Mayor Steve DeBrum believes there is a cheap solution to improve flood protection for Manteca and neighboring Lathrop — dredging the San Joaquin River.
DeBrum during Tuesday’s council meeting said he plans to approach Congressman Jeff Denham to see if he could help persuade the Army Corps of Engineers to push for dredging between Mossdale Crossing and Vernalis just south of the confluence of the Stanislaus and San Joaquin rivers south of Manteca.
And while dredging may enhance overall flood protection especially in 100-year flood events that have happened nine times south of Manteca since 1928, it likely would not get Manteca and Lathrop off the hook for 200-year flood protection work expected to cost upwards of $168 million. That’s because the existing levees along the only two rivers are rated for 100-year flood protection. That said, dredging could reduce the potential for future flooding.
It’s a longshot at best, however given the severe aversion state and federal agencies have to river dredging due to environmental concerns. The positon of many state and federal agencies is that sediment build up is part of the environment and therefore dredging would constitute a major disruption.
There is anecdotal evidence that silt build up has raised the river bottom six tor so feet since the 1950s. That was primarily due to silt run-off from water being returned to the San Joaquin River from the valley’s Westside.
The late Alex Hildebrand — a Manteca farmer as well as engineer and former Sierra Club president who was considered an expert on water issues by many state leaders and water agency bureaucrats — believed that to be the case as well after solutions were being sought to stop a repeat of the 1997 floods south of Manteca.
Then State Senator Mike Machado pushed to have language added to the $2 billion state water bond approved by voters as Proposition 13 in 2000 that earmarked $45 million for flood control projects and studies. Nothing was spelled out specifically about conducting a feasibility study of dredging the San Joaquin River segment between Vernalis and Mossdale. Machado encouraged local leaders to push for such funding if the bond was approved.
The state legislature ended up commandeering $1 billion to help cover Department of Water Resources’ operational budgets during Sacramento’s cyclical budget crises. At the same time, though, local leaders never mounted an organized effort to get the state to do a feasibility study to determine if dredging was an effective way to increase the river volume.
Back in the 1860s and 1870s the river was deep enough for river barges to take grain loaded at San Joaquin City that was located south of the Airport Way crossing and transport it to San Francisco.
Hildebrand also warned that the decision by River Islands at Lathrop to put in place 200-year flood protection levees would put pressure on the levees protecting farmland to the east of the San Joaquin River south of Manteca. Dredging could help reduce pressure on the levees protecting the east side of the river during the more common 100-year flood events that are occurring more often due to silt build-up and increased paving upstream that in turn increase runoff on the San Joaquin River watershed.
Longtime South Manteca residents that live near the San Joaquin have noted tree and vegetation buildup between the levees have also impacted water volume capacity that is critical when flows approach flood stage.
Replacing, extending and heightening the secondary levee south of Woodward Avenue currently ending west of Airport Way is considered the most economic way to secure the state mandated 200-year flood protection for Manteca.
Experts have the existing secondary levee is essentially a “pile of dirt.” The replacement levee would need to meet current standards for flood protection.
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