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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Bosl~y,}Vash f!,rainage Maste~ Plan

Phase 2: Alternative Drainage Facility Evaluation

Highline Canal. This inadvertent storage significantly reduces the flow through the
culvert and therefore the discharge downstream in the Watershed.

2.3.3 Government Highline Canal

The inadvertent storage at the Government Highline Canal joins that behind G Road.
While it appears significant when viewed together, the contribution of the Government
Highline siphon is negligible when compared to the ponding resulting from G Road. This
is the result of the limiting capacity of the G Road culvert. However, there is some
ponding at the Government Highline Canal crossing. This ponding merges with the pond
above G Road, forming one large area of ponding (see Figure 5).

2.3.4 Hwy 6/U PRR

The limited capacity of the culvert at Hwy 6/UPRR creates a large area of inadvertent
storage (see Figure 6). This area has been identified as having about 55 AF of storage.
The storage is widely distributed and inundates a large area of land to the north, west and
east of the crossing. Several structures and areas of agricultural land are impacted. As
with most other areas of inadvertent storage, there is a beneficial reduction in
downstream discharge that reduces the flood hazard south of the crossing.

2.4 Public Meeting Input

A public meeting was conducted on July 10, 2002. This meeting was structured to present
an overview of the project to date and to provide a forum for the discussion of problems
within the Watershed.

During the meeting, the project sponsors and HDR presented their delineation of the
floodplain for discussion. It was generally concluded that the floodplain was a reasonable
depiction of the flood hazard. It was noted that the storms of 1999 did not cause the
extensive damage predicted by the floodplain delineation presented. One explanation
presented was that the storms of 1999, while severe, lacked the broad coverage assumed
for the design storm. The actual storms covered a smaller area of the Watershed and did
not create the broad areas of flooding predicted by the assumptions used in this study.

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