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Without $175M levee upgrade thousands in Manteca, Lathrop would need flood insurance
levee map

Failure of levees during a 200-year event would send three feet or more of water to cover almost all of Lathrop, flood neighborhoods in Manteca southwest of the Airport Way and 120 Bypass interchange as well as inundate the first floor of the 500-room Great Wolf Lodge and indoor water park.

Areas in the City of Manteca where almost 4,000 homes have been approved west of McKinley Avenue along with the existing Oakwood Shores development could be covered with more than 10 feet of water. The same applies for the new Wayfair distribution center in Lathrop along the 120 Bypass.

Protecting against a 200-year flood — which references the magnitude and not the frequency of flooding — is a state mandate that must be in the process of physically be constructed by 2025 or else all development will stop in the identified areas. It should be noted that continued development of non-porous surfaces from rooftops to pavement throughout the San Joaquin Valley is increasing the likelihood and frequency of flooding.

 Manteca and Lathrop are protected against 100-year floods by Reclamation District levees. After the state raised the minimum protection standards to 200 years, the two cities had to show progress toward meeting the 2025 mandate each year or the state could order a moratorium on building in affected areas.

The two jurisdictions have meet annual progress standards by implementing funding mechanisms, adopting a time table, and executing various steps that move the flood protection work closure to breaking ground.

While the bulk of the $175 million goes toward addressing seepage issues along San Joaquin River levees, a dry levee in southwest Manteca plays a key role in making sure potential breaks along the San Joaquin south of RD-17 or levee failures on the Stanislaus River don’t flood portions of either city.

The exact alignment of the dry levee envisioned to protect thousands of Manteca homes now being built south of the 120 Bypass west of Union Road against the possibility of a 200-year flood should be known later this year.

The San Joaquin Area Flood Control Agency (SJAFCA) is in the process of moving toward environmental work needed as a precursor to the $12 million levee that is part of an overall $175 million levee upgrade aimed at protecting all of Lathrop and much of southwest Manteca as well as along the Airport Way north toward French Camp Road. The cost of the levee work will be borne by those owning property in the impacted area.

The general alignment was selected several years ago by the Manteca City Council.

It strengthens an existing dry levee that runs from Weatherbee Lake at the western end of Woodward Avenue and “disappears” when it blends in with the grade level of the ground at a point west of Airport Way midway between Peach and Fig avenues.

The general alignment that Manteca Councilman Gay Singh who also serves as the SJAFCA chair said at last week’s council meeting “wasn’t yet cast in stone” would turn south at a point of the existing dry levee roughly midway between McKinley Avenue and Airport Way. It would then swing east forward Fig Avenue and would shift farther to the north to run behind existing homes along Fig Avenue to end at a point just west of Tinnin Road.

The local share of the overall $175 million project will be covered by benefit assessment districts involving property within the 200-year floodplain, development impact fees, and tax increment financing,

It was noted property owners may balk at approving a benefit assessment district. However, if such as district wasn’t approved making the project finically unfeasible anyone with a mortgage on their homes or other structures would be required to purchase flood insurance that would cost in excess of $2,000 per home on an annual basis.


What would impacts

of 200-year flood be

Should a 200-year flood occur with multiple levee failures along the Stanislaus and San Joaquin rivers south of the Interstate 5 bridge before the merger with the 120 Bypass, engineers have indicated it would:

*flood 5,200 existing homes with 3 feet or more of water.

*endanger and force the overall evacuation of 50,000  residents in Lathrop outside of River Islands, Weston Ranch in Stockton, southwest Manteca, and rural areas

*force the evacuation of San Joaquin Hospital — the county’s major trauma center — as well as the county jail.

*force first responders at five fire stations, the Lathrop Police Department and the county sheriff to abandon their stations and key communication centers in the middle of a major emergency.

*Lathrop High and Weston Ranch High would have water flowing through their campuses as would six other Manteca unified elementary schools.

*force the closure of portion of Interstate 5 — the major West Coast freeway running from Mexico to Canada — and the 120 Bypass.

*water would swamp the wastewater treatment plant serving 85,000 existing Manteca residents and more than 13,000 of Lathrop’s nearly 24,000 residents.

*disrupt Union Pacific Railroad train movements as well as damage tracks that Altamont Corridor Express relies on.

*182 commercial and industrial properties from Costco to the Lathrop Target and Tesla Motors to Simplot would be flooded.

And that’s just for starters. Modeling shows a number of existing homes would likely suffer water damage in fringe areas that could receive upwards of three feet of flood water.

Manteca, Lathrop, and Stockton aren’t the only communities impacted by the Senate Bill 5 mandate. There are 85 cities in 33 Central Valley counties that have to comply.


To contact Dennis Wyatt, email