference 1). The digitized sections
were also supplemented with cross sections taken from this mapping as needed.
All bridge cross sections were obtained from actual field measurements.

Major stream crossings (footbridges not included) were assumed obstructed by
25 percent, allowing for the potential threat of debris blockage. This was
accomplished in the HEC-2 model by lowering the low cord elevation of the
superstructure such that the effective area of the bridge opening is 75 percent
of the actual bridge area according to survey.

Five main stem study reaches were identified for modelling purposes. The
five reaches are described as follows:

Roughness coefficients (Manning's "n") for the channel and overbank areas
were estimated by field inspection of the study area. The ranges for these

Reach No.1. Beginning immediately downstream from the Valley View
Road crossing and ending upstream of the Lower Arapahoe
Avenue crossing (Station 1011+20 to 1099+80).

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