Abstract
The 244.3-km-long, mostly concrete-lined Friant-Kern Canal, an early key facility of the Central Valley Project of the U.S. Bureau of Reclamation, extends from Friant Dam to the Kern River near Bakersfield and is situated in the southeastern part of California's San Joaquin Valley. The canal has a diversion capacity of 170 m3/s. About 48.3 km of the alignment, between MP (milepost) 95 and MP 125, are on the periphery of the major Tulare-Wasco subsiding area. The postconstruction subsidence within the reach locally exceeded 1.5 m. The exact causes of subsidence along the canal are unknown, but most of the subsidence is probably caused by a distant irrigational overdraft of confined ground-water aquifer systems. Some tectonic movements are also possible. Postconstruction ground-water levels along the canal are generally rising. Data on past subsidence are incomplete, and the results of some 12 construction and postconstruction canal levelings are confusing.
Subsidence interfered with canal operation, resulting in rehabilitation of a 26.67-km-long reach of the canal in 1976/77 and in the raising of three subsided pumping plants in 1979–81, at a total cost of about $4,700,000. Prerehabilitation estimates of future subsidence were made in 1975 and in 1979. Due to the uncertain nature of subsidence and the poor quality of basic data the method and results of estimates are questionable. The 1975 estimates for the 37-km-long canal reach, based on data obtained at the compaction recorder site at Terra Bella (MP 101.67), in an area of maximal subsidence of an apparently exponential character, yielded values of 0.61 or 0.91 m for ultimate subsidence after January 1975. These values were prorated for the entire reach, assuming the values of past and future recorded subsidence to be proportional. The 1979 estimates made for three pumping plant sites were based on past subsidence and on past and estimated future decline of ground water in 1990, 2000, and 2020. The total amount of future subsidence at the sites ranged from about 1.2 to 1.7 m in 1990, from 2 to 2.9 m in 2000 and from 4.2 to 6.1 m in 2020.
The quality of future levelings along the canal is questionable because of apparent instability of reference bench marks. It is recommended, therefore, to conduct systematic (once every 2–3 years) measurements of depth of water at check structures and selected bridges in the standstill canal (1979–80 measurements of this type indicate the possibility of some local tectonic uplift and downwarp).
