Reading: Summary, Note & Table Completion: Question 7

When waves approaching a coastline encounter a small island lying a short distance offshore, they do not pass it evenly. Wave energy travelling on a direct path toward the island refracts, or bends, around both ends of the obstruction, while the section of wave front travelling directly behind the island is partially blocked. This creates a comparatively calm, low-energy zone in the water immediately landward of the island, sometimes called a wave shadow, where waves arriving from either side lose much of their force and largely cancel out rather than reinforcing one another.

Because wave energy is what keeps sediment suspended and moving along a coastline, the sheltered wave shadow behaves very differently from the open water on either side of it. Sand and shingle carried by longshore drift (the gradual movement of sediment along a shore, driven by waves striking the coast at an angle) continue travelling steadily past the exposed sections of coastline, but slow down and begin to settle once they enter the calmer water directly behind the island. Over time, repeated deposits of sediment build upward and outward from the seabed, eventually forming a low ridge that breaches the surface at low tide before becoming a permanent bar of sand or shingle.

Once this ridge grows tall enough to remain exposed even at high tide, it forms a continuous strip of land joining the island to the mainland, a landform geomorphologists call a tombolo. A long-term survey near Bellhaven Point tracked one such feature over a fifteen-year period, recording that the bar widened from an initial three metres to over twenty metres as sediment supply from a nearby eroding headland increased. Researchers noted that the tombolo's orientation closely tracked the dominant wave direction recorded that year, confirming that the ridge builds directly along the axis of the wave shadow rather than at a fixed angle to the coast.

Not every offshore island develops a tombolo. Where two islands lie close together and each casts its own wave shadow that overlaps with the other, sediment can instead accumulate into a double-tailed spit rather than a single connecting bar, a variant that coastal engineers say is far less commonly documented than the classic single tombolo.

Complete the table below. Choose NO MORE THAN TWO WORDS from the passage for each answer.