Many divers think the craters are made by bat rays, big, winged fish that expertly dig up the sand while foraging for prey. However, bat rays hunt beyond the surf zone. It turns out that sand crater formation near the water’s edge is not biological, but physical. Officially called “scour holes,” the depressions are made by waves and currents, which shift the sand, especially at the base of a shoreline. Geology is also involved. Sandy beaches almost always have layers of cobbles beneath them, and these are commonly exposed during the higher waves of winter, which carries the sand further offshore. The smaller waves of summer return the sand onshore in large, smooth sheets that eventually blanket the cobbles. Though only a thin layer of sand is needed to hide the cobbles, at least a foot-thickness of sand needs depositing before the cobbles are no longer influenced by wave action. And since the cobbles don’t layer evenly as does sand, sand grains fill the pockets around the stones but do so irregularly, covering the cobbles faster in some areas than others. Rip currents may also scour sand, exposing cobbles in some areas of the beach more than others, particularly near where the waves break. In a nutshell, scour holes are a sign that sand levels are changing and water is in motion.
Interestingly, scour holes have a deep connection to the levee breach from Hurricane Katrina. When storm water overran the levees, the normally dry side of the levee structures was pounded by torrents of water, which led to scour holes developing in the sand below. Keep in mind that while the levee foundations support the levee, it is sand that supports the foundations. Floodwater turmoil scoured away the sand, exposing and, so, weakening the foundations until they collapsed.
Scour holes may be a nuisance at La Jolla Shores, but they can also highlight a failure in civil engineering design. It is both human nature and human folly to want to tame the ocean’s edges but time and again, the ocean decides when to take it back.
— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org.