The Nature of Sand Dunes
Like the tides, dunes come and go and come again. But these seemingly simple features of the beach do more than just absorb the force of hurricanes and nor’easters. Exquisite physics gird their stoic nature. During a major storm, the frontal dunes sacrifice their sand to raging waves. Then the healing begins. The dune exists in a cycle of destruction and reclamation. The breaking waves of a storm surge eat away at the front face of a dune, but the sand doesn’t disappear. Instead, those same waves create underwater currents that move the dune’s dislodged sand just offshore. Most of it piles up in submerged bars, waiting for nature to take its course. Normal post-storm currents reverse course and gradually push the offshore sandbar back toward the beach. The berm at the toe of the dunes begins to rebuild. It widens as more of the original dune sand rides the waves shoreward. Meanwhile, vegetation begins to take root on the eroded face of the dune, either when seeds sprout or by the spreading of rhizomal root mats. Each sprig of growth, no matter how small, seines sand from the wind. The trapped sand stimulates more growth from dune-adapted plants, resulting in taller, fuller vegetation. And even greater quantities of trapped sand. Depending on the storm’s severity, it might take 10 years for a dune to recover from a single eroding event. But recover it will, provided the natural process isn’t stymied by seawalls, groins, jetties, sandbags, dredging, construction, or the deposition of ill-fitting sands through beach renourishment programs. —T. Edward Nickens
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