Questions: Coastal Processes: Wave Refraction, Erosion, and Deposition

Question 1

A headland juts into the sea between two bays. Where does wave energy concentrate, and what coastal process results?

• A. The bays receive concentrated wave energy because they directly face the open ocean, producing rapid erosion
• B. The headland receives concentrated wave energy because refraction bends wave crests toward its shallower flanks, driving erosion
• C. Wave energy distributes equally along the coast regardless of coastal geometry
• D. The headland accumulates sediment because converging wave crests carry material toward it from both sides

Answer: (B)

Wave refraction bends the part of the wave crest that enters shallow water first, which is the water around the headland's flanks. This pivots the crests toward the headland, concentrating energy on it — making it an erosion zone, not a deposition zone. Option D is the classic misconception: convergence of waves sounds like it should bring sediment in, but it actually concentrates destructive energy. Bays, where the wave crests spread out over a concave coastline, receive less energy per unit area and are therefore zones of deposition.

Question 2

A jetty is built perpendicular to shore to protect a harbor entrance. What will likely happen to the beach on the downdrift side of the jetty?

• A. It will grow because sediment accumulates in the calmer water behind the jetty
• B. It will remain unchanged because wave energy is unaffected by a single structure
• C. It will erode because the jetty intercepts longshore sediment transport, starving the downdrift beach of its sand supply
• D. It will erode because the jetty reflects waves that directly scour the downdrift face

Answer: (C)

Longshore drift is a continuous sediment conveyor moving sand along the coast. A jetty perpendicular to shore acts like a dam in that conveyor: it traps sediment on the updrift side while the downdrift side receives no replacement sand. The downdrift beach then erodes because the sea continues to remove sand that is no longer being replenished. This is one of the most common and costly unintended consequences in coastal engineering — nearly every hard structure that interrupts the shoreline starves beaches downdrift of it.

Question 3

Because wave refraction bends crests toward shallow water, a headland jutting into the sea experiences wave energy convergence from multiple directions simultaneously.

• T. True
• F. False

Answer: True

This is correct. A headland projects into the sea on all sides, so waves approaching from various directions all refract toward its flanks, converging energy on the headland from multiple angles simultaneously. This is why headland erosion is so powerful and relentless — the headland is attacked from all sides, which is why headlands are progressively cut back into sea stacks, arches, and eventually stumps.

Question 4

Sandy beaches are permanent landscape features that form where conditions are favorable and remain stable once established.

• T. True
• F. False

Answer: False

Beaches are in constant transit — sand is continuously arriving via longshore drift from updrift sources and departing downdrift. A beach exists only as long as sediment supply matches removal rates. Seasonal changes in wave direction can shift dominant longshore drift, causing the same beach to grow in summer and shrink in winter. Storm events move sand offshore into temporary submarine bars. Blocking sediment supply (by damming rivers, building groins, or cutting off eroding sea cliffs) can cause beaches to disappear entirely. There is nothing stable about a beach on geological — or even decadal — timescales.

Question 5

Explain why building a jetty on one side of a harbor entrance often leads to erosion of the beach on the other side.

Model answer: Longshore drift carries sand continuously along the coast in the downdrift direction, driven by waves arriving at an angle to shore. A jetty perpendicular to the coast interrupts this sediment conveyor: sand accumulates on the updrift side of the jetty but cannot pass through to continue downdrift. The beach on the downdrift side of the harbor entrance continues to lose sand to longshore currents but receives no replacement supply, so it erodes.

This question tests whether students understand longshore drift as a system — a continuous flow of sediment — rather than just a definition. The jetty doesn't need to directly cause erosion; it simply removes the sediment supply that previously counterbalanced natural erosion. The same logic applies to any structure that impedes sediment transport: breakwaters, sea walls, even natural changes to sediment sources. Understanding this system logic is essential for coastal management and for predicting how any intervention will affect adjacent shorelines.