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

direct- them back to the creek. Acquisition and construction of a raised north-south 9th Street,
along with some additional grading of the natural swale to the creek, would provide a collection
system for directing floodwaters to the drainageway. An alternate location to collect the flood
flows would be several hundred feet upstream of 9th Street extended. This alternate location
would collect flood discharge upstream of existing or proposed development.

AL TERNA TE EV ALUA TION AND RECOMMENDED PLAN

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Flood spills to the south of the creek need to be prevented for the design event - Enlarging
and improving the drainageway upstream of Broadway could confine floodwaters to the creek
section to prevent flood spills to the south for the design event. To avoid impacting or relocating
existing mobile homes, an urbanized drainageway section could be constructed. The addition of
vertical elements, such as rock walls and expanded drop structures would create a more formal or
hardened cross section, but would provide greater capacity in a narrowed width.

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INTRODUCTION

Master plan alternates were identified in Chapter 6 and environmental assessment and opportunities
of these alternates were presented in Chapter 7. This chapter presents three selected alternates which
were analyzed in more detail and provides estimates of probable construction costs, maintenance
requirements, residual flood damage estimates and a benefit/cost analysis. A recommended alternate
solution is provided within this chapter. The three alternates selected for more detailed analysis are:

8.1

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The drainageway downstream of Broadway needs to be improved to pass flood waters
collected and concentrated from upstream - Drainageways are typically improved from
downstream to upstream to adequately convey flood flows that are concentrated by the project.
Collecting floodwaters to eliminate southern flood spills results in a need to improve the
downstream drainageway conveyance to prevent adverse floodplain impacts. The need to
increase downstream drainageway conveyance may extend downstream as far as the Fourmile
Canyon Creek confluence with Boulder Creek. The following drainageway conveyance
improvements would be needed within the study reach

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50-Year Channel
1 00- Year Channel
Maintain existing floodplain configuration

AND 2 GENERAL DESCRIPTION

Alternate 1:
Alternate 2:
Alternate 3

ALTERNATE 1

8.2

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Increased channel capacity would need to be constructed through the Village Center just east
of Broadway in a 120-foot wide easement acquired as a part of site annexation. An alternate
"bowl-type" drainageway section incorporated into the "Village Green" proposed in the North
Boulder Subcommunity Plan could be provided to create a more aesthetic greenway feature.
This conveyance system could be coupled with site fill to eliminate high hazard zone flooding
of properties south of the creek and allow redevelopment of the site.

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The overbank area south of the creek riparian corridor located on the city-owned park site
along Violet Avenue could be excavated to increase flood conveyance across the property.
This mitigation approach would preserve the existing creek riparian habitat and continue to
allow a developed and level park use of this site.

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o The creek corridor between Violet Avenue and 30th Street would need to be acquired or
easement obtained and modifications need to be constructed to provide a clear path for
floodwaters without adversely affecting residential structures. This mitigation approach
would eliminate high hazard structures and provide overbank contouring to increase
floodwater conveyance without adversely affecting the riparian corridor along the creek bank.

An expanded drainageway corridor must be secured - West of Broadway, Fourmile Canyon
Creek traverses through a narrow channel section located between two roadways both known as
Rosewood Avenue. In order to increase conveyance, a wider corridor needs to be acquired for
channel improvements. Use of the Rosewood roadway section on the north creek bank offers an
opportunity to widen the creek without impacting existing mobile homes in the Ponderosa Mobile
Home Park on the south side of the creek. East of Broadway, annexation agreements provide for
a 120-foot wide easement corridor through the Village Center area to the city-owned Violet park
site. The Violet park site offers a wide corridor that would allow flood mitigation to occur in the
overbank to the south without altering the creek riparian zone. Further downstream property
acquisition or easements between Violet Avenue and 30th Street would be required to secure a
drainageway corridor for the proposed channel improvements. Between 30th Street and the
railroad tracks below Highway 119 no additional right-of-way would be required.

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Alternates 1 and 2 have many similarities, since they both would construct a natural type of waterway
following the general historic channel alignment. As discussed in Chapter 6, the purist definition of a
"natural channel" would be impossible to construct since road crossings have been constructed, the
channel has been straightened, the banks of the creek have been filled, and development has
encroached on the natural floodplain. Therefore the term "natural channel" is defined as a channel
designed by an inter-disciplinary team of engineers, geomorphologists, and biologists to emulate a
"natural" channel. Refer to Chapter 7 for environmental issues related to this type of channel design.
The common similarities for Alternates 1 and 2 include the following:

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Widespread flood flows from the northwest need to be collected and directed to the creek
corridor - Flooding west of 9th Street spreads several hundred feet north of the creek. Capturing
these flows and conveying them into a narrowed drainageway may be accomplished by taking
advantage of a large natural channel draining from the north. This channel is a former creek
channel, now cut off, and offers a "natural conditions" opportunity to help collect flood flows and

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