Facts Needed on Delta Sea Level Rise Projections
Friday, August 12th, 2011
By Barbara Barrigan-Parrilla
Restore the Delta
Among many other things you will find in the fifth staff draft of the Delta Plan, issued last week, is the subsidence/earthquake/sea level rise mantra repeated over and over and over and OVER.
Roger Patterson of the Metropolitan Water District of Southern California (MWD), among others, is going around chanting the same mantra.
Facts hardly matter in the face of this kind of religious conviction.
But we’ll put some facts out there anyway. These are from the July 21 draft of the Delta Protection Commission’s Economic Sustainability Plan, Chapter 4 (“Flood, Earthquake and Sea-Level Rise Risk Management”) and Appendix D (“Clarification of Some Basic Issues with Regard to Delta Levees”).
Recent estimates of levee fragility rely on the Delta Risk Management Strategy (DRMS) Phase 1 report, which is based on older test borings and insufficient testing, and which doesn’t take into account ongoing improvements to Delta levees. (People in the Delta have not just been sitting around waiting for the rest of the state to decide what to do with their levees.) Use of decades-old data led to erroneously high failure probabilities. DRMS has been criticized by a number of reviewers, including an independent review panel assembled by the Cal-Fed Science Program.
The Economic Sustainability Plan (ESP) notes that although there were many levee failures in earlier years, the majority resulted from overtopping, not collapse. “Improved flood management, in addition to other improvements in the levees, has significantly reduced the rate of failure. Today’s levees, which retain water 24 hours a day, have demonstrated an ability to withstand normal tidal and typical flood loadings regardless of their variability [in construction and composition].”
Regarding vulnerability to tides and floods, “High water elevations resulting from tides and floods can . . . be seen days or weeks in advance so that appropriate emergency measures can be taken. The probabilities of failure due to overtopping that are calculated in DRMS appear to be inconsistent with these realities.”
Regarding subsidence, it is worth remembering that this is a river delta with land protected by levees, and it is not abnormal for the land side to be lower than the water side. Interpretation of the data suggests that problems associated with subsidence, such as impaired drainage, are occurring on less than 14 percent of the area of the Delta; continued subsidence is not a Delta-wide problem.
And anyway, subsidence of even several more feet has little impact on the stability of levees that are already 20 to 30 feet high on the land side.
Sea level rise, meanwhile, can be addressed by routine maintenance of levees improved to the state and federal standards already in place for Delta levees.
But aren’t the Delta levees vulnerable to earthquakes? No more vulnerable than other portions of California through which water transfer infrastructure moves, and maybe less vulnerable.
“Meticulous work by Drexler et al. (2009) indicates that the oldest peat deposits [in the Delta] are in the order of 7,000 years old so that the underlying sands are at least this old.” These sands are not especially susceptible to liquifaction. “The repeated citing of levee deformations that were sustained in the Kobe and Christchurch earthquakes, which had higher ground motions and where levees were founded on very loose and recent alluvial soils, is not particularly helpful. However, although these case histories are not directly applicable to the Delta, they do illustrate that levees do not necessarily breach and release water, even when they are quite badly deformed. In fact, to the extent that the Delta levees are largely composed of peat [which is fibrous], they may be expected to perform better than levees in general under earthquake loadings.”
What about “sunny-day failures”? There has only been one in 30 years due to weaknesses in levees. Two other sunny-day failures in that period resulted from operation of the PG&E gas storage facility under a Delta island.
The Economic Sustainability Plan recommends reducing risks resulting from levee failure by building more robust levees; improving regular maintenance and monitoring; improving flood-fighting and emergency response; and improving preparedness for dealing with failures when they do occur. All this can be done at far less cost and with far less disruption than building a big canal or tunnel.
Much of the money to do the necessary work on Delta levees has already been allocated in voter-approved bond issues. So why hasn’t it been spent?
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