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$3.6M needed to avoid outlet canal flooding

It could cost $3.6 million to reduce the chances of the French Camp Outlet Canal on Manteca’s western flank from spilling its banks during a future 48-hour storm event that has a 1 percent chance of happening in any given year.

That is based on increased runoff that would be generated under the proposed updated general plan for Manteca that envisions a population of possibly 119,000 by 2040.

The determination was made in a study Carrollo engineering services commissioned jointly by the South San Joaquin Irrigation District that owns and operates the canal and the City of Manteca that the SSJID allows it to use to send storm runoff to the San Joaquin River. It is just one puzzle in many storm runoff related issues that the city hasn’t physically addressed.

It also underscores the challenges Manteca is up against with water.

*There is a large tab for the city’s share of $180 million needed for regional 200-year flood protection that includes the southwest portion of Manteca and a long swath of development straddling the Airport Way corridor north of the 120 Bypass.

*Reoccurring drought conditions and a looming state mandate to not pump more groundwater from a basin than is recharged in a given year will trigger a need to expand or stretch the city’s future water supply. That can be done by recycling wastewater, triggering the phase of the South San Joaquin County Surface Water Treatment Plant, or both. Either which way, it will be an expensive proposition.

*Treating water before it is returned to the river — either wastewater or storm water — requiring meeting ever tightening federal and state standards.

*The city needs to impose higher water rates not just for operation, maintenance, and pipeline/well replacement costs moving forward but to also pay back almost $20 million from other city accounts spent for water system work over the past decade.

*Making sure that as the city grows the current aging storm system pipelines and pumps stay functional. That will require capital outlay that likely will be significant over a period of time.

Storm water removal becomes a bigger issue every time another rooftop, street, sidewalk, patio, or any other impermeable surface is created in Manteca.

That’s because the ability of rain to seep into the ground is eliminated.

The accumulative effect is the more land that is paved over the more runoff from storms of specific sizes.

Currently the French Camp Canal Outlet is the only way city storm runoff reaches the river. The city’s system of storm retention basins has effectively prevented the canal from overflowing its banks so far. Water is stored in the basins until such times there is capacity in storm drains and the canal to flow it to the river.

The city is working with the Oakwood Shores project to get a second outlet to the river.

The use of the SSJID canals over the years has been a tremendous cost savings for the city. The outlet canal was put in place to ferry overflows of irrigation canals to the river. During the rainy season when irrigation flows are not running it allows any runoff that ends up in the SSJID canal system to move to the river.

The French Camp outlet canal has never overflowed. Models show that won’t necessarily be the case going forward as Manteca continues to grow.

There are 12 projects estimated to cost $3.6 million to reduce future overdose risk.

Two culverts need to replace culverts at Roth Road and the Union Pacific Railroad crossings with box culverts for $1.7 million, removing debris beneath a railroad trestle bridge, raising the banks in five places, and monitoring devices in four places.


To contact Dennis Wyatt, email