Tidal inlets are breaks in barrier island systems that allow water, nutrients, organisms, ships, and people easy access and exchange between the high-energy open ocean and the low-energy back-barrier environment consisting of bays, lagoons, tidal marshes, and creeks. Most tidal inlets are within barrier island systems, but others may separate barrier islands from rocky or glacial headlands. Tidal inlets are extremely important for navigation between sheltered ports on the back-barrier bays and the open ocean; thus they are the sites of many coastal modifications such as jetties, breakwaters, and dredged channels to keep the channels stable and open.
strong tidal currents move water into and out of tidal inlets as the tides wax and wane, and also carry out of the channel large amounts of sediment brought in by waves and long shore transport. Never try to swim in a tidal inlet. As tides rise on the ocean side of tidal inlets the water rises faster than on the inside of the inlet, since the inlet is narrow and it takes a long time for the water to move into the restricted environment behind the barrier. The difference in elevation causes the water to flow into the inlet with a strong current, called a flood-tidal current. As the tide falls outside the inlet, the reverse happens—as the tide falls quickly on the outside of the barrier, the sea surface is higher inside the inlet and a strong current known as an ebb-tidal current then flows out of the inlet, returning the water to the ocean. Considering the amount of time that it takes for water to flow into and out of tidal inlets, it is apparent that times of high and low tide may be considerably different on the two sides of barrier systems connected by tidal inlets.
The sides of tidal inlets are often marked by curved sand ridges of recurved spits, formed as waves are refracted into the barrier and push the sand into ridges. The strongest currents in tidal inlets are found where the inlet is the narrowest, a place with the deepest water called the inlet throat. Water rushes at high velocity into and out of this throat, carrying sand into and out of the back-barrier environment. since the velocity of the water decreases after it passes through the throat, large lobes and sheets of sand are typically deposited as tidal deltas on both the inside and the outside of tidal inlets. The delta deposited by the incoming (flood) tide on the inside or landward side of the inlet is known as a flood-tidal delta, whereas the delta deposited on the outside of the inlet by the ebb tide is known as an ebb-tidal delta.
Tidal inlets form by a variety of mechanisms. The most common is during the formation and evolution of barrier systems along coastal platforms on passive margins, where barrier islands emerged as glaciers retreated and sea levels rose onto the continental shelves in Holocene times. sea levels rose more slowly about 5,000 years ago, and enhanced coastal erosion provided abundant sand to create the barrier island systems. Continued rising sea levels plus diminished sediment supplies and the many modifications of the shoreline by humans has led to increased erosion along much of the world's coastlines. With
Sketch showing features of a barrier island, tidal inlet, and lagoon coastal system. Note the positions of the small deltas on either side of the tidal inlet, the coastal march, and beach-dune ridge. (modeled after R. Davis and D. Fitzgerald).
this trend many barriers have been breached or cut through during storms. Typically this happens when an incoming storm erodes the foredune ridge, and waves top the barrier island, washing sand into the back barrier region, often making a shallow channel through the barrier. As the storm and elevated tides recede, the water in the back barrier bay, lagoon, or tidal marsh is left high, then begins to escape quickly through the new shallow opening, deepening it rapidly. If the tides can continue to keep this channel open, a new tidal inlet is established. Many tidal inlets along the outer Banks barrier islands of North Carolina have formed in this way. Any homes or roads that were in the way are gone.
Tidal inlets may also form by longshore currents, building a spit across a bay or drowned river valley. As the spit grows across an open bay, the area open to the sea gradually becomes narrower until it begins to host strong tidal currents, when it becomes deep and reaches an equilibrium between the amount of sediment transported to the inlet by longshore drift and the amount of sand moved out of the inlet by tidal currents and waves.
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