Tombolo

A tombolo is a ridge of beach material (typically sand), built by wave action, that connects an island to the mainland. Tombolos are often formed formed where a spit continues to grow by long shore drift, joining land to an offshore island.



Some tombolos continue to build up as waves are refracted around the island at the end. This happens because when the waves near the island they are slowed down by the shallow water surrounding it. As a result they are then refracted or “bend” around the island to the opposite side as they approached. The wave pattern created by this water movement causes a convergence of long shore drifting on the opposite side of the island.

The beach sediments that are moving by lateral transport on the lee side of the island will accumulate there conforming to the shape of the wave pattern. In other words, the waves sweep sediment together from both sides. Eventually, when enough sediment has built up the beach shoreline, known as a spit, will connect with an island and form a tombolo.

A well know example of a tombolo is Chesil Beach in Dorset. Chesil Beach is an 18 miles long, pebble beach, stretching north-west from Portland to West Bay. For much of its length it is separated from the mainland by an area of saline water called the Fleet Lagoon. However it is far more likely that Chesil Beach was formed when sea levels rose after the last ice age, and collected beach material from across Lyme Bay to form a barrier beach, parallel to the coast. Chesil beach stopped before it hit land as the sea levels stopped rising far enough for the beach to move right against the shore, and hence the Fleet lagoon was formed

Chesil beach is, because of this reason, not a standard orthodox tombolo, as it just happened to join to the north side of the Isle of Portland due to currents and the shape of the sea bed between Portland and the mainland.

The pebbles, mainly a mix of flint and chert, with some quartzite pebbles from Buddleigh Salterton are, graded in size from fist-sized near Portland to pea-sized at Bridport. The lagoon behind Chesil Beach is known as the Fleet. The Fleet is 13.1 km (8.2 miles) long and covers an area of 480 hectares. The width varies from 900 metres at Littlesea down to just 65 metres at The Narrows The Fleet contains brackish water with the salinity varying throughout. It connects to the sea via a narrow channel at Ferrybridge into Portland Harbour and sea water can also enter the Fleet by percolation through Chesil Beach under some tidal conditions. Fresh water can enter the Fleet from a number of small streams and by run-off from the surrounding fields. The Fleet is an important area for wildlife and has received a number of designations

including; Special Area of Conservation (SAC), ·Special Protection Area (SPA )under the EU Birds Directive and a Ramsar site. The Fleet is home to a number of important rare plant species including Compact brome and Little-robin.

Another much smaller tombola can be found at Kynance in South Cornwall. The `Tombolo`beach, connects Asparagus Island to the mainland.