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Emergency levee repair
Concern rises about Sierras spring runoff
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Emergency repairs are being made on a levee on a bend in the San Joaquin River southwest of Manteca about a mile north of the confluence with the Stanislaus River.

Crews started work Friday on an eroding levee on the Manteca side of the river that is being damaged by usually heavy December river flows triggered by extensive rains.

It is one of nine spots south of Manteca that failed during January of 1997 along the Stanislaus and San Joaquin rivers triggering the flooding of 35 square miles. There was $100 million in property damages, 2,000 people were evacuated, 20,000 head of cattle were relocated, and 800 homes and other structures sustained some form of water damage.

The crumbling levee is also where emergency crews worked in May of 2006 when spring runoff started eroding the integrity of the earthen levee. No flooding occurred in 2006.

The San Joaquin Office of Emergency Services stressed there is no imminent danger of collapse.

The concern though is what will happen when the snowpack melts this spring.

The northern Sierra that feeds the Sacramento River that has a much wider channel plus a series of bypasses is at 164 percent of normal in terms of the snowpack. The southern Sierra that runs off into the San Joaquin River, though, is at 260 percent of normal. It is an extraordinary high percentage for January snowpack. The San Joaquin River channel lacks bypasses, has a narrower channel, and has more levees under stress.

The weakest part on the entire San Joaquin River section is just south of Manteca.

The flooding in 1997 was caused by unusually warm weather in early January that triggered an unexpected rapid run-off after a heavy December snowfall.

If the temperatures warm up rapidly this time instead of staying cold in the upper elevations, most reservoirs can handle a surge in water storage.

The concern though, is what will happen when the run-off starts in earnest.

The area south of Manteca has flooded 11 times since 1929.

Property owners two years ago approved parcel taxes to fund efforts to strengthen the levee system to develop a higher degree of protection. There is a misconception that emergency work down to restore levees after they are broken increase the level of protection. That is not the case. Under law, the Army Corps cannot improve levee protection beyond what was in place at the time of the failure. The non-emergency levee enhancement work that is underway will ultimately strength the levees and enhance flood protection in rural Manteca as well as Lathrop plus the southwest portion of Manteca.

Manteca is protected somewhat from levee failures along the Stanislaus and San Joaquin rivers by a dry cross-levee that parallel Woodward Avenue to the west of Airport Way.

That, however, was the one spot that state emergency personnel feared would fail after the initial levee breaks back in 1997. It was covered with tarp and sand bags and monitored 24/7 for several days until water receded enough for the danger to pass.

There was only a smattering of homes that would have been impacted had that dry levee failed back in 1997. Now there are more than 400 homes that would be threatened with most of them saddling Airport Way south of the 120 Bypass.

The last line of defense for Lathrop and parts of southwestern Manteca are the freeways. McKinley Avenue that passes beneath the 120 Bypass was plugged with dirt during the 1997 floods while crews had also blocked off Louise Avenue under Interstate 5 in Lathrop and were prepared as well to plug Lathrop and Roth roads. The fear was a levee break could happen where a number of boils - bubbling water spots - popped up in the levees during the 1997 flooding. The state feared if that happened water would reach Weston Ranch within seven hours.

This time around a failure at the levee on the west side of Interstate in Lathrop would imperil hundreds of homes built in the Mossdale Landing neighborhood.

Also different is the level of protection on Stewart Tract which in the past has been the perennial scene of flooding. The developer of River islands at Lathrop have created super levees that are 300 feet wide to afford maximum protection for the first phases of the planned community of 10,800 homes. No homes have been built yet.

Reclamation District 17 work now underway

The chances of flooding in Lathrop and southwest Manteca will ultimately be reduced significantly thanks to $62.4 million in state bond money awarded to Reclamation District 17.

The work will maintain protection against a 100-year flood event and go a long way towards 200-year protection.  The 100-year and 200-year monikers are a bit misleading as they are described as flooding that has a chance of happening every 100 years based on natural conditions. As more valley soil has been paved or built on run-off has increased significantly reducing the times between major flooding.

The money is being wedded with $1 million being raised from a $37 parcel tax assessment over five years that a majority of 4,000 home owners in Lathrop, Weston Ranch, and the extreme southwest edge of Manteca voted to put in place. The parcel tax made it possible to secure the state bond money.

The parcel tax also allowed the district to secure a $10 million U.S. Army Corps of Engineers grant to improve flood protection for a large swath of land generally west of Interstate 5 from Mossdale north to Weston Ranch and a large chunk of the developed portion of the City of Lathrop.

It initially involves building a series of berms next to 21 crucial spots in the levees. The 50-foot long by 4-foot high berms will serve to strengthen the levees if water manages to seep through.

The biggest recorded flood in modern South County history started in 1950. It caused flooding west of present-day Interstate 5 in Lathrop. Flood waters threatened San Joaquin County Hospital and came within four miles of downtown Manteca. There were 2,000 people evacuated. Today, if the same flooding occurred, it would force 20,000 people to flee.

The large area southwest of Manteca that flooded in 1997 is not protected by Reclamation District 17 work. Instead, a separate district is moving forward with plans to upgrade levees along the Stanislaus and San Joaquin rivers in that area.

The City of Manteca is exploring the possibility of pursuing levee improvements as far south as Nile Garden School or extending the current levee that parallels Woodward Avenue west of Airport Way and extending it to a point about midway between Union Road and Tinnin Road.

The levee would be 80 feet wide and six feet high to the west of Airport Way, 50 feet wide and four feet high east of Airport Way, and 40 feet wide and four feet high east of Union Road.