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Quake + weak levees = Flooding
Model says Lathrop, part of Manteca at risk
Some of the homes in southwest Manteca that could see water as high as 10 feet should a 200-year flood occur. - photo by HIME ROMERO

A major earthquake and levees protecting Manteca, Lathrop and Weston Ranch may not mix well.

Manteca municipal leaders Tuesday were told liquefaction — where saturated soils can readily move with respect to each other — is a major concern in regards to sections of the levees along the eastern side of the San Joaquin River between the confluence with the Stanislaus River and the French Camp Slough. Many of the levees when they were created in the Delta over 130 years ago were made from nearby soil that tends to have properties conducive to liquefaction.

It is concern over liquefaction and the possible collapse of levees in the Delta that is one of the selling points being made by backers of the Twin Tunnels to have Sacramento River water bypass the Delta and go directly to the California Aqueduct as they argue it would protect Los Angeles’ water supply in a major earthquake.

The closest fault zone is the Tracy-Stockton Fault running from southwest near Tracy to the northeast near Linden passing just north of Manteca. Two quakes at 4.0 on the Richter scale occurred on Sept. 19 and 20, 1940, five miles south of Linden. The fault is expected to have the potential for a 5.0 quake. That may not be large enough to cause a problem. The concern is that a big quake in excess of 8.0 on the Richter scale along the San Andreas or Hayward faults would have a severe impact on the Delta.

In a worst case scenario, the Manteca City Council was told almost all of Weston Ranch, much of Lathrop, and parts of southwest Manteca would be under as much as 10 feet of water should multiple levee breaks occur.

The flood modeling was part of a $150,000 study of the 200-year floodplain bankrolled jointly by Manteca and Lathrop. It is in response to a state mandate by July 1, 2016 that cities and counties must make significant progress toward putting in place 200-year flood protection or else face a mandated curtailment of issuing building permits for new development in flood zones.

In Manteca’s case, doing the study along with upcoming work to identify improvements needed and how to fund them coupled with work that Reclamation District 17 is already doing to the levees may get Manteca past the first deadline.

Initial work on dry levees needed south of Woodward Avenue could cost $3 million. Other levee work expected to be required is expected to cost significantly more.

Manteca leaders made it clear they expect Lathrop to pay their fair share of the need for protection and not simply split it 50-50 as they did for the flood modeling study.

Virtually all of Lathrop east of the river is within the 200-year floodplain. Only the southwest section of Manteca — basically the area southwest of the Airport Way and 120 Bypass interchange  — falls into the 200-year flood category.

Several council members thought it wasn’t fair that River Islands at Lathrop wasn’t included in the 200-year floodplain. Mayor Willie Weatherford noted that Stewart Tract where River Islands is being developed had flooded numerous times over the years including in the 1997 floods.

The developers of River Islands — Cambay Group — nearly a decade ago spent $70 million to create super levees that are 300 feet wide compared to nearby levees that are 60 feet wide at the base. They are the only levees in California — and possibly the nation — certified to withstand a 200-year flood. Some 11,000 homes are being built in the planned community that is being protected by the levees.

Ironically the late Alex Hildebrand — considered by many to be one of the state’s foremost lay water experts — expressed concern in 2002 that allowing River Islands to have 300-foot wide levees without upgrading levees on the east side of the river would put additional pressure on them. In short, he believed that the presence of the significantly upgraded levees on the west side of the river ultimately would increase the chance of levees failing on the east side of the river that protect Manteca, Lathrop, and farmland south of Manteca.

Manteca leaders at the time opted to stay out of the levee dispute arguing that it was a Lathrop issue and not a Manteca concern. Since then the California Legislature passed the 200-year flood Legislation requiring protection or else a forced suspension of development in flood prime areas in the Central Valley watershed.

The presentation Tuesday was a progress report on the city’s effort to comply with the state mandate.