Had Manteca followed its own general plan adopted in 2003, all of the homes built south of the 120 Bypass straddling Airport Way would have been built on berms or stilts so that the lowest floor was at least one foot above the 200-year flood plain.
In order to comply with the general plan the homes west of Airport Way south of the 120 Bypass would have had to have been elevated three or more feet higher than they are right now. The homes east of Airport Way should have been one to three feet higher.
At the same time non-residential projects such as the Stadium Retail Center was supposed to be “anchored and flood-proofed in accord with Federal Emergency Management Agency standards” or elevated to at least one foot above the 200-year flood level. That was not done.
Much ado has been made about Senate Bill 5 forcing Manteca’s hand when it comes to 200-year flood protection but Manteca elected officials 14 years ago in adopting the general plan acknowledged the problem and adopted policies that said certain things “shall” happen with new construction but they never did.
The safety element of the general plan speaks volumes about issues with actually turning adapted policies cobbled together over a three year study period into reality. The subdivisions and commercial approved in the 200-year floodplain clearly reference that they are in compliance with the general plan when they were not when it comes to the 200-year floodplain general plan policies.
Part of the problem was the 200-year floodplain wasn’t identified until 13 years later in an amendment to the general plan’s safety element spurred by passage of the state mandate that communities had to have such protection in place or make sure development was above the 200-year flood level in order to allow development to continue.
Outside of a storm of non-stop storm of biblical proportions over 40 days and 40 nights, the absolute worst case flooding scenario for Manteca is the unthinkable — the failure of the 2.4 million acre foot New Melones Reservoir. It goes way beyond a 200-year event and enters the territory of being a 500-year flood.
Such a catastrophe would flood all of the City of Manteca and all of the surrounding rural area to a point running roughly a mile north and parallel to French Camp Road.
Right behind it in the No. 2 worst scenario flood would be the collapse of the San Luis Reservoir west of Los Banos in Merced County. It would flood a third of the urbanized parts of Manteca and come within a mile of downtown.
Flooding potential is the most serious safety concern delineated in the city’s state mandated general plan designed to serve as a blueprint for growth. Other major safety concerns identified are the transportation of hazard materials by rail and truck, earthquakes and subsidence caused by groundwater pumping.
Earthquakes and subsidence are downgraded a bit as major concerns for the Manteca general plan area. There are no faults in the Manteca area although the county has two known faults — the Tracy-Stockton Fault that runs east to the Calaveras County line and the 18-mile Corral Hollow Fault running along the county’s southwestern border near the Coastal Ranges. The last Tracy-Stockton Fault quake that happened in 1940 registered 4.0 on the Richter scale. Scientists believe the Corral Hollow Fault detected in 1991 is capable of a quake between 6.3 and 6.7. The 1991 Loma Pieta Quake was 6.9.
Subsidence is not one of 19 soil types found in the Manteca area.
The high risk of flooding — major flood damaging events involving the San Joaquin River have happened in 1861-1862, 1867, 1881, 1890, 1904, 1907, 1909, 1911, 1928, 1950, 1955, 1959, 1962-1963, 1968-1969 and four times since 1980 including January 1997 — is the reason why 200-year flood protection is identified as a need in the city’s existing general plan. The passage of State Senate Bill 5, however, created a situation where the city could not allow ant additional building after a certain point unless levees protecting against a 200-year flood — an event that has a 1-in-200 chance of happening in any given year — was put in place.
Manteca’s share of the $180 million protection could come to between $40 million and $60 million.
The 2003 general plan did specifically address development issues regarding public safety and floods in the 200-year flood zone were never followed.
The Manteca City Council that adopted the general plan agreed that new residential development including mobile homes “shall be constructed so that the lowest floor is at least one foot above the 200-year flood level.” Roughly the same requirement was in place for commercial uses as well.
To contact Dennis Wyatt, email firstname.lastname@example.org