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Update on Manteca dry levee project to protect thousands of homes being presented on Monday
SJ river airport
The San Joaquin River southwest of Manteca at the Airport Way Bridge on January 2017 as water reached the monitoring stage.

South San Joaquin County as well as the rest of California is in severe drought.

But one of the big worries of more than a few people including rural South Manteca farmers and residents, elected leaders in Manteca and Lathrop, and the handful of city residents not oblivious to the fact they reside in one of the most flood prone areas in California is too much water.

On Monday when the forecast calls for a high of 93 degrees with absolutely no chance of rain, the San Joaquin Area Flood Control Agency (SJAFCA) is updating the community on the Manteca dryland levee project. It is part of an overall effort to protect all of Lathrop on the east side of the San Joaquin River as well as a wide swath of western Manteca along much of the Airport Way corridor, French Camp, the county hospital and jail, as well as the Weston Ranch portion of Stockton from the ravishes of a 200-year flood.

The project update will be presented Monday at 6:30 p.m. at the Manteca Transit Center, 220 Moffat Blvd.

The overall project has an estimated price tag of $180 million. The bulk of that will be spent upgrading the levees along the eastern banks of the San Joaquin River from a point just south of the massive Wayfair distribution center to the French Camp Slough. The goal is to obtain the same 200-year certification the platinum level of levee construction at River Islands at Lathrop has earned.

The project along the San Joaquin River won’t replicate the River Islands super levee design that at 300 feet is roughly four times wider than typical river levees. Instead it will work primarily within existing right-of-way.


Dry levee will create

numerous impacts

As a result the location and scope of the work ranging from height to width for traditional “wet” levees is far from controversial given there are minimum negative impacts to people with the work needed to upgrade to 200-year protection.

The dry levee is a different story.

Depending upon its location it will require the taking of property and in some cases cut parcels in two. It will disrupt rural neighborhoods. It can also make farmland that could not be urbanized for sitting in the 100-year floodplain developable for more tracts of homes.

And depending on where you live south of the eventual alignment it could increase the amount of damage your home and property sustains in the next flood even if it is much lower on the scale of a 200-year event.

The 200-year moniker given to the level of flood protection the state has mandated be in place — or under construction — by 2030 in specific areas of California or else all new construction whether it be residential subdivisions or a room addition would be banned is misleading.

The measurement of flood events such as a 50-year flood is based on the probability of a flood of a specific magnitude happening in any given year. The area a 50 year-flood would inundate, as an example, would have a 1 in 50 chance of happening in a given year.

While the designation of a 100-year flood conveys a sense of a rare occurrence, it isn’t. There have been two 100-year floods in the Manteca-Lathrop-Stockton area in the past 65 years. The most recent was 1997 when 70 square miles between Manteca and Tracy flooded.

The 1955 flood covered almost every acre west of present-day Interstate 5 in Lathrop and sent floodwaters as deep as 6 feet into some sections of 125 blocks that flooded in Stockton.

The frequency of significant floods has changed in the past century. That is due to the creation of more impermeable surfaces such as rooftops, streets, parking lots, sidewalks, and driveways that reduces opportunities for rainfall to soak into the ground. The end result is more storm runoff.


Frequency of flooding

threats is picking up

And now that hydrology is changing in the Sierra due to the return of nature’s cycle of mega droughts coupled with climate change, experts anticipate a shrinking snowpack in the coming years that will be replaced with more rain in the upper and lower elevations. That would increase runoff and raise the potential for more frequent flooding.

The area south of Manteca has flooded 11 times since 1929. The most flood-prone is the area nestled against the banks of both the San Joaquin and Stanislaus rivers.

Dry levees — also known as cross levees — work to try and contain the spread of floodwaters after a river levee breaks. It operates much of the same principle as water tight compartments on ships that contain water inflow when the bulk is breached.

It is that function of the 200-year flood protection plan that has the most people in rural south Manteca on edge.

Their concern is that as flooding increases — even if it is on a scale that is much less severe than a 200-year event — so does the odds of the water backed behind the emergency levees goes higher and lingers longer than if nature were left to its own devices.

The result would be more property damage from floods south of the dry levee.

The stark reality for Manteca elected leaders is they need a more robust dry levee to protect over a thousand homes they’ve allowed to be built in the 100-year-floodplain since the flood of 1997 in the vicinity of Airport Way and to the west.

It was the dry levee now in place that was covered with plastic sheeting and sandbags in the 1997 flood by California Conservation Corps crews working around the clock that U.S. Senate Dianne Feinstein walked in high heels while escorted by then State Senate Mike Machado to inspect.

Whether that dry levee that had water lapping at its top would hold was a big question. California’s Office of Emergency Services at the time said it was touch and go for 48 hours.

But if it failed, they had a fallback. The McKinley Avenue underpass of the 120 Bypass had been plugged with dirt and covered with plastic sheeting weighted down by sandbags.

The same was true of the Louise Avenue and Lathrop Road underpasses in Lathrop.

Both the 120 Bypass and Interstate 5 had been designed to serve as emergency backup levees.

Should the river levees on the San Joaquin River at its most vulnerable point at the Mossdale bend fail today or the dry levee south of Manteca be breached during a flood event, they could still serve as emergency levees. But at that point thousands of homes and other structures would already been flooded.

Various Manteca dry

levee alignment options

Originally the initial alternative advanced for the Manteca dry levee was extending the existing dry levee across Airport Way and run it between Peach and Fig avenues with it being closer to the south side of Peach Avenue. It would run as far as Tinnin Road in the east.

Engineers studied and then plotted four other alternative routes that in reality are variations of extending the dry levee and swinging the levee alignment farther south.

Research was able to dismiss assumptions a more southern route would be cost prohibitive. That’s because an analysis showed the actual height of the levee would be increased 1½ to 2 feet by a more southern alignment.

A more southern levee would extend roughly half a mile less to the east. It would end before reaching Union Road.

It would require a seepage berm footprint of 300-foot width along with a 20-foot wide access road on the north side of the levee. That seepage berm would end where hydraulic loading is less than 2 feet that occurs just west of Oleander Avenue. The 300-foot width is a maximum. Hydrology may allow the seepage berm to be as narrow as 100 feet in spots.

The most southern basic alignment would require a levee starting at 3.8 feet and then dropping to just over a foot at its eastern most point.

Most of the property south of the southernmost levee alignments is at an elevation where it is currently subject to somewhat greater than 3 feet of water during a 200-yead flood. As you head east the flood risk is between 1 and 3 feet.

An auxiliary issue for property owners is where the levee alignment would send the east-west Raymus Parkway alignment that would run from the new McKinley Avenue/Woodward Avenue intersection to as far east of at least Austin Road.

The southern alignment would set the stage for more residential development that the general plan update now being processed would allow by taking floodplain out of development.

Based on the land areas involved in the proposed Lumina neighborhood on the southwest corner of Airport Way and Woodward Avenue that is proposed for 827 homes, the area being taken out of the floodplain west of Airport Way and south of the existing dry levee could accommodate upwards of 1,700 more homes.

That would help make financing for the local share of the $180 million project to provide enhanced flood protection along the San Joaquin River for parts of Manteca, Lathrop, Stockton (Weston Ranch), and almost all of French Camp. All new construction within the 200-year floodplain in Manteca and Lathrop has an add-on fee for flood protection.

River Islands at Lathrop —with 300-foot wide super levees — isn’t expected to have issues if water flows in the San Joaquin River triple by 2065.


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 5,000 existing homes with 3 feet or more of water.

*endanger and force the overall evacuation of 52,000  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 88,500 existing Manteca residents and more than 14,000 of Lathrop’s nearly 30,000 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