CHAPTER 10: COASTAL PROCESSES, PATTERNS AND LANDFORMS IN DEPTH – Tombolos
Tombolos are formed in two ways by the action of wave refraction and longshore drift. Firstly, as waves approach the shallow water near an island they slow down and begin to wrap around (refract) the island. The wave patterns caused by the island trigger longshore drift currents on each side of the island. These longshore drift currents then collide in the sheltered area between the island and the mainland. This causes deposition to occur leading to the formation of a ridge of sand or shingle. Over time the ridge connects the island to the mainland and a tombolo has formed (Fig. 32). Secondly in some places an island is located in
the path of the prevailing longshore drift current. Longshore drift produces a sand spit where the coastline changes direction. The sand spit may grow along the coast until it reaches an island forming a tombolo (Fig. 33). Tombolo deposits are often layered with coarser
sediments at the base and finer sands at the top of the tombolo. Tombolos are easily eroded by storms and destructive waves. They are often reinforced to provide access to islands for tourists and the local population who may live on the island.
Mainland Tombolo
Longshore drift currents collide leading to deposition
Island
Wave refraction around the island
Fig. 32 Exam Diagram: The formation of a tombolo (a)
Change in direction of coastline resulting in the formation of a spit
Spit
Island
Prevailing wind direction
Fig 33 Exam Diagram: The formation of a tombolo (b)
Fig. 34 A growing tombolo visible at low tide at Shenick’s Island near Skerries, County Dublin. See map on page 178.