FLOOD10390

• County: Boulder
• Community: Boulder
• Stream Name: Fourmile Canyon
• Basin: South Platte
• Title: Fourmile Canyon Creek - Major Drainageway Planning - Phase A Report - Alternatives Analysis
• Date: 6/1/2000
• Prepared For: UDFCD, Boulder
• Prepared By: Love & Associates
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

published following selection of the preferred alternative. The Phase B report will serve as planning tool
for the Project Sponsors and private development within the Fourmile Canyon Creek floodplain.

FOURMILE CANYON CREEK
MAJOR DRAINAGEW A Y PLANNING
PHASE A REPORT
ALTERNA TIVES ANALYSIS

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Back2round Information

The study area, shown in Figure 1-1, includes approximately four (4) miles of the Fourmile Canyon
Creek floodplain from the mouth of the canyon to just downstream of the Boulder-Longmont diagonal
(SH 119).

EXECUTIVE SUMMARY

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Major drainageway planning documents were previously developed for Fourmile Canyon Creek by
Greenhorne & Q'Mara, Inc in 1984 and 1987. Due to recent development that has occurred in the
floodplain and inaccuracies in the original studies the City of Boulder commissioned a re-study of the
Fourmile Canyon Creek Floodplain in 1997. Love & Associates was retained by the City to complete a
re-study and draft results were presented in Fourmile Canyon Creek Floodplain Study and Letter of Map
Revision, dated May 18, 1999.

mitigate existing

to

The development, evaluation, and recommendation of floodplain alternatives
flooding within the Fourmile Canyon Creek floodplain are presented in this report.

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The draft restudy indicated significant problems with the original study, resulting in additional properties
located within the Fourmile Canyon Creek Floodplain. Of particular importance, the re-study identified
reaches of Fourmile Canyon Creek west of 19th Street, where, once the channel capacity is exceeded,
flood flows overtop the south bank of the creek and flow in a southeasterly direction towards
Wonderland Creek. These "spill flows" result in a reduction of flows in Fourmile Canyon Creek
downstream of 19th Street but also result in an increase in the flows in Wonderland Creek. During the
lOa-year flood, approximately 3,300 cubic feet per second (cfs) is in Fourmile Canyon Creek near the
mouth of the Canyon, of this amount approximately 1,600 cfs will overtop the south bank spill and flow
to Wonderland Creek. The remaining 1,700 cfs will remain in the Fourmile Canyon Creek Channel.

This effort is jointly sponsored by the Urban Drainage and Flood Control District (UDFCD) and the City
of Boulder (City). Love & Associates, Inc. was the lead consultant for this project that, in cooperation
with the Project Sponsors and Anderson & Company, has prepared this alternative evaluation report.

Purpose and Objectives

The purpose of the study is to analyze the existing and future drainage conditions with the Fourmile
Canyon Creek floodplain, develop alternate drainageway planning concepts to mitigate existing flood
damages, and prepare a preliminary design of an alternative selected by the Project Sponsors

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The study is divided into two phases. The first phase (Phase A) of which this report is a part covers the
hydrologic, hydraulic, and alternate evaluation aspects of the project. The second phase (Phase B) will
cover the preliminary design of the selected alternate.

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Dama

All property, structures and infrastructure within the SOO-year floodplain were included in the damage
analysis. The damage analysis was calculated using the UDFCD Methodology for Evaluation of
Feasibility: Multi.Jurisdictional Urban Drainage and Flood Control Proiects. Flood damages were
calculated for the 2-, 5-, 10-, 50-, 100-, and SOO-year flood events. For the economic analysis, a 50-year
project life was chosen, which corresponds to a typical structure life. A 6% annual interest rate was
selected, which corresponds to the cost of borrowed capital for the entities involved and a 3% annual rate
of inflation was assumed. The net discount rate used in present value calculations was 3%. The total
present value of damages in the entire Fourmile Canyon Creek SOO-year floodplain was estimated to be
approximately $ 26 million dollars.

Process

The planning effort began in June of 1999. Since that time, a series of progress meetings have taken
place to exchange information and discuss ideas and findings of the study. The progress meetings were
regularly attended by representatives of the sponsoring agencies as well as Boulder County. Concurrent
with the progress meetings, a series of public meetings were conducted to discuss the planning effort and
solicit public input. An Independent Review Committee (IRP) was convened to review the data,
methodologies, assumptions, and recommendations of the planning effort

Plannin

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ation Alternates

Flood Miti

Identification of Potential

A discussion of the various alternatives investigated to mitigate flood damages and a recommendation of
a preferred alternative were presented in a Draft Phase A Report, date March 10, 2000. Many of the
review comments on the Draft report received from the Project Sponsors, Boulder County and concerned
citizens were incorporated into this Final Phase A Report, dated June 9, 2000.

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identified. These alternates were screened following
meetings on a reach by reach basis.

An environmental assessment and opportunities analysis was undertaken by P. Kaia Anderson of
Anderson & Company, Inc. This analysis utilized an extensive biological database, which had been
collected on the City of Boulder stream tributaries, including Fourmile Canyon Creek. Habitat

Early in the study, a broad range of alternates were
an analysis and detailed discussions held during progress

The Project Sponsors will now review the Phase A report and make a decision on the alternative that is to
be studied in greater detail in Phase B of this planning effort. In Phase B, the consultant will prepare a
preliminary design of the alternative selected by the Project Sponsors. The type, size, location of various
improvements will be developed in greater detail. In the process, cost estimates will be refined and the
relative priorities and phasing of these facilities will be identified A final Phase B report will be

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