Hidden Dam

Challenges

Hidden Dam is a 184-foot-high zoned earthen embankment dam located on the Fresno River, in Madera County, California.  The purposes of the dam are to provide flood control, recreation, fish and wildlife habitat, and irrigation water supply.  The dam consists of a main embankment, grout curtain, six earthen dikes on the right reservoir rim, a detached open-channel spillway, and an outlet works with a gate tower.

Geotechnical data from over 70 instruments had been collected over approximately 40 years but had not been organized and evaluated since the early 1980s.  Also, it was unknown if the existing instrumentation system was functioning properly and if modifications were required.

Solution

As part of a larger seepage evaluation for the dam, the U.S. Army Corps of Engineers – Sacramento District retained RJH to prepare a Periodic Instrumentation Report that presented a complete evaluation of all project instrumentation and data for the 40-year period of operation.  Instrumentation included piezometers, observation wells, weirs, Parshall flumes, survey monuments, and accelerographs.  RJH performed a site visit to inspect the condition of the instruments and become familiar with the site.  Evaluations of data reliability and instrument functionality were performed using a system of double checking and plotting.  The reliability of all data was classified as reliable, somewhat reliable, or unreliable.

RJH prepared a comprehensive Instrumentation Report that clearly presented the instrumentation data, evaluation of data reliability, trends from collected data readings, and key conclusions.  As part of this report, RJH provided recommendations for modifications to the current instrumentation program at the dam.  Recommendations included abandoning unreliable instruments; performing slug tests, video inspections, and clearing of piezometers and observation wells; and establishing detailed monitoring instructions.