Infrastructure v. Public Works 3: The tunnel option

updated and revised August 2
This week there are reports that the DWR is studying the feasibility of constructing a 35-mile long tunnel as an alternative to the Peripheral Canal.

There are at least two major reasons why this is a really bad idea and a waste of time for engineers to bother with even very preliminary study.

First of all, a tunnel option of either 5,000 or 15,000 cfs capacity would not “sweeten” intersecting Delta rivers and sloughs. Tunnel proponents, therefore, cannot make a claim that the tunnel would help stabilize the Delta’s ecosystem, something that PC proponents can at least claim and possibly prove. If the tunnel option were framed as being a way to have 5 or 15 thousand cubic feet per second of Sacramento River water completely bypass the Delta, would it attract any support among environmental groups?

Since a tunnel would bypass the Delta entirely, why some seem to be suggesting that this option “might” be a better one than the PC is unclear. It might be, but so might a PC. If the reduced capacity of 5,000 cfs is thought to be a victory, it might still be better for the Delta if advocates push for that to be the capacity of the PC, plus an amount intended to be injected into the eastern sloughs and rivers the PC would intersect with.

Second, the costs to build a tunnel would be, as Matt Weiser estimates, “ginormous.” DWR spokesmen acknowledge that costs would be higher than the PC, but according to Lester Snow, “[w]e have to look at the trade-off between the extra costs of tunneling and how it compares to a canal.” The good news for tunnel supporters is that the absentee landowners of many of the islands that sit above the likely tunnel path would listen to offers for their property investment.

There is little technical information available at this point, but I thought it might be useful to crunch a few numbers that compare the tunnel option to the Canal. I am no hydraulic engineer, but accept as accurate an estimate that a 5,000 cfs tunnel would need to be “25- to 30 feet” across. Let’s assume that the number of 15,000 cubic feet per second remains the design capacity of the tunnel. Such a tunnel, if a cylinder, would be on the order of 55 feet in diameter. More likely, the tunnel would be a flatter, box culvert design of some sort, made of huge, poured-in-place or precast concrete sections, reducing the depth of excavation necessary.

In order to minimize the excavation depth, let’s assume a culvert cross-section depth of 20 feet. That would make the width of the culvert approximately 120 feet. Keep in mind that this culvert/tunnel will cross at least 10 sloughs and rivers, including the San Joaquin River Deep Water Ship Channel, which must be maintained to a depth of 35 feet. The San Joaquin is approximately 1/4 mile wide at any likely point of a tunnel, and a two-stage (to allow for continued passage to the port of Stockton) sheet piling dam operation would be necessary to excavate a 120 foot-wide “cut and cover” tunnel to an absolute minimum depth of approximately 60 feet below water level.

The tunnel would be about 15 miles shorter than the PC but about three times deeper. The PC would still require excavation to a depth similar to the tunnel at the siphons and culverts needed to pass below infrastructures and waterways. A rough estimate might be that excavation costs for a tunnel would be twice as much as for the PC.

And, like the Peripheral Canal, the tunnel would have to contend with intersecting railways, pipelines, and roads. It is difficult to come to any conclusion other than one that very roughly estimates that constructing this tunnel would cost at least twice as much as a peripheral canal. Something on the order of 17-25 billion dollars. This is a rich-man’s through-Delta alternative, and given the state’s grim financial future, will never happen.

Needless to say, one of the reasons why such a bad option might be the subject of an even cursory study is to make other options appear more appealing.