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

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More Flood

Less Structural 1m

lain

All development within the existing floodplain should be in accordance with the City, County and
Federal permitting requirements. In additional we recommend that any development immediately
adjacent to Fourmile Canyon Creek be required to dedicate drainage easements, construct the
recommended flood improvements along the length of their property or put money in escrow for
future construction of the flood improvements as a condition of the development.

ment within the Flood

Develo

rovements

The independent review committee suggested a fourth alternative be considered which would consist
of less structural improvements and greater emphasis on floodplain management approaches, such as
improved maintenance, early warning systems, emergency response plans, flood proofing and
expanded flood insurance for those properties within the floodplain.

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rovements

Regardless of the floodplain mitigation alternative effort chosen, management of the Fourmile
Canyon Creek floodplain is likely to be an ongoing expense. Funding mechanisms should be planned
and implemented to fund floodplain improvements and, in the event of a damaging flood, fund the
emergency response, cleanup and repair of damages

Fundin

Fan

Fourmile Canyon Creek, in the area east of the mouth of the canyon, is perched atop an alluvial fan.
During the public review process, concern was expressed that due to the nature of an alluvial fan, a
major flood event could radically change the course and flow direction of the Fourmile Canyon Creek
channel. Concern was also expressed that the 'fixed bed" methodology used to define the existing
floodplain was inappropriate.

Alluvial

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land Avenue

One possible alternative identified and discussed during the planning process was the elimination of
the Upland Avenue roadway crossing of Fourmile Canyon Creek. Elimination of this roadway
crossing of the creek would eliminate the cost of a bridge or culvert and would improve the flood
conveyance of the creek in the vicinity of Crestview Elementary School. Discussions identified two
scenarios for Upland Avenue. One, Upland Avenue would terminate in a cul-de-sac west of the creek
and secondly, Upland Avenue could be re-routed south to connect with Sumac Avenue or Tamarack
Avenue as part of the development ofa park site immediately west of Crestview Elementary.

Eliminatin

The Independent Review Committee investigated this concern and stated, in their review comments
dated May 2, 2000, that in their opinion most of the alluvial fan is currently inactive, Fourmile
Canyon Creek has a defined channel, a mappable floodplain and the "fixed bed" methodology
utilized to define the existing floodplain is appropriate. If a catastrophic flood should occur that
results in a change in the course of Fourmile Canyon Creek, the City will have to respond according
to the specifics ofthe flood event. The IRP report and recommendations can be found in Appendix E
of this report

FUTURE PLANNING AND DESIGN CONSIDERATIONS

8.13

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Below is a list of additional items that should be taken into consideration during future planning,
design and implementation of flood mitigation improvements along Fourmile Canyon Creek.

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the environmental

with

accordance

In

Concerns

Flood improvements should be design and implemented
recommendations and guidelines presented in Chapter 7.

Environmental

Maintenance

lssues,

groundwater

address

Any planned
including:
. Potential impacts on the existing groundwater.
. Potential impacts on existing wells,
. Potential impacts to the existing contaminated groundwater plume
. Potential opportunities for ground water recharge

specifically

should

improvements

mitigation

flood

Plan, fund and implement a maintenance program that will monitor the Fourmile Canyon Creek
corridor, make spot repairs, repair erosion, and conduct weed control and removal of debris and
sediment as necessary. Over the years, lack of maintenance has allowed an extensive sediment
deposition and woody debris to minimize channel capacity. Regular maintenance will be a key in all
situations.

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Miscellaneous recommendations

Ditch crossings should be designed to not accept or convey flood flows

rovements

Due to economic constraints phased implementation of flood improveme likely. In general
flood improvements should start in the downstream reaches first and proceed upstream.
Upstream improvements may be implemented prior to downstream improvements if no adverse
impact to downstream properties will occur.

nts is
then

of Flood 1m

Phasin

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8-21

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