The complaint, filed Tuesday in Maricopa County Superior Court, claims the agencies either knowingly or negligently used properties in the Emerald Acres area for "overflow and storage of storm water collected throughout the City of Mesa and a portion of U.S. Highway 60" after Sept. 8's record downpour.
Residents are seeking an injunction to prevent it from happening again.
The historic rainstorm slammed the Valley in the early-morning hours, stranding residents in their homes and cars as water rose throughout the region. By early afternoon, the rain had eased. But in Mesa, the mammoth Emerald Park retention basin had begun to overflow, flooding an elementary school and nearly 200 nearby homes.
"Defendants continued to cause or allow water to be diverted into Emerald Park even though they knew or should have known that there was no effective outlet for the water entering Emerald Park other than in and through the plaintiffs' ... homes and property," the residents' complaint says, asking that a judge intervene so properties won't again be inundated if a comparable storm hits.