What happens to nearshore sand when cyclones migrate along coasts?

What is it about?

The recent Cyclone Hillary caused disruption in the southwestern USA, but what happened offshore? We studied an old set of measurements made with current meters in the 1970s to investigate how sand off the Southern California moved during passage of cyclone Norman. The meters were installed in 15 to 60 m depths off Del Mar. The cyclone caused most of the sand to move along-coast, carrying sand particles suspended in the water. That movement was slightly oriented offshore, so it led to a small movement of particles from 15 m to the greater depths, where it may have deposited. It would have contributed a small amount to the outer edge of a wedge of sand we see in the bathymetry (Figure 2).

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Photo by KC Welch on Unsplash

Photo by KC Welch on Unsplash

Why is it important?

Clinoforms are classical geological structures comprising dipping surfaces (Figure 1). The results contribute to other studies suggesting that many of these features form by sediment carried primarily along-slope. In outcrop (Figure 1), it might be tempting to interpret the particles as having moved from right to left but without current direction data the particles may instead have largely been moved perpendicular to the section. Del Mar wave data also suggest that more extreme waves would have remobilised sand on a large part of the clinoform foresets (dipping part) there, hence current direction indicators that we typical look for in ancient outcrops, such as current ripples, may get destroyed. In the case of Hillary, the cyclone's centre lay inland, in contrast to Norman, which was offshore. The water movement would have been opposite to that of Norman but otherwise still likely involved mostly along-slope movement with minor offshore components leading to sand moving offshore. Many of us have been interested in studying how geometrical characteristics of sedimentary bodies vary with possible causative parameters, the subject of "morphometrics". However, these new results illustrate that morphometrics are unlikely to be so powerful when applied to clinoforms. In situ measurements of currents, sediment properties, etc., are needed to work out how clinoforms develop.

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