The story of our coastlines is a tale of sediment in motion. How it moves, where it moves, why it moves – all chronicle the forces that shape our shorelines.
At its heart, this is a (not so) simple tale: Waves, winds, water and weather work together (most of the time, at least) to keep coastal sediment in constant motion. These forces ensure the coast is an ever-changing environment, albeit with changes that can be minutely subtle some days and drastically shocking on others.
Amid this fluidity, however, there are some coastal constants when it comes to sediment:
While sediment works to continue its southward slog, inlets – through both interruption and hydrology – interrupt that flow, pushing sediments both seaward into shoals and landward into the inlet and the back bay. Often, efforts are undertaken – jetties, groins, channelized dredging, for example – to keep an inlet open and/or in place, which can further exacerbate the natural flow of sediment along the coast.
There are a variety of options to alter and affect the flow of sediment – all with benefits and risks. That’s where the advent of modeling – the ability via data collection and computer simulation to reasonably predict the consequences of coastal changes on sediment movement – has proven a boon to inlet management and a benefit to adjacent beaches.
Once the current movement of both sediment and water in and around an inlet is captured for analysis, numerical models can be run to predict (with all the usual caveats) how altering the inlet or shoreline will affect the coast. Variables such as dredging, channel depth and alignment, sediment placement or removal, adding or subtracting coastal structures and other changes to the shoreline or tidal inlet can all be run through the model, with simulation results that can be fine-tuned and improved using filed data to ground-truth the model results.
The model results can tell you what should happen, if conditions evolve as predicted. Of course, nothing is ever so simple along the coast. Unanticipated changes to the conditions wrought by waves, winds and water – usually driven by the fourth W, weather – bring unexpected changes to the sediment they shape – and that means major changes to coasts and inlets.
The impacts from storm events can be most dramatic, opening new passes or reshaping existing ones almost overnight. Once these shock waves subside, the coastal system begins its endless quest toward equilibrium once more… but these large episodic changes in sediment movement take a long time to return to “normal,” if they ever do.
This adjustment usually plays out most prominently on the adjacent beaches and inside inlets, where the always-moving sediments try to adapt to the changes to their distribution. Shoaling and erosion reshape the shoreline on both sides – all four sides, actually – of an impacted inlet. Sometimes, these radical reshaping events resulting from storms can alter a larger hydrologic system, changing the way water moves from the back bay to open ocean along a series of inlets in ways that will affect the functioning of each one.
All of this ignores the upland changes that can affect… not how sediments moves, actually, but what that movement means the further you go in from the water and waves. Look at images of inlets over decades or even longer, from their formation or wanderings when allowed to move at will, then later to how those changes are modified once the inlet’s upland is hardened to limit how much it is allowed to roam.
The coast doesn’t care if there’s a bulkhead or a building or a bridge there. Sediment still wants to move, and the inlet still impacts how all that happens.
While the science and engineering to predict what COULD happen gets better and better, it can never tell you what WILL happen over the years. That ultimately falls to waves, wind, water and weather, and how they all work to keep that coastal sediment moving along.
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