Naturally Kiawah Magazine Volume 34 | Page 14

The Dunes: Barrier for a Barrier Island Story and photographs by Paul Roberts The Sand If asked how the sand dunes along our beaches got there, most people would answer that they obviously come from the sea. Clearly sand is required to build a sand dune and it does come from the ocean. However, the ocean does not produce the sand; it only sorts it by size and pushes it up on to the beaches. Consequently, you could say that the process of building dunes actually begins onshore. The sand is brought to the oceanfront by fast-running streams from the mountains and hills, where larger rocks are broken down by freezing and thawing, by the grinding actions of glaciers, and by the steady action of running water. Rivers disgorge this load of sediment into the sea where offshore banks of sand are deposited underwater and moved grain by grain to the beachfront by wave movement, where it is eventually exposed by tidal action to drying in the sun and the wind. The Wind The smaller grains are then blown by the wind across the surface of the beach, bouncing and rolling until they fall into a place where they are protected—where the wind is no longer strong enough to move them, and there they sit, behind a log, a bush, a bottle, a small rise, or a piece of beach flotsam until something changes. Some very common windbreaks on our beaches are 12 bundles of dead spartina grass (referred to locally as “rack”) sloughed off the banks of tidal creeks by undercutting currents and carried into the ocean by the receding flow—eventually washed up on the beach by the tide and waves. Rack serves as a common point of beginning for the formation of a dune. It provides that little windbreak that disrupts the wind and catches more and more grains of sand in the quiet spot in the lee of the wind. As the wind continues to blow, more and more grains are pushed up the dune and fall off the backside expanding the base, and the dune gets higher and higher, creating an even larger windbreak. Eventually, the wind stops blowing, or changes direction, and the dune may shrink or change shape to conform to the new conditions, changing daily. The Valleys Up and down the beach, different conditions exist locally. Where the beaches are wider and prevailing winds stronger, higher dunes can be built. Where there is beachfront erosion, new dune formation is impeded. Seasonal storms may reshape the dune field dramatically. Rising water may push through the low spots between dunes into the valleys behind the beach ridge (or foredune) with its load of sand, plant debris, and fine sediment, building up the land and creating conditions favoring the growth of larger plants and bushes.