FLOOD11759

• County: Mesa
• Stream Name: Bosley Wash
• Basin: Colorado Mainstem
• Title: Bosley Wash Drainage Master Plan - Phase 2: Alternative Drainage Facility Evaluation - Draft
• Date: 11/1/2002
• Prepared For: Bosley Wash Watershed, CWCB
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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,Bosley .Wash !?-r.ainage Master Plan

Phase 2: Alternative Drainage Facility Evaluation

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little freeboard was incorporated into this alternative. The alternatives did include an
estimate of the required right of way (ROW) acquisition to make the identified
improvements.

3.2.1 Improve channel along Stub Ditch

Improvements to the drainage channel along the Stub Ditch have a dramatic affect on
flooding along the Government Highline Canal. The ditch improvements were sized to
convey 50 cfs without overtopping. Improvements in the capacity will be accomplished
by excavating the channel slightly and then placing the excess material on the southern
channel bank. The estimated cost of these improvements is $207,000. This alternative
may be particularly effective in combination with storage at Re.;:ervoir M&N.

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3.2.2 Improve channel along F % Road

Improvements to the channel along F % Road will dramatically reduce the flooding
impacts to the south, all the way to the Hwy 6/UPRR embankment. In this case, only
minor channel improvements are required, since a majority of the flooding is the result of
undersized culverts (see Section 3.3.3).

3.3 Culvert Improvements

Undersized culverts are the biggest contributors to flooding problems in the Bosley Wash
basin. The improvement of some of these undersized culverts has the potential to
significantly reduce flooding upstream and downstream. Upstream improvements result
from the reduced headwater, which occurs at many culvert entrances. This causes much
of the inadvertent flooding and damage to properties upstream. Downstream flooding
often occurs as a result of overtopping of the roadway or canal under which the culvert
crosses. When these facilities overtop, the uncontrolled overflow downstream often is out
of bank and impacts adjacent properties.

Unfortunately, the enlargement of many of the undersized culverts has the potential to
increase downstream flood hazard. While these structures currently impede the flow of
water and cause upstream increases in flood hazard, they also reduce the amount of water
flowing downstream. Enlargements are likely to increase downstream discharges. This

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