6.12.2. Bars, Barriers and Spits

A ridge of sand and shingle formed in the sea in the off-shore zone lying approximately parallel to the coast is called an off-shore bar. An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.

Off-shore bars are underwater ridges made of sand and pebbles that run parallel to the coastline. They form when waves drop sand just off the shore, creating a raised area. If more sand accumulates and the bar rises above the water level, it becomes a barrier bar. Barrier bars are important because they create sheltered areas behind them, which can lead to the formation of lagoons.

Think of off-shore bars like a fence made of sand just offshore, which stops some waves from hitting the beach directly. When the fence is tall enough to be seen above the water, like a barrier bar, it creates a calm area behind it where water can pool, similar to how a fence can create a quiet zone in your backyard.

The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay. Sometimes such barrier bars get keyed up to one end of the bay when they are called spits.

Spits are landforms that form when sediment, carried by waves and currents, accumulates along the shoreline. They extend from the mainland into the water, often towards the mouth of a bay or river. As waves hit the coast, they can push sand and shingle along, leading to the formation of a spit. Over time, these spits can grow longer and can block water from flowing in the bay, making it more shallow and turning it into a lagoon.

Picture a spoon scooping up some sugar. As you move it around, some sugar spills off the edge. That’s similar to how a spit forms: just like the sugar spilling out, materials from the shoreline get pushed into the sea and accumulate, creating a new landform extending into the water.

The barriers, bars and spits at the mouth of the bay gradually extend leaving only a small opening of the bay into the sea and the bay will eventually develop into a lagoon.

As spits and barrier bars grow, they trap sand and sediment on the inside, leading to the reduction of water flow into the bay. This can create a lagoon, a shallow body of water separated from the sea by the spit or barrier. Over time, these lagoons can fill with sediment from rivers or land, transforming the area into a coastal plain.

Imagine a bathtub with a small drain. As you fill the bathtub, the water level rises but stays within the edges. If the tub had a lid that blocked some of the water from overflowing, the water inside would become more still, representing how a lagoon forms behind a barrier. The edges form a ‘lid’ keeping some water in and creating a calm environment inside.

--