FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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INTRODUCTION

A meteorologic and hydrologic study of the Boulder Creek watershed has
been performed. The objectives of the study are to develop simplified guid-
ance for mountain canyon flash flood warning, and to assess the relative mag-
nitude of flood hazard from infrequent, severe storms that may occur in the
watershed. The study was performed by developing a rainfall-runoff model of
the watershed, and, then,' applying both synthetic rainfalls and transposed his-
toric rainfalls to estimate the corresponding flood magnitudes. Synthetic
rainfalls are based on the design rainfall criteria from the Urban Drainage
and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual. His-
toric rainfalls are based on the reconstitution and transposition of three
historic storms that have occurred along the front range.

The model development, model verification, and meteorologic analyses are
described in this report. The results are presented along with a graph of
peak discharge of Boulder Creek at the mouth of the canyon as a function of
maximum I-hour point rainfall in the watershed. The graph can be used to
estimate flood discharges that can be expected from a forecast of rainfall.

MODEL DEVEWPIIENT

The watershed was modeled using the HEC-l Flood Hydrology Program (U.S.
Army Corps of Engineers, 1988). The model development and selection of model
input is described in this section.

The watershed drainage area and subbasin delineation are shown in Figure
1. This map was provided by UDFCD. The subbasin areas and percent of
directly connected impervious area (RTIMP) are shown in Table 1. The areas
and RTIMP were obtained from file data that was obtained from UDFCD. An RTIMP
of 2 percent for all subbasins was also used in one model run.

The 10-year and 100-year rainfalls were developed according to the proce-
dures in the UDFCD Urban Storm Drainage Criteria Manual. Rainfall depths were
determined for three elevation bands; the low elevation generally from 5,600
ft to 7,500 ft (subbasins 10, 14 and l5), the middle elevation generally from
7,500 ft tp 8,500 ft (subbasins 2, 4, 6, 7, 8, 9, 12 and l3), and the upper
elevation generally above 8,500 ft (subbasins 3, 5 and II). The rainfall
depths were adjusted for area reduction (based on NOAA Atlas 2 depth-area