FLOOD05356

• County: Jefferson
• Community: Jefferson County
• Stream Name: Cub Creek
• Basin: South Platte
• Title: Request for Conditional Letter of Map Revision for Cub Creek
• Date: 6/1/1992
• Prepared For: State of Colorado
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1.

II.

INTRODUCTION

The purpose of this report and supporting documentation is to request a

'Conditional Letter of Map Revision' (CLOMR) to the 1989 published Flood

Insurance Study (FIS) for a portion of Cub Creek which encompasses the area

adjacent to the Jefferson County Public Library just south of Evergreen.

Colorado.

This CLOMR includ,es hydraulic analyses to reflect proposed changes in overbank

topography due to the proposed construction of the new Jefferson County

Library. The affected reach extends from approximately 1,000 feet upstream

of its confluenCE! with Little Cub Creek, south approximately 1,200 feet. The

study area is shown on the attached copy of the existing Flood Insurance Rate

Map (FIRM), Figuz.e A in the Appendix.

Supporting documentation includes hydraulic computations for the existing and

proposed conditions and a calibration run, which demonstrates that our

hydraulic model (HEC-2) is operational.

BACKGROUND

The existing FIRM maps show detailed flooding for Cub Creek through the study

reach under construction (Figure A). The information used for the FEMA Study

was completed by Gingery Associates in 1983. Since that time, Gingery

Associates has gone out of business and no previous project files are

available. A review of County records, records held by the Colorado lIater

Conservation Board, and records held at the Regional FEMA office show that no

hydraulic backup data is available (such as HEC-2 computer runs). An original

work map of the a.rea was located which shows approximate HEC-2 cross-section

locations on Cub Creek (Figure B). Since no HEC-2 data was available, a

duplicate run which matches the FEMA flood profiles could not be assembled.

In order to demonstrate that our HEC-2 model is working properly, we have

duplicated the floodplain hydraulics on another drainageway in Jefferson

County for which hydraulic data was available. This 'calibration run' was

completed on a reach of Upper Bear Creek. The water surface elevations from

the summary printout of our computer run matches the FEMA flood profile

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