FLOOD10469

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon Creek, Wonderland Creek
• Basin: South Platte
• Title: Request for LOMR - Boulder
• Floodplain - Doc Type: Correspondence

REQUEST FOR LETTER OF MAP REVISION
FOR FOURMILE CANYON CREEK AND WONDERLAND CREEK,
CITY OF BOULDER WITHIN BOULDER COUNTY, COLORADO

1.0 INTRODUCTION

Community Name:
State:

City of Boulder
Co lorado

1.1 Purpose of Study

The purpose of this study is to update the Flood Insurance Study (FIS) for Fourmile
Canyon Creek and Wonderland Creek within the City of Boulder and Boulder County,
Colorado with a Letter of Map Revision (LOMR). A LOMR is required because:

(1) New and better resolution topographic information has been obtained for the City
compared to the information used for the regulatory flood study;
(2) The hydrologic analysis has been updated to reflect current land use within the
City;
(3) The hydrologic analysis of Wonderland Creek was updated using as-constructed
information and the rating curve for Wonderland Lake obtained from records on
file with the Colorado State Engineer's office;
(4) Vertical datum in the regulatory model was NGVD 1929 and the LOMR updates
this datum to NA VD 1988;
(5) The HEC-2 hydraulic analysis in the regulatory model was updated in the LOMR
using HEC-RAS,
(6) On both Creeks, divided flows which were not analyzed in the regulatory HEC-2
model are analyzed using HEC-RAS; and,
(7) The spill from Fourmile Canyon Creek to Wonderland Creek was quantified
using HEC-RAS and a new hydrologic model developed for the spill and
Wonderland, which was not analyzed for the regulatory model.

The appropriate FEMA Forms (Forms 1, 2, and 3) are included in Appendix 1 of this
document; the hydrologic analyses for Fourmile Canyon Creek and Wonderland Creek
are located in Appendix 2; the HEC-RAS analysis for Fourmile Canyon Creek,
Wonderland Creek and the Fourmile Canyon Creek spill to Wonderland Creek are
included in Appendix 3; and supplemental information is found in Appendix 4; and the
Work Maps are in 24" x 36" print format for this report. A vicinity map is shown on
Figure 1. It was determined from the onset of the two floodplain studies that they should
be combined into one LOMR submittal because of their significant connectivity (flows
leaving one basin (Fourmile Canyon Creek) and diverting into the lower basin
(Wonderland Creek) of the two streams. The decision to combine the studies was
discussed with both Bill DeGroot, P.E. (Urban Drainage and Flood Control District) and
Dave Jula (Michael Baker, Jr. - Denver Office).

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