FLOOD09573

• County: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

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SECTION III

PAST HYDROLOGY MODELING REVIEW

INTRODUCTION

The principal objective of this Phase II effort was to prepare new modeling for the
South Boulder Creek spill at Highway 36 and the West Valley Overflow
downstream to the Wellman Ditch.

The best way would have begun with the identification of a watershed subbasin
delineation map with a corresponding model schematic. Also, Phase 1 questioned
many of the parameters used in the Corps' SWMM model, noted that there were
gauge data upstream of Eldorado Springs which presumably had been used for
calibration purposes, and concluded that model verification studies probably had
been conducted and should be reviewed.

A formal letter requesting such information was sent to the Omaha Corps of
Engineers Office, A reply letter provided further study data such as rainfall; basin
of infiltration, percent impervious; and model sensitivity testing for the Boulder
Creek watershed, Unfortunately, no watershed map for the South Boulder Creek
watershed area below Gross Reservoir was received. Therefore, major and some
minor subbasins were delineated sufficient to correspond to the model data
provided by the Corps.

BASIN AND SUBBASIN DELINEATION AND COMPARISON

Detailed subbasins were delineated on the 1993 City topography and on the latest
versions of the USGS quadrangle maps, Major nodes were identified by reviewing
general area magnitudes of basins and subbasins, identifying subbasins that had
unique and larger magnitude areas such as Martin Gulch below Gross Reservoir,
South Draw below Scar Top Mountain which joins South Boulder Creek above the
gaging station; and Shadow Canyon, Bull Gulch, and Dowdy Draw below Eldorado
Springs. Figure 111-1 delineates many of these features, For basins above Highway
93 directly tributary to South Boulder Creek the model schematic and model
subbasin areas corresponded, with a slight overage in area by the Corps for a few
subbasins in and just above Eldorado Springs (380 acres),

Unfortunately, the subbasin basin areas and total area downstream of Highway 93
did not check, As best as we can decipher, changes to some basins were made to
the model, as indicted such basins being out of order in the input data under a
"change card" that modified other hydrologic characteristics. The model appears to

III - 1