FLOOD04855

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 7/1/1986
• Prepared For: Boulder County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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- flOOD RELATED STUDIES -

In March 1984, Simons, Li and Associates, Inc. (SLA) completed a Major
Drainageway Planning Study for the confluence area of Boulder and South Boulder
Creeks (Reference 6). The SLA study ended just above the C&S Railroad crossing
(which is the downstream limit for this study). Results from the SLA report were
utilized to establish starting water surface elevations for the current study.

A "Flood Plain Information" report was published by the Army Corps of Engineers
in August of 1969 (Reference 2). This report included floodplain analysis for
the Intermediate Regional Flood (approximately 100-year flood) and Standard
Project Flood (between a 200-year and 500-year floDd). Peak discharges for these
events were 5,000 cfs and 9,400 cfs, respectively. Flood profiles were developed
based on stream and valley cross-sections surveyed in 1968. Flooded areas were
defined on USGS 7.5 minute series topographic maps with contour intervals ranging
from 10 to 40 feet.

- HYDROLOGIC AND HYDRAULIC DETERMINATIONS -

Hydrologic Analysis

In December 1973, R. W. Beck and Associates published a Major Drainageway
Planning Study outlining major components and preliminary designs for flood
contro 1 improvements along South Boul der Creek (Reference 3). Inc 1 uded in the
R. W. Beck and Associates study effort was the evaluation of hydrology and
hydraulics and delineation of the 100-year floodplain on I-foot contour
topographic mapping. The flood control measures were designed for a 100-year
discharge of 5,600 cfs. The U.S. Bureau of Reclamation's backwater analysis
model was utilized to evaluate the hydraulics.

A hydrologic analysis was completed by the U.S. Army Corps of Engineers to define
the magnitude of the 10-, 25-, 50-, 100-, and SOD-year flood events along South
Boulder Creek. Discharge-probability relationships for South Boulder Creek were
developed using the Environmental Protection Agency's Storm Water Management
Model (SWMM).

In November 1977, Leonard Rice Consulting Water Engineers completed a hydraulic
analysis and floodplain definition along South Boulder Creek and Dry Creek No.2
Ditch in the vicinity of the Flatiron Industrial Park (Reference 4). The report
included evaluation of the C&S Railroad bridge openings on both Dry Creek No.2
Ditch and South Boulder Creek. Results indicated that a split flow condition
would occur with approximately two-thirds of the South Boulder Creek discharge
passing into the Dry Creek No.2 Ditcl1 drainageway.

To model hydrologic response, the 132 square mile South Boulder Creek basin was
divided into 236 subcatchments averaging 358 acres in size. Rainfall input data
for the SWMM model was obtained from the National Oceanic and Atmospheric
Administration's Atlas 2, Volume 3, Colorado, dated 1973. Rainfall hyetographs
having 6-hour storm durations were input into the model for each respective
frequency storm. Adjustments for depth-area and expected probabil ity were
reflected in all hyetographs. The "Standard Project Storm" breakdown
distribution procedure outlined in Civil Works Engineer Bulletin 52-8 was used to
distribute the twelve 3D-minute hyetograph values.

In August 1978, the Federal Insurance Administration pUblished a Flood Insurance
Study for unincorporated areas of Boulder County (Reference 5). Hydrologic and
hydraulic analyses for this study were performed by the U.S. Soil Conservation
Service in August 1974. The Army COE HEC-2 water surface profile model was
utilized in this study with a 100-year flood peak of 5,000 cfs.

Soil infiltration rates of 1.0 inch per hour in the mountains and 0.5 inch per
hour in the plains were used for the pervious areas in the hydrologic model.
Selected loss rates were consistent with other hydrologic studies made in this
region. The Manning's "n" coefficient used for channel and overbank generally
ranged from .045 to .10, respectively.

The impact of Gross Reservoir, a 43,060 acre-foot reservoir constructed in 1955
to provide water conservation storage, was evaluated by routing storm runoff
through the reservoir pool. Significant reductions in peak discharges resulted
with the various frequency routed storms. The 6-hour, 10-, 25-, 50-, and

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