FLOOD11733

• Designation Number: 257
• County: Boulder
• Community: Longmont
• Basin: South Platte
• Title: Proposed Amendments to the FBFMs for Dry Creek No. 1
• Date: 2/1/1985
• Designation Date: 9/4/1986
• Prepared For: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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Introduction

~rhe lower reaches of the Dry Creek No. 1 drainage basin are located at
the southern edge of the Ci ty of Longmont, \o1i th the headwater regi on
extending into the foothi lIs southwest of the Ci ty (see Project
Vicinity Map, Figure 1).

Substantial increases in the areal extent and densities of residential
and ccmnercial develoPTIent in the basin during recent decades, in
conjunction wi th the now grossly undersi zed culvert structures along
the drainageway, has created a considerable hazard of floed inundation
over large areas in the lower reaches of the basin.

Previous floodplain/floodway mapping reports pertinent to this map
amendment subni ttal have been published in a "Flood Insurance Study"
:Eor the City of Longm:mt (FEMA, 1977) and in the "Floodplain
Information and Flocd Control. and Drainage Plan for Dry Creek No.1"
(WRC, 1980). These publicati ons illustrate the extensive floodplain
areas presently existing in the lower reaches of Dry Creek No. 1.

~['hi S J113P arrendment suhni ttal "vi 11 summari ze the detai Is of three
adjacent flood irnproverrent projects, each in varying stages of design
or construction, which will substantially improve the channel
conveyance capabilities of the lower sections of Dry Creek No. 1. The
resultant shrinkage of floodl::;,lain area left existing in the vicinity
of these channel irnprove.ilent projects is the subject of this report.

Coordination and Authorization

1~ "Flood Insurance Study" was initially catlPleted for the City of
Longrront by Gingery Associates in 1977. Revisions to the St. Vrain
Creek analyses necessitated a revi sion of the FIS in 1983 (FEJ'1A,
1983).

Ccmnunity base map materials used in this amendment submittal were
prepared by Parker and Associates (Denver, Colorado) in 1980, prior to
the start of detailed hydrologic investigations of Dry Creek No. 1.
'I'he subsequent hydrologi c study conducted in 1980 by Water Resource
Consultants, Inc. (Denver, Colorado) was authorized by Boulder County,
i,n cooperation with the City of Longrnont and the Colorado \'later
Conservati on Board (OiCE).

'T'he City of Longmont subsequently authorized Taranto, Stanton and
'J~agge, Consulting Engineers ('rST) of Fort Collins, Colorado to provide
the engineering design and construction managerrent for channel
improverrents in the lower reaches of Dry Creek No. 1 in .January, 1984.

llClditional channel improvement projects have been initiated for two
crljacent channel reaches immediately upstream of the section designed
by TST (see Project Layout I'lap, Figure 2). The first of these,
located between the lower section and Hover Road, has been designed by
Charles H. Sain Associates (Birmingham, Alabama) for the developers of
the "'!Win Peaks Hall" site. Another section, located upstream of
Hover Road, is being designed by Rocky Mountain Consultants (Longrront,
Colorado).

The City of Iongmont is presentlv sponsoring thi s map areendment
sul::m i tt a1.

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