All of Lathrop west of Interstate 5 and east of the San Joaquin River, every dwelling in Weston Ranch as well as southwest Manteca neighborhoods could end up under upwards of 10 feet of water in the event of a major flood rated as a 200-year occurrence.

More than 30,000 people would be forced to flee to higher ground including the entire community of Lathrop.

That’s what a 200-year flood composite floodplain of maximum flood depths being prepared for the cities of Manteca and Lathrop by Folsom-based Peterson, Brustad Inc. Engineering Consultants. State law requires 200-year flood plains to be identified and steps made to enhance protection or else face severe restrictions on new housing, retail and industrial development after July 2016.

At the same time the federal government will use such flooding models to require mandatory flood insurance that, based on a proposal that was narrowly defeated this year in Congress, could cost homeowners with government backed mortgages in excess of $2,000 a year.

The Manteca City Council will review the map when they meet Tuesday at 7 p.m. at the Civic Center, 1001 W. Center St. 

If the city shows they are making “adequate progress” they may be able to defer for several years the July 2016 deadline that essentially stops all development within identified 200-year floodplains.

Those steps include:

• identifying required levee improvements.

• obtaining design criteria approval from an independent panel of experts.

• establish a financing plan and mechanism to fund the needed levee improvements.

River Islands at Lathrop would have been included in the 10-foot high floodwater area if it hadn’t been for the fact Cambray Group invested $70 million in creating super levees that are 300 feet wide. They received the highest possible rating by federal disaster agencies for being able to withstand a 200-year flood event.

A 200-year flood event doesn’t mean it will happen only once every 200 years. It simply refers to the worse that could happen every 200 years. The Northern San Joaquin Valley has been hit with three 100-year flood events in the past 60 years. 

Part of the reason for the increased frequency is development is increasing impermeable surfaces such as roof tops, parking lots and streets making is impossible for storm run-off to soak into the ground. 

Also, there is silt build-up in a large swath of the San Joaquin River between the confluence with the Stanislaus River and Mossdale Crossing that hasn’t been dredged.

The modeling is based on both rivers overflowing their banks or punching through levees as they did in the 1997 flood. Back then cross or dry levees kept water from reaching homes in Manteca. Crews were able to secure breaches before they got substantially worse. Had leeves failed, the state was preparing for a mandatory evacuation of Weston Ranch but not Lathrop. That’s because there was no development west of Interstate 5 and east of the San Joaquin River in Lathrop in 1997 nor was there south of the 120 Bypass along Airport Way. That is not the case today.

Back in 1997 the underpass on 120 Bypass for McKinley Avenue and the underpass on Interstate 5 at Louise Avenue were plugged with dirt by Caltrans to create makeshift levees to protect Manteca and Lathrop.

In today’s worst case scenario some 300 plus homes in Manteca south of the 120 Bypass and straddling Airport Way would have four to 10 feet of floodwater.

The city has tentatively looked at requiring developers of the Trails of Manteca to improve a cross levee that eventually would be extended to the east to a point near Tinnin Road to enhance flood protection for the area.