Gillespie Dam
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| Gillespie Dam | |
| Official name | Gillespie Dam |
|---|---|
| Impounds | Gila River |
| Creates | Gillespie Dam Reservoir (former) |
| Locale | Southwestern Arizona |
| Length | 1,700 feet (520 m) |
| Height | 20 feet (6.1 m) |
| Opening date | 1921[1] |
The Gillespie Dam is a concrete gravity dam located on the Gila River between the towns of Buckeye and Gila Bend, Arizona. The dam was constructed during the 1920s for primarily irrigation purposes. A portion of the dam failed unexpectedly in 1993 during unusually heavy rains.
[edit] History
The Gillespie Dam was constructed in 1921 by a local rancher to replace an existing structure.[2] As the dam was located at an important river crossing that would later become US Route 80, the Arizona Highway Department - the predecessor to the Arizona Department of Transportation - constructed a concrete apron at the foot of the dam to allow for vehicular crossings. As the dam was a simple spillover construction, during times of heavy runoff cars would have to be pulled through the flow by trucks, and during floods could not cross at all.[1]
In anticipation of the formation of the United States Highway System in 1926, the Highway Department commissioned the construction of a steel truss bridge just downstream from the dam. The bridge was completed and opened to traffic on August 1, 1927 at a cost of US$320,000 (US$3,950,000 in 2007). The bridge, which was at the time the longest highway bridge in the state of Arizona, was immediately incorporated into the highway system as Route 80.[1] The bridge carried US 80 traffic until 1956 when the highway was decommissioned, devolving to a county highway, thus placing the bridge under Maricopa County care. The bridge was added to the National Register of Historic Places on May 5, 1981.[3]
[edit] Failure
The winter months of 1993 saw unusually high rainfall amounts that resulted in record flows through central Arizona rivers and streams, including the Salt River, a major tributary to the Gila upstream from the Gillespie Dam.[4] At approximately 10:30 on the morning of January 9, the dam failed when a segment approximately 120 feet (37 m) in length collapsed into the river. While the precise cause of the failure is unknown, the extreme flooding was almost certainly a contributing factor. The precise size of the flood was not recorded due to equipment failure, but an estimate based upon a high water mark recorded on USGS equipment yielded a peak flow of approximately 200,000 cubic feet (5,700 m³) per second, corresponding to a predicted 65-year flood, or a flood of a magnitude anticipated only once per 65 years. The previous high, recorded during similarly disastrous floods in 1980, had been 178,000 cubic feet (5,000 m³) per second.[5]
As a direct consequence of the failure, 3 underground natural gas lines were excavated and later severed by the flood by January 11. The bridge downstream survived the failure, and was deemed safe for travel.[1]
The remnants of the dam have not been removed and is the area is largely accessible to the public. A small earthen embankment exists to divert water into nearby canals.
[edit] References
- ^ a b c d Historic Gillespie Dam. Town of Gila Bend. Retrieved on 2008-06-03.
- ^ Field Excursion Reports. United Nations University. Retrieved on 2008-06-03.
- ^ Gillespie Dam Bridge. Arizona State Parks. Retrieved on 2008-06-03.
- ^ Arizona's Most Notable Storms. National Weather Service. Retrieved on 2008-06-03.
- ^ Larson, Robert; James Slosson (1997). Storm-induced Geologic Hazards: Case Histories from the 1992-1993 Winter. Geological Society of America, 32-33.
