The BDCP unveiled much anticipated new cost estimates for a peripheral canal, and the new proposal for a tunnel conveyance under the Delta. Here is a powerpoint of the proposal.
I am no engineer, but I can intuitively see how there would be big economies of scale in constructing a large capacity surface canal. However, it seems that when you are boring giant tunnels, the marginal cost of increasing the capacity would be relatively high.
Thus, it is interesting to me to see that only a 2 bore parallel tunnel (with the same total 15,000 cfs capacity as the surface canal) was presented. I would be interested in seeing a single bore tunnel proposal. It seems plausible that it the cost of the project would decrease by more than the benefits, if you had only 1 bore. It also seems that a physical constraint on export capacity would also go a long way towards building trust with Delta, Northern California, and environmental interests that freshwater Delta flows would be sustained.
I don't claim to know the answer, I would just like to see the 1 bore option evaluated. I have no doubt that others inside the process have asked the same question, and it will be interesting to see if it is answered.
Many people criticize desalination because of cost and other issues, and these are very legitimate issues. However, one of the advantages of desalination over a peripheral canal for improving water supply reliability is that you can buy desalination capacity in smaller increments rather than a single $10 billion chunk like a peripheral canal. Given the multiple uncertainties involved in all strategies, I think that flexibility is valuable. If tunnels are not wildly more expensive (as this BDCP presentation suggests) maybe it is possible to add alternative conveyance in smaller increments that have less economic and environmental risk.