FLOOD02246

• County: Boulder
• Community: Boulder
• Stream Name: Bullhead Gulch
• Basin: South Platte
• Title: Bullhead Gulch Hydrology Report
• Date: 8/22/1997
• Prepared For: FEMA
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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BULLHEAD GULCH HYDROLOGY REPORT

FEMA instructed Taggart Engineering Assoicates, Inc. (TEA) to use the hydrology
model from the Urban Drainage and Flood Control District (UDFCD) Flood Hazard
Delineation Report, by Advanced Sciences, Inc. (ASI), dated July, 1991, as a
base model for the watershed. ASI used the Urban Drainage Storm Water
Management Model, UDSWM2-PC, for routing and CUHPE/PC for rainfall-runoff
calculations. TEA used the updated UDSWM386, because of its capability to
handle more elements and diversion and storage tables than previous versions.
TEA ran tests of the two models to be sure that they would give the same
results. The ASI models used were "bhg100fr.sin,.sot,.chi,.chd,.cho", provided
by UDFCD and dated August 1991.

TEA modified the ASI models four ways (the appendix contains detailed
calculations) :

1. The percent impervious was decreased for areas zoned as Open Space or
agricultural (ASI used future conditions). A field check confirmed that
these areas are not developed.

2. The area downstream of the Colorado and Southern Railroad and upstream
of Arapahoe Road was removed from the models and replaced by the
UDSWM386 and CUHPEPC models for the Indian Peaks Golf Course.
These models were provided by Hurst and Associates, Inc., and are
recorded in their Master Drainage Reoort for the Indian Peaks Golf Course.
for McStain Enterprises, dated April 30, 1992. Please note that the
models were modified since the April 1992 report; TEA used the modified
models dated February 20, 1997: "ta100.sin,.sot", and "tapr.chi,.cho,.chd".
Tuttle Applegate, Inc. provided the SWMM curve for Pond X, upstream of
Baseline Road, as reported in Indian Peaks Detention Pond X. Lafayette.
Colorado, dated February, 1997. TEA modified the Pond X curve to
approximate constructed conditions (Tuttle Applegate's curve described an
ultimate future condition).

3. The storage at Arapahoe Ridge, per Hurst and Associates' Final Drainage
Report. Arapahoe Ridce Filing No. 1. Erie. Colorado, dated February 2,
1996, was added to the model.

4. The larger of the two storage ponds along the railroad tracks in Louisville
(pond no. 206) was removed because the City of Louisville will not commit
to maintain that facility for flood control. The City did write a letter
agreeing to maintain the smaller pond along the railroad tracks (pond 205);
therefore it is included in the FEMA model. The City also agreed to

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