Picture it as a narrow capital 'V' that sits mostly beneath the surface of the water, but stretches across the mouths of bay inlets. The top of the stem closest to the shore could double as a dock, and the edges might include locks for small craft. The oceanside stem would be capped with a mechanical wall that tilts higher as sea levels rise, creating a waterfall that drops into the space between.
Anyone standing on the shore would see a nearly natural maritime scene: a dock, the bay and a small waterfall when the levee is at work.
A system of "pump ventilators" would be built into the walls, returning excess water to the sea while mimicking the effects of tidal exchange. Natural pressure would force ocean waters and small sea life into the estuary through one side of these tubes, while the mixed water would be pumped back out the other side.
Protecting all the low-lying areas at risk along San Francisco Bay would require 10 to 15 levees, each of which could be calibrated to maintain the natural ecology of a specific cove.
Folding Water would operate autonomously, using a system of sensors, smart software and clean energy from tidal turbines and geothermal wells. Working out the technical details will be a challenge, but outside observers say the basic concept is sound.
It would surely be substantial, however. A 2009 report by the Pacific Institute in Oakland estimated that it would cost $14 billion (in year 2000 dollars) plus $1.4 billion in annual maintenance to armor the California coast line for sea level rise, with the majority of the spending in the Bay Area.
BCDC, however, estimates that the projected sea level rise by 2100 puts $62 billion in shoreline development at risk.