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General plan touches on dam failures

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POSTED October 30, 2017 2:00 a.m.

New Melones Dam failing — a highly unlikely event — is one of the safety hazards that are outlined in the Manteca general plan designed to guide growth decisions.

But should the 625-foot high earth and rock fill dam that has a maximum capacity of 2.4 million acre feet of water fail, it would flood the entire city of Manteca with water reaching points hundreds of yards north of French Camp Road.

The general plan being put together by DeNova Group consultants identifies four dams that, should they fail, would pose a flooding risk to Manteca. Besides New Melones they are the Tulloch Dam on the Stanislaus River, the New Exchequer Dam (Lake McClure) on the Merced River, and the San Luis Reservoir on San Luis Creek. A fifth dam — the Pine Flat Dam on the Kings River — would flood a small portion of southwest Manteca that has been designated as open space if it should fail.

Dam inundation maps have been required in California since 1972 when the Lower Van Norman Dam came close to failing during the 1971 San Fernando Earthquake in Southern California.

Inclusion of such information in the general plan illustrates how thorough the state law requires the planning documents to be even dealing with issues that are considered highly unlikely to happen.

State experts have said the only way that dams such as New Melones could fail during a major earthquake. That safety assessment was made, however, before the Oroville Dam incident this past winter when a failing spillway forced the hasty evacuation of more than 200,000 people downstream. The spillway had emergency repairs made that prevented a catastrophe. There is a push to re-assess spillway and other structural conditions at the state’s numerous dams.

The quake concern doesn’t focus just on the impacts of a major tremblor on the massive San Andreas Fault. It includes smaller faults that often times have gone undiscovered until dam site work started.

It was the concern of earthquakes that stopped the building of the Auburn Dam in the 1970s after work on a coffer dam — the temporary dam that was needed to divert the American River so foundation work could be done for the main dam — led engineers to come across a previously uncharted fault almost right under the proposed base for the dam.

Seismologists determined the fault was still active and had the potential of unleashing a 6.1 quake on the Richter Scale.

The discovery came after the August 1975 Oroville quake that had an epicenter 7 miles south of Lake Oroville delivered five foreshocks, a main shock of 5.7 and numerous aftershocks including seven earthquakes of a magnitude of 4.6 or greater. 

While the state sharpened its standards for dams being built in regards to quakes after the Oroville shaker, it never changed the status of the dam being safe in an earthquake.

The next biggest remote threat to Manteca after New Melones in the highly unlikely event of a dam failure is San Luis Reservoir. Its failure could send water as far east as Union Road north of Alameda Street, and as far east south of the 120 Bypass to a midway point between Union Road and Main Street.

Next in severity would be the failure of the Tulloch Dam that soars 205 feet above the Stanislaus River. Should it fail, it would flood a large swath of Manteca south of the 120 Bypass. The farthest north water modeled to come is several hundred yards short of Peach Avenue in the west and Tannehill Drive in the east.

Dam failure at Lake McClure creates an exposure limited to a portion of southwest Manteca.

In the case of Lake McClure and Pine Flat, the proposed 200-year levee would likely stop a worst case scenario. That may not be the case for San Luis should it fail when it is at near capacity.

A failure of Tulloch or New Melones would be extremely problematic for Manteca as flood water would come from the east.

While the state is now only requiring protection for a 200-year flood — and event that has a 1 in 200 chance of happening in any given year — the general plan addresses a 500-year flood that has a 1 in 500 chance of happening in any given year. Such an event could flood Manteca to a point just west of Union Road north of the 120 Bypass and in some areas east of Union Road south of the 120 Bypass.

The next general plan advisory committee is Monday, Nov. 6, at 6 p.m. at the Manteca Transit Center, 220 Moffat Blvd. On the agenda are discussions about conservation, open space, and air quality.

 

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com

 

 

 

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