FLOOD03253

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: South Boulder Creek Draft Hydrology Study Phase III
• Date: 2/10/2000
• Prepared For: UDFCD
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Mr, Bill DeGroot
February 10, 2000
Page 2

After Phase II, the University of Colorado joined the effort as a sponsor. The
sponsors directed both a Hydrology Study (Phase III) and a Master Planning
(Phase IV) effort,

We observed several discrepancies in the previous Corp's model during the Phase
I and II studies. The two most important deficiencies include the drainage area in
the model which is significantly less than the actual watershed area, and the peak
one-hour rainfall, which is less than that prescribed by local, regional, and
National Oceanic and Atmospheric Administration (NOAA) procedures. In this
Phase III effort we also found that the Gross Reservoir storage and spillway
characteristics in the previous model were under stated, while the stream
hydraulics and land runoff characteristics were inadequately depicted. A variety
of changes were required to address each of these discrepancies.

During Phase III, we have constructed an essentially new hydrology model using
the UDFCD UDSWMM computer program, but using portions of the Corp's model
as a starting point. The following have been incorporated.

1. Revised rainfall using National Oceanic Atmospheric Agency (NOAA), City,
County, UDFCD, and U.S. Bureau of Reclamation {USBR} procedures.
2. Electronic base mapping using City, County, and U.S. Geologic Survey
(USGS) sources. Basins, streams, and other model features are delineated
on this base.
3. Basin wide review of National Resource Conservation Service (NRCS or
formerly SCS) and U.S. Forest Service (USFS) soils and ground surface
mapping to identify infiltration and other rainfall loss characteristics.
4. Updating and Phase IV model data downstream of Highway 36.
5. Detailed new modeling for the area from Eldorado Springs to Highway 36.
6. Correction of all Corp's modeling upstream of Gross Reservoir, including
redelineation of all basins downstream of Gross Reservoir, and of all
secondary basins upstream of Gross Reservoir.
7. Reasonableness comparisons with Eldorado Gauge Data, Colorado Unit
Hydrograph Procedure (CUHP) modeling of representative secondary
watersheds, unit discharge (cubic feet per second (cfs) per acre), and
regional discharge relationships.

The modeling was performed in accordance with regulatory requirements which
do not permit incorporation of storage that exists in Gross Reservoir below the