Transportation and Deposition by Sea
Transportation by Sea:
Most of the waves approach a shoreline at an angle. Consequently, the ”uprush” of water from each breaking wave is oblique. However, the ”back-flow is straight down the slope of the beach. The effect of this pattern of water movement is to transport particles of sediment in a zig-zag pattern in a direction along the shore. This movement is called ”Beach drift”. The beach drift can transport sand and pebbles hundreds of meters each day.
When a wave strikes a shoreline at an angle, the kinetic energy of the wave is only partly spent in the impact, the remaining energy is used up in forming currents that flow parallel to the shore. These currents are called ”long-shore currents”. The long-shore current transport sediments along the shore. When the sediment transported by the longshore current is added to the quantity moved by beach drift, the total amount can be very large.
Where the incoming waves strike at right angles to the shoreline, the undertow (water waves) currents are returning which are formed below the oncoming waves. They transport finer sediment out to the sea.
Deposition by Sea:
Where beach drift the longshore currents are active, several depositional features may develop along the shore. The principal depositional features are as follows;
(i). Beach: A beach is the flat mass of sand and gravel that is deposited on seashores. The sediment of the beach is derived from erosion of adjacent cliffs and forms alluvium contributed by rivers.
(ii). Wave Built Terrace: Under suitable conditions, a part of the sediment is carried beyond the rock bench and is deposited there. In this way, a flat platform-like feature is formed, which is called ”wave-built terrace”. Towards the shore, it merges with the beach.
(iii). Spits: Where a straight shoreline takes a sharp turn, the longshore currents are not able to flow parallel to it. Such a shore directs the currents into the water of increasing depth, where deposition takes place. This results in the formation of a submerged bar, one end of which is attached to the mainland. This bar is known as ”spit”. The end of the spit often becomes curved landward in respect to the wave action. Such a spit is called ”hooked spit”, or simply ”hook”.
(iv). Sand Bars: Sand bars are the low offshore ridges of sand, which extend parallel to the coast to the coast. They commonly enclose a lagoon. A lagoon is a stretch of salt water separated from the sea by a low sandbank or coral reef.
(v). Tombolo: In the lee side of islands spits are often formed. If such a ridge connects an island to the mainland or joins two islands together, it is called a ”tombolo”.