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MESA, Ariz. — The completion of the final leg of the Loop 202 Red Mountain Freeway gave residents of Mesa and the far East Valley more than an additional commuting route. The project also paved the way for the Flood Control District of Maricopa County (District) to rehabilitate one of its flood control dams.
Spook Hill Flood Retarding Structure (FRS) is a four-mile-long earthen dam located in Mesa, stretching from McDowell Road and Power Road to near Brown Road and Hawes Road. Spook Hill FRS is the key component of a series of three District dams in northeast Mesa and Apache Junction. The three dams, along with interconnecting floodway channels, protect downstream development and the Central Arizona Project Canal from stormwater runoff from a 110-square-mile watershed fed by the Usery and Goldfield mountains. The flood control structures capture floodwater from the watershed and re-route it to the Salt River.
The District determined that overall rehabilitation of the dam was required following field investigations which found cracking within the embankment of the 28-year-old structure. Due to the parallel alignment of Spook Hill FRS with the new freeway segment between McDowell Road and University Drive, the Arizona Department of Transportation (ADOT) and the District entered into an agreement to include the rehabilitation of the dam, at the District’s cost, as a part of the freeway construction contract. Coordinating upgrades to the dam while building the freeway was less expensive than waiting until after the highway was in place.
Three new road crossing bridges were constructed over Spook Hill FRS by ADOT. The bridge construction facilitated the rehabilitation project, which commenced in January 2006 to complete the following: 1) the construction of a sand material zone built into the center of the dam as a defensive measure to prevent dam failure due to cracks in the structure; and 2) the construction of a new principal outlet drain. Both upgrades were built to current dam safety standards. Updated hydrology was developed to determine the most accurate stormwater flow paths and utility lines were properly relocated. The rehabilitation addressed all of the issues the District noted in its earlier dam assessment.
The new Red Mountain Freeway segment runs along the east side of the FRS through the dam’s flood pool. To protect the freeway from floodwater, ADOT designed and built a soil cement levee in the flood pool on the east side of the freeway to hold back the stormwater of a 100-year rainfall event. If floodwater overtops this levee, Spook Hill FRS is designed to contain and re-direct this floodwater. The District reviewed ADOT’s floodplain use permit to build the levee and confirmed the design would not adversely affect the function of the FRS flood pool.
In the interest of public safety and as part of the agreement with ADOT, the District’s Flood Warning Branch installed a new ALERT gage at each of the bridge crossings over the dam and four additional rainfall gages in the watershed upstream of the new freeway. The gages are part of ADOT’s freeway monitoring system as well as the District’s countywide ALERT warning system measuring rainfall and stormwater runoff. ADOT officials will be notified by the District if rain gages show the potential for flooding along the Red Mountain Freeway corridor.
Spook Hill FRS is one of 22 flood control dams in Maricopa County and neighboring Pinal County operated and maintained by the District. The dam’s rehabilitation was performed through the District’s Dam Safety Program, which evaluates the existing condition of each structure and formulates planning alternatives to extend the service life of dams deemed to need rehabilitation or replacement. Spook Hill is the first of eight District dams slated for future overall rehabilitation or replacement.
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