waves in the shoreline






When waves approach shore and "feel the bottom", water piles up and breakers form. This process proves that a shoreline is not static. Primarily these waves, breaking at an angle to the shoreline, are what generate a long shore current that parallels the shore. The long shore current not only moves water in the surf zone, it also moves sediment parallel to the shoreline. Long shore current is a function of the angle of wave approach. For example, if the waves approach the shoreline from the south, long shore current moves from south to north
Although, long shore current and the resulting transport of sediment is only one of the dynamic processes that constantly alter the shoreline. As waves repeatedly hit the shore, water moves onto the beach and then retreats in a continuous cycle. However, the waves are not all that moves on the shoreline. In fact, the sediment on the shore is also always on the move. Great energy is expended on the beach as waves crash against the shoreline. This energy allows the water to transport sediment. The grains are lifted as the waves in the swash zone move onto the beach, and then the grains are deposited again as the water retreats. As long as the waves hit the shoreline “straight on”, the wave crests are parallel to the shoreline, the sand grains will be picked up and redeposit in the same general area. In this case, no real net movement of sand occurs in the swash zone.
Waves generally do not form parallel to the shoreline, and thus, usually approach the shore at an angle. Consequently, beach sand will have a net movement up or down the beach, depending on the direction of incoming waves. Sand grains are transported as the waves move onto the beach. Therefore, the sand grains are being carried in the same direction that the waves are moving. However, when the water retreats, due to gravity it goes straight back perpendicular to the shoreline and not back in the opposite direction from which it came. Since grains are redeposit as the water retreats, the grains are actually deposited further down the beach or down long shore current. The net overall movement of sand is down current. The sediment that has moved down shore is replaced by sediment from further up shore, as long as sediment is available there. If the waves hit the beach from the south, the net movement of sand will be to the north as we saw last week at the Loyola beach. Net beach sediment movement can change back and forth with shifts in wave direction. In spite of this, along any particular shoreline one direction for incoming waves usually dominates resulting in a preferential net movement of sediment.
Sand and water work together consistently on the shore. Many years of this procedure of waves movement will result in one side with more sand than the other. The side with less sand, in conclusion, will be replaced with sand poured in by humans.




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