FLOOD00385

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: South Boulder Creek Interim Hydrology Study
• Date: 7/1/2000
• Prepared For: Boulder County
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Mr. Bill DeGroot
July 2000
Page 2

We observed several discrepancies in the Corps' SWMM model during the Phase I
and II studies. The two most important deficiencies include: 1) the drainage area
in the Corp model is significantly less than the actual watershed area, and 2) the
peak one-hour rainfall is less than that prescribed by local, regional, and National
Oceanic and Atmospheric Administration (NOAA) procedures. In this Phase III
study we determined that modifications were required for the Gross Reservoir
storage and spillway characteristics, the stream hydraulics, and the land runoff
characteristics. 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 Corps' 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 SCSI and U.S. Forest Service (USFS) soils and ground surface
mapping to identify infiltration and other rainfall loss characteristics.
4. Updated Phase II model data downstream of Highway 36.
5. Detailed new modeling for the area from' Eldorado Springs to Highway 36.
6. Correction of all Corps' modeling upstream of Gross Reservoir and
redelineation of all basins downstream of Gross Reservoir.
7. Reasonableness comparisons with Eldorado Gage 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
spillway and upstream of the rail line at many south tributary locations from the
foothills to upstream of Gross Reservoir. The large floodplains below Eldorado
Springs provide dynamic storage which is not well simulated by UDSWMM.
Under actual conditions, this storage would attenuate the 1 OO-year flood peaks
below the figures in this report for South Boulder Creek downstream of Gross
Reservoir. For this reason and others elaborated herein, we believe this hydrology