Coastal Processes and Landforms
AdvancedWaves sculpt the coast: constructive waves build beaches while destructive waves erode them; hydraulic action, abrasion and solution cut notches, arches and stacks along weak rock lines, leaving wave-cut platforms as cliffs retreat; longshore drift deposits beaches, spits and tombolos, and refraction straightens the coast.
- Wave energy comes from wind: speed, duration and fetch decide the force
- Constructive waves build beaches with strong swash; destructive waves erode them with strong backwash
- The erosional sequence: notch, arch, stack, while the retreating cliff leaves a wave-cut platform
文體小知識：自然系統說明文——剖析海浪的侵蝕與沉積作用如何塑造海岸地貌，並以海蝕崖至海蝕柱的演化序列訓練圖解能力。
§Text Passage
1Waves are born of wind: blowing across the sea, wind transfers energy to the water by friction, raising ripples into waves. Wave size depends on wind speed, how long the wind blows and the fetch — the distance of water the wind crosses. The wave form travels forward, but the water particles merely circle in place, passing energy towards the shore wave by wave.
2Waves arriving at the shore fall into two types. Constructive waves are gentler and less frequent; their swash is stronger than their backwash, carrying sediment up the beach and building it higher. Destructive waves are steep and frequent; their stronger backwash drags sediment seaward and erodes the beach. As the two types alternate, beaches grow and shrink with the seasons.
3Waves erode coasts mainly by hydraulic action, abrasion and solution: water pressure blasts air into cracks and loosens rock, hurled sand and stones grind the cliff face, and seawater slowly dissolves soluble rock. Along weak lines the waves cut a wave-cut notch, then a sea arch through a headland; when the arch collapses a sea stack is left. As the cliff retreats step by step, a gently sloping wave-cut platform is exposed in front.
4When waves approach obliquely and run straight back, sediment zigzags along the shore — longshore drift. Where the coastline bends or an island shelters the water, the current slows and deposits: beaches grow on the shore, spits extend along the coast, bars close off bays, and a tombolo ties an island to the mainland.
5Hong Kong has both rocky and sandy coasts: Tung Ping Chau in the north-eastern waters shows a broad, gently sloping wave-cut platform famed in the Geopark; the tombolo at Kiu Tsui Chau in Sai Kung emerges at low tide and can be walked; the sea arch at Cape d'Aguilar on Hong Kong Island records the sea's long carving. Erosion and deposition side by side make these coasts a natural classroom.
1Summary 重點摘要
Waves sculpt the coast: constructive waves build beaches while destructive waves erode them; hydraulic action, abrasion and solution cut notches, arches and stacks along weak rock lines, leaving wave-cut platforms as cliffs retreat; longshore drift deposits beaches, spits and tombolos, and refraction straightens the coast.
2Vocabulary cards Vocabulary
A wave whose swash is stronger than its backwash, carrying sediment ashore and building the beach.
A steep, frequent wave whose backwash drags sediment seaward, eroding the beach.
Erosion as waves force water and compressed air into cracks, blasting rock apart.
Erosion as hurled sand and stones grind the cliff and platform like sandpaper.
A hollow cut by concentrated attack at a weak line at the cliff foot; as it deepens the rock above is left overhanging.
An arch-shaped opening cut through a headland; when its roof collapses a sea stack remains.
An isolated rock column left in the sea after an arch collapses, later worn away.
A near-level, gently sloping rock surface left at the cliff foot as the cliff retreats, covered at high tide and exposed at low tide.
The zigzag movement of sediment along the shore as waves swash obliquely and backwash straight back.
A sand ridge tying an island to the mainland, such as the tombolo at Kiu Tsui Chau, Sai Kung.
3Step by step Steps
- Step 1: Where wave energy comes fromWaves are the product of wind: the stronger the wind, the longer it blows and the longer the fetch, the bigger and more powerful the waves. The wave form travels while water particles merely circle in place, handing energy towards the coast; in shallow water seabed friction slows the base, the crest outruns it, and the wave pitches forward and breaks into swash and backwash. For example, as a typhoon nears Hong Kong, long fetch and strong winds raise waves taller than a person at Big Wave Bay, while calm days bring only ripples.
- Step 2: Constructive and destructive wavesCompare the two waves through the duel of swash and backwash. Constructive waves are gentle and infrequent; their swash pushes sediment ashore, making them the beach's builders. Destructive waves are steep and frequent; their strong backwash drags sediment seaward, making them the beach's demolition crew. For example, with calmer summer seas Hong Kong beaches are generally wider, while winter storms let destructive waves narrow them — a seasonal change of command.
- Step 3: The three modes of erosionErosion works along three lines. Hydraulic action forces water and air into cracks and blasts rock apart by sudden compression; abrasion grinds the cliff face with hurled sand and stones; solution slowly dissolves limestone and other soluble rock. Rock type sets the pace: loose sedimentary rock wears fastest, granite resists longest. For example, Tung Ping Chau is made of thin-bedded siltstone that the sea peels layer by layer — hence its broad, gentle wave-cut platform.
- Step 4: The evolution sequence of a sea cliffThe advanced skill is reading erosion landforms as a time sequence. Waves first cut a wave-cut notch at a weak line at the cliff foot; as the notch deepens the overhanging rock collapses and the cliff retreats. If notches on the two sides of a headland meet, a sea arch forms; when its roof falls, a sea stack is left offshore; continued retreat exposes a gently sloping wave-cut platform at the cliff foot. For example, seeing a stack lets you infer that an arch once stood nearby — sequence reasoning earns marks.
- Step 5: Wave refraction and longshore driftWaves refract on an irregular coast: water is shallower off the headland, the wave slows first there, and the crests bend to concentrate energy on the headland, sharpening erosion; in the bay energy spreads out and deposition rules. With oblique swash and straight backwash, sediment zigzags alongshore as longshore drift. For example, because energy gathers on headlands and spreads in bays, an irregular coastline is straightened over time — the key concept behind coastal evolution.
- Step 6: The family of depositional landformsWhere longshore drift meets a bend in the coast, a sheltered bay or an island's wave shadow, the current slows and deposits: sand piled against the shore makes a beach; a ridge still tied to the land at one end is a spit; one sealing a bay's mouth is a bar; and sand tying an island to the mainland is a tombolo. For example, the tombolo at Kiu Tsui Chau in Sai Kung is made of pebbles and emerges at low tide, when visitors can walk across to the islet — Hong Kong's most accessible lesson in coastal deposition.
- Step 7: Rocky coasts versus sandy coastsSum up by contrasting two coasts. Rocky coasts are dominated by erosional landforms — cliffs, wave-cut notches, arches, stacks and wave-cut platforms — steep in outline. Sandy coasts are dominated by depositional ones — beaches, spits, bars and tombolos — gentle in outline. Their distribution is decided together by rock type, wave direction and longshore drift. For example, a strong answer first classifies a landform as erosional or depositional, then names the process and conditions to complete the causal chain.
5Worked example Example
Diagram question: the sketch shows four stages of a headland coast — A: an intact cliff; B: a wave-cut notch at the cliff foot; C: the headland cut through as a sea arch; D: the arch collapsed, leaving a sea stack offshore. Put the stages in time order and explain each change.
- Read the question: for a sequence, order the stages first by the direction of erosion, then explain each change as 'which process plus how' — never stop at names
- Order: A, B, C, D. At B, waves concentrate on the weak line at the cliff foot, cutting a wave-cut notch by hydraulic action and abrasion
- Explain C: the notch deepens and widens until the two sides of the headland meet, forming a sea arch; the roof, losing support, is weakened by weathering along joints
- Explain D: the arch roof collapses and the more resistant column remains offshore as a sea stack; meanwhile the cliff retreats and a wave-cut platform develops at its foot
- Conclude: the sequence is the work of hydraulic action and abrasion, aided by weathering and solution, steered by the weak lines; noting that energy concentrates on weak lines earns the higher-level marks
- Answer: The four stages run in the order A, B, C and D. At A the cliff is intact and waves strike it without concentration. At B the waves find a weak line at the cliff foot: hydraulic action forces water and compressed air into the cracks while abrasion grinds the rock with hurled stones, cutting a wave-cut notch. At C the notch deepens and widens until the two sides of the headland are cut through, forming a sea arch; the roof, deprived of support and loosened by weathering along joints, grows fragile. At D the roof finally collapses, leaving the more resistant rock standing offshore as a sea stack, while the retreating cliff exposes a gently seaward-sloping wave-cut platform at its foot. The sequence shows wave energy concentrating on weak lines in the rock, with erosion advancing from point to line to volume and landforms succeeding one another in order; pointing out energy concentration and the control of weak lines completes the explanation.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three main ways waves erode?
Q2Put in order: wave-cut notch, sea arch, sea stack.
Q3How do constructive and destructive waves differ?
Q4How does a tombolo form?
Q5Why do headlands erode while bays deposit?
8Recap Recap
- Wave energy comes from wind: speed, duration and fetch decide the force
- Constructive waves build beaches with strong swash; destructive waves erode them with strong backwash
- The erosional sequence: notch, arch, stack, while the retreating cliff leaves a wave-cut platform
- Longshore drift moves sediment zigzag, depositing beaches, spits, bars and tombolos
- Refraction makes headlands erode and bays depose; Tung Ping Chau's platform and Kiu Tsui Chau's tombolo are living textbooks