Posted by: Deirdre Des Jardins | February 27, 2017

Urban Delta – 264,000 people at risk from flooding in Stockton area

The largest cities in San Joaquin County are on the eastern edge of the Sacramento-San Joaquin Delta, and include Stockton (population 292,000), Lathrop (population 18,000), and Manteca (population 72,000.) The Stockton metropolitan area is protected by a system of levees that was built between the gold rush and the 1920s.  Parts of the system were improved by the Army Corps of Engineers after the Flood Control Act of 1944.   After Hurricane Katrina, the San Joaquin Area Flood Control Agency worked with the Army Corps of Engineers to create a plan for upgrading the levees to 200 year levels of protection.  The Corps released the Draft Lower San Joaquin Interim Feasibility Study and Environmental Impact Statement / Environmental Impact Report in February 2015. The final feasibility study for the $803.7 million plan has yet to be completed.

In the interim feasibility study, the Army Corps of Engineers noted that 264,000 people live in floodplains in the Stockton metropolitan area, with $21 billion in damageable property and 23 critical structures:

The existing levee system within the study area protects over 71,000 acres of mixed-use land with a current population estimated at 264,000 residents and an estimated $21 billion in damageable property. In addition to the residents and property, the levee system protects approximately 23 structures considered to be critical infrastructure (hospitals, police and fire stations, etc.) as well as the Interstate 5 and State Highway 99 corridors.

and concluded that “[t]here is significant risk to public health, safety, and property in the project area associated with flooding” (p. 90, emphasis added.)

1Lower San Joaquin plan area levees         Source: Army Corps of Engineers feasibility study, p. 89

Seepage endangers levees

The Army Corps also explained in the feasibility study that the lower San Joaquin levees have problems with seepage, and are at risk of breaching at river stages below flood:

The potential for seepage problems to occur along the existing levees in the project area is created by discontinuous layers of coarse‐grained pervious soils (i.e., sands and gravels)… During high‐water events, water from the river can enter the pervious soil layers and then move laterally through these layers under/through the levee. Excessive seepage can erode soil within the levee and lead to a rapid collapse and subsequent breach… The risk of levee failure is not due to design deficiency or to lack of O&M of the existing levees, but to a better understanding of the mechanics of under‐seepage in the Central Valley. The project levees within the study area do not meet current USACE levee design criteria and are at risk of breach failure at stages considerably less than levee crest elevations. This is evidenced by historical levee boils and heavy seepage at river stages less than design flows.  (p. 90, emphasis added)

Because of these problems, the Army Corps estimated that the “risk and uncertainty” of levee failure was 2% a year for many levees on the lower San Joaquin.  The map below shows the levees that are at risk of failing, and the depth of flooding in a 500 year flood.   While it is extremely unlikely that all the levees would fail simultaneously, the ACE estimated that they are all at risk of failing during high flows and need remediation.

2 500 year flood depths                   Source: Army Corps of Engineers EIS/EIR, p. 86

High water perils

During the high-water periods in January and February of 2017, locals have been patrolling the Stockton metropolitan area and Delta levees, looking for signs of levee seepage or boils. Flooding southwest of Manteca was narrowly averted when a firefighter from the Lathrop-Manteca Fire District spotted a sinkhole as it was occurring.   While these efforts to protect the levees have been heroic, the stressed levees could easily fail in a flood.   Below is a picture of flooding of Stockton in 1955, from the Army Corps, giving an idea of the amount of flooding that would be experienced with major levee breaches.

3 Stockton flooding, 1955                                         Source: Army Corps of Engineers

State funding problems

The federal Water Infrastructure Improvements for the Nation Act (WIIN), passed on December 11, 2016, mandated that the Secretary of the Army expedite and give priority funding for the final Lower San Joaquin feasibility study, and authorized proceeding to planning, engineering, and design once the final feasibility study is completed.

However, there is no similar mandate at the state level for priority funding for completion of the Lower San Joaquin plan.  Proposition 1E accounting shows that line items for Lower San Joaquin planning totaled only $3.8 million in bond funds.    This was slightly more than 1/10th of 1% of the total of $2.895 billion authorized in Prop 1E for improving state plan of flood control facilities, increasing levels of flood protection in urban areas, and reducing risk of failure of Delta levees.   Almost all of the $2.895 billion in Prop 1E flood control funds has been obligated, and there is only $24.6 million in remaining unallocated funds.

Part of the funding issue was that the levees in North Stockton, which are most at risk of failing, were not part of the State Plan of Flood Control when Prop 1E was passed.   Although Prop 1E allowed the non-SPFC levees to be upgraded and added to the State Plan of Flood Control, the 2012 Central Valley Flood Protection Plan committed only to evaluating whether there was a “state interest” in upgrading the levees.   The following is from p. 51 of the 2012 Central Valley Flood Protection Plan:

Stockton Metropolitan Area – Improvements for this area include the following:

»» Improve SPFC levees along the San Joaquin River and tributary channels.

»» Evaluate the potential benefits of and State interest in local floodgates and control structures, as they relate to facilities of the SPFC in and around Stockton, and contribute to achieving an urban level of flood protection.

The Draft 2017 Central Valley Flood Protection Plan now includes upgrading both SPFC and “appurtenant” non-SPFC levees in the Stockton  area in the “urban portfolio” of projects. However, funding for the $803 million project will need to come from other bonds. Governor Brown has proposed to allocate $65 million in Proposition 1 funds for flood control in urban areas in the Delta, but that is the entire amount allocated in Prop 1 for urban flood control, and is likely also needed for emergency repairs.

This post was updated on March 1 at 5:00 pm.


Responses

  1. What is needed is attention to development of the Paradise Cut Bypass. Completing this project would relieve pressure on downstream levees, protecting Stockton, Lathrop and Manteca.

    Like


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