LATHROP – There hasn’t been a dam failure in Northern California for almost half a century.

But that’s exactly what planners are expected to brace for if home construction anywhere within the 200-year floodplain is to continue unabated for the foreseeable future.

And that means shelling out a lot of cash.  

The Lathrop City Council on Monday agreed to greenlight the geotechnical engineering work required as the next step to get approval from the State of California that would allow new home construction to continue within the confines of Reclamation District 17 – an area that includes a large portion of Lathrop and parts of Manteca, Stockton and San Joaquin County. 

The work will cost approximately $2.58 million and Lathrop will be on the hook for roughly two-thirds of that amount. All but $250,000 is being picked up by the major developers in the areas that would be adversely affected if construction wouldn’t be able to continue as planned. The cost doesn’t include any of the actual levee improvement work that is expected to cost significantly more.

River Islands, which built its levees from the ground-up to comply with standards that will likely easily clear the 200-year stipulation, is essentially its own reclamation district and will handle the work associated with getting state approval on its own.  The 11,000-home planned community is expected just to need certification as the levees were improved to handle a 200-year flood. Cambay Group spent $70 million nearly seven years ago to enhance the levees on Stewart Tract

The flood rating system, as explained by Lathrop City Engineer Glen Gebhardt, assigns a 1 percent chance of a given flood happening in that calendar year. For a 200-year flood that means that the percentage point is cut in half – noting that there has never been one recorded in this area, and the likelihood of something like that happening this far downstream would be extremely low.

The results of the engineering report would likely be shared with the other agencies that comprise the reclamation district provided that the necessary split in funding is received. 

Only then can the paperwork be filed with the state that shows the scope of the work that would be needed to be completed in order to meet the requirements and allow new home construction to continue. If approval is not granted then building will be halted in June of 2016. 

Gebhardt said that a variety of factors need to be addressed in order to create the sort of scenario in which a 200-year flood would even be possible – creating the sort of hydrological models that would include water spilling over Sierra dams and likely over the banks of the San Joaquin River if flows could not be maintained. The only other thing that could create that sort of a scenario would be the total collapse of an upriver dam, and there hasn’t be one of those recorded in California since a construction dam failed outside of Auburn in the early 1970s.

The report will also determine the best way to proceed with the work required to bring District 17’s levees into compliance. Gebhardt said that there’s the possibility that cross-levees – built abutting the current levees – could be built to prevent the water from flowing north-south in the event of a failure. Going through and strengthening all of the levees throughout the district could be another possibility.