The biggest threat to Manteca and Lathrop: Flood or drought?
As a light rain fell Tuesday night hopefully putting a small dent in the three-year drought griping the Northern San Joaquin Valley and the rest of California, Manteca’s elected leaders were getting an update on steps being taken to avoid a catastrophic flood
All of Weston Ranch, most of Lathrop, and parts of southwest Manteca would be under at least three feet of water should a 200-year flood occur if levees along the San Joaquin and/or Stanislaus rivers fail.
Hydrologists retained by the cities of Manteca and Lathrop have determined where water would flow in the event of a 200-year flood event. The moniker refers to flooding that odds based on historic data indicates could happen every 200 years. But in reality the frequency of such events can be closer than that depending upon conditions. And given the fact that more and more of the valley is being paved over and having roof tops added, the amount or runoff based on reduced areas where water can be absorbed increases with every passing year.
The engineering firm of Peterson Brustad is working with the two cities in their bid to comply with a state mandate that they have 200-year floodplains identified, plans developed to protect the area, and a funding strategy in place to make those plans a reality by July 1, 2016. If not, all building activity must cease with 200-year floodplains. All improvements have to be done by July 1, 2025.
Some 20 miles of levee improvements needed are expected to cost $145 million of which Lathrop will pick up two thirds and Manteca the remaining third or roughly $48 million.
The last flood was in 1997 when seven levee failures occurred along the San Joaquin River and Stanislaus River. Waters flooded 70 square miles south of Manteca and east of Tracy causing $100 million in property damages. At one point a 24-hour watch was placed on a dry levee covered with tarp just south of the Highway 120 Bypass where the McKinley Avenue undercrossing had been plugged with dirt by Caltrans. Had that levee failed flood waters would have made it to the bypass and up the Airport Way corridor.
Weston Ranch was on voluntary evacuation as boils — the precursor to actual breaks — on the San Joaquin River levees had been detected. At the time, there was no development in Lathrop west of Interstate 5 that would have been flooded. The Interstate 5 freeway had been designed as a makeshift levee allowing crews to also plug the Louise Avenue undercrossing as a precaution with additional dirt placed at the Lathrop Road undercrossing to be put in place in the event of an actual break.