DDHC Engineering Inc.

Fanning Wash at the Intersection of Steves Boulevard Drainage Study for the City of Flagstaff Stormwater Divison

Project Description

The culverts at the intersection of the Fanning Wash and Steves Boulevard in Flagstaff, Arizona
were analyzed to determine whether the current conditions in the channel meet the City of
Flagstaff requirements. The culverts were the primary area of interest initially since the existing
culverts showed debris, sediment and other obstructions preventing flow of stormwater. This led
to concerns of possible flooding issues for the surrounding area and potential failure of the
drainage network. Through collecting topographic data of the channel, culvert, and roadway
necessary characteristics were determined and modeled to show how the current infrastructure
performs. From the existing conditions new infrastructure was designed to improve the
performance of the channel and culverts found at the intersection of the wash and roadway. To
improve the drainage conditions the design team proposed to remove the existing three
corrugated metal pipes and instead implement a four foot by seven foot double barrel, concrete
lined box culvert. With this implementation the box culverts through models and calculations
improve drainage conditions in the wash, and decrease the likelihood of the culvert overtopping
onto the roadway in flooding stages. Overall, the purpose of the design was to mitigate flood
risks, while improving the drainage network at the upstream portion of the Fanning Wash as it
travels underneath Steves Boulevard by analyzing existing and proposed conditions of the
channel and culverts.