Spook Hill floodwater retarding structure emergency spillway overtopping and piping breach failure floodwater routing with FLO-2D

The Spook Hill Flood Retarding Structure (FRS) is an earthen embankment approximately 25 feet high and 4 miles in length, running northwest-southeast in the City of Mesa, Arizona. The structure was designed by the U.S. Natural Resources Conservation Service (formerly Soil Conservation Service), and constructed in 1978, to protect development from flows originating in the Usery Mountains. Operated and maintained by the Flood Control District of Maricopa County, the FRS provides protection for developments including residential, commercial and industrial land uses. In addition, the Central Arizona Project Canal is located approximately 500 feet downstream from the FRS emergency spillway. Floodwater overtopping the FRS emergency spillway, or a breach of the FRS embankment, would result in an area of inundation to the west of the structure. The area of inundation, and magnitude of depth and velocity of flow, is dependent on the rate and volume of flow passing through the FRS. Two earlier Flood Inundation Studies were completed in 1985 and 1990. However, future construction of a new section of the Red Mountain Freeway through previously identified areas of inundation required updating potential flood limits and hazards. Stanley Consultants used the two-dimensional floodwater routing model FLO-2D to evaluate flooding scenarios based on the FRS emergency spillway overtopping and a piping breach structure failure. FLO-2D modeling provided floodwater depths, velocities and limits of inundation for selected time increments at particular locations within the study area. Results from FLO-2D analyses will be compiled by the Flood Control District of Maricopa County and provided to the Maricopa County Department of Emergency Management for updating the Spook Hill FRS Emergency Action Plan. 14 pp., 3 table, 4 figures, 3 references.