Des Moines, Iowa -- (July 19, 2010) A thorough U.S. Army Corps of Engineers’ inspection of Saylorville Lake’s inflatable pneumatic crest gates following the recent flood event has determined that the air bladders used to lift the gates are experiencing separation of the rubber ply layers resulting in the loss of an airtight seal.
Until all bladders are replaced along the 430-foot-wide concrete spillway, the pneumatic crest gates will not be operated for future flood events. Although the pneumatic crest gates will not be placed into operation until repairs are made, outflow from the Lake during normal and flood operation will not be affected. Cost estimates and repair schedule are being developed at this time.
In 1994, pneumatic gates were added to the spillway to reduce the number of overflow events eroding the unlined portion of the spillway and NW 78th Avenue across the spillway channel. The addition of the gates did not change the release rate at the dam; only the conduit versus the spillway location of the release.
Without the pneumatic crest gates, as the pool rises above elevation 884 feet (spillway crest) the discharge through the Lake’s conduit is reduced so that the total of the conduit discharge and the spillway overflow remains constant at 21,000 cubic feet per second (cfs). By the time the pool reaches 889 feet, the conduit is completely closed and 21,000 cfs flows over the spillway. Between elevation 889 and 890 feet the discharge in the conduit is gradually increased, similar to the lowering of the pneumatic crest gates. As the pool level reaches elevation 890 feet, the conduit is again fully open and the combined discharge from the conduit and the spillway totals 42,000 cfs. Above elevation 890 feet, the dam’s conduit is kept fully open and uncontrolled flow passes over the spillway.
With the pneumatic crest gates, if the pool approaches (or is forecast to exceed) elevation 884 feet, the pneumatic crest gates are inflated, raising the spillway crest to elevation 890 feet. As the pool rises from elevation 884 to 890 feet, the conduit remains fully open to release 21,000 cfs and no overflow is allowed at the spillway. If the pool is forecasted to exceed elevation 890 feet, the crest gates are lowered depending on the forecasted maximum pool. Normally, the gates start lowering at pool elevation 889 feet depending on the pool crest forecast. The gates are completely lowered when the pool reaches (or is forecast to exceed) elevation 890 feet and the combined discharge from the conduit and the spillway totals 42,000 cfs. Above elevation 890 feet the conduit is fully open and uncontrolled flow passes over the deflated pneumatic crest gates.
Release no. 10-07-37