by Keith Heyer Meldahl
Walk on a Southern California beach, and a sense of permanence may come to mind. The sand scrunches predictably underfoot, the coastal bluffs loom seemingly unchanged, and the sea brushes the shore with its same ageless rhythm. Yet the scene can quickly change. Waves from a single storm may erase that beach. Portions of the bluff may collapse without warning. A large earthquake might elevate the coast several feet in an instant. And if we flip back through just the last few million years, the coastal scene, far from appearing stable, looks like frenetic animation. The sea bobs up and down, earthquakes crackle without letup, tsunamis wash ashore, and islands lurch up from the sea.
Does it matter to know these things? I think yes. Probing Southern California’s geologic past can inform decisions we make today. The past tells us that earthquakes and tsunamis will strike the coast again, and although we cannot predict when or where, we can prepare. It also tells us that Southern California’s beaches are in constant flux, with sand arriving and leaving in vast quantities every year. But river dams and seawalls have choked off sand arrivals to the beaches, so now more sand leaves than arrives. Shrunken beaches give coastal bluffs less protection from wave attack. Today, miles of rock and concrete armor much of Southern California’s coast, but these only postpone the sea’s advance. And what of the sea itself? Here too, the past is clear. In recent geologic time, the sea has risen and fallen hundreds of feet as polar ice sheets have come and gone. By happenstance, much of human history has unfolded during a time of unusually stable sea level. That is changing. We presently face a probable sea rise of two to six feet over the next century.
These developments—shrinking beaches and rising seas—point to a looming coastal erosion crisis for Southern California. How will we handle it? Perhaps through a combination of managed retreat and beach replenishment (importing sand to depleted beaches). But the scale of such replenishment will necessarily be enormous. We will need to import enough sand onto our beaches to make up for ongoing losses from dams and seawalls and to keep up with the rising sea. I’m reminded of Alice in Wonderland, where the Red Queen explains to Alice, “You see, it takes all the running you can do to keep in the same place.”