US Army Corps of Engineers’ (USACE) plans to rehabilitate the Cherry Creek dam in Colorado are to reviewed, in deference to the wishes of home owners whose property could be affected by the Corps’ plans to modify the dam. Property owners have asked USACE to carry out further analysis of weather data and hydrology studies that predict that the dam does not have the capability to handle a Probable Maximum Flood (PMF) that could be generated by 25in of rainfall across the 385m2 Cherry Creek Basin in fewer than 72 hours. Rainfall of that magnitude is the rarest of possibilities, but if it were to occur, the dam’s failure could claim as many as 10,000 lives and cause US$15B in damage, said USACE.
In the late 1980s, the Weather Service revised its maximum possible rainfall standards and the Cherry Creek dam was listed as deficient. The revised standards increased the maximum possible rainfall by more than 50%. An independent study to be conducted by the Colorado State University will examine the Cherry Creek Basin and the derivation of probable maximum precipitation data, and create a model based on 20 extreme weather events that can be used to predict extreme rainfall in basins above 5000ft.
USACE has not yet come up with a firm proposal to remedy the dam’s deficiencies. The list of alternatives under consideration include building upstream dry dams and raising the existing dam. The most controversial of the proposed upgrades is the construction of dry dams, which hold water only during flooding. Six sites have been identified, including one at the site of the old Castlewood Canyon dam, which breached in 1933. The dry dams, however, would displace dozens of residents. A group of residents from Cherry Creek Basin is preparing for a legal battle should the Corps choose to build dry dams.