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OLMOS DAM MODIFICATIONS,
ASCE TEXAS SECTION OUTSTANDING CIVIL ENGINEERING ACHIEVEMENT (OCEA) AWARD 1982
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The City of San Antonio has suffered numerous floods on the San Antonio River throughout its history. The city finally was spurred into action when an 18-inch rain in September 1921 left most of the business district submerged and drowned about 50 residents. The prescribed remedy was a vertical wall of concrete with decorative arches, topped with a road that let motorists cruise between Olmos Park and Alamo Heights over scenic but boggy Olmos Basin. Olmos Dam, finished in 1929 and located six miles north of downtown, proved to be one of the city's most enduring public works, a landmark that helped tame the San Antonio River and made feasible the River Walk concept.
Olmos Dam was built as a flood retarding concrete gravity dam with no emergency spillway. Flood releases were made through six outlet tunnels regulated by slide gates. In 1974, an engineering study indicated that the dam did not have sufficient discharge capacity to prevent overtopping at PMF and the structure did not meet acceptable safety factors for events larger than the 100-year flood. As a result, modifications began in 1978 to replace 1,500 feet of the non-overflow section with an uncontrolled spillway and to add post-tensioned anchors to the non-overflow sections. As early as 1984 problems were reported with the bar anchors, but this subsequent refit is not the subject of the OCEA. Curious readers are pointed toward a 2012 USSDAMS Proceedings article “THE THIRD TIME’S A CHARM” (
visit link) and mySA 12/9/2010 “Olmos Dam gets upgrade” (
visit link)
Meanwhile, back in 1974: rabid – excuse me – rapid development in the Olmos Basin Area and the region’s vulnerability to heavy rainfall raised growing concern that the basin’s water collection could overtop the dam. A 1973 study indicated point blank that flood routings produced by severe storms would in fact overtop the dam – overstressing the structure and leading to possible failure.
The modifications that followed included: post-tension cables that extended down through the dam and anchored into the solid limestone strata below, modification and/or replacement of the control gates, construction of a new control house, and relocation of the two-lane roadway from its original location across the top of the dam to below the dam.
Prestressing of the east and west non-overflow segments used a hard limestone layer (7-10 ft thick) 50 feet below the base of the dam as anchorage. Reconstruction of the roadway also included two bridges – one over the outlet channel and one to connect the new roadway to the remaining existing street, and replacement of the roadway on the west end with an ogee spillway.
One innovative method utilized during construction included the use of explosives to remove the upper portions of the dam, clearing the way for construction of the spillway crest. This type of demolition – partial removal of increments of an existing dam using explosives -- had never been attempted before its use on the Olmos Dam Modifications. Engineers were inspired by presplitting techniques used in quarrying. This method saved considerable time and money and actually produced less shock to the structure than from a hydraulic ram.
The completion of the project in 1982 marked a moment of (temporary) relief for the community from the growing anxiety of an impending dam failure.
Construction management and coordination of engineering was handled by the San Antonio River Authority (SARA), which recorded total cost of the project at $8,661,483.90.
Records:
Owner- City of San Antonio, Department of Public Works, Director of Public Works Frank R. Kiolbassa PE
Project Manager – San Antonio River Authority (SARA)
Engineer- JV: W. E. Simpson Co., Inc., Executive Vice-President H. Douglas Steadman PE, and Hensley-Schmidt, Inc.
Contractor- Clearwater Constructors, Inc.
FURTHER READING:
San Antonio River Authority (SARA) Past Projects: Olmos Dam (
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Texas Water Development Board Olmos Reservoir (San Antonio River Basin) (
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Olmos Dam Modifications Civil Engineering—ASCE, Vol. 52, No. 6, June 1982, pp. 54-55 (
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Texas Beyond History Olmos Dam (
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SUBSEQUENT REFITS:
USSDAMS 2012 Proceedings “THE THIRD TIME’S A CHARM” (
visit link)
mySA Olmos Dam gets upgrade, DECEMBER 9, 2010 (
visit link)
Arias GeoProfessionals OLMOS DAM REHABILITATION (
visit link)