Waves and tides

Height at the coast alone cannot reliably distinguish a tsunami from an ordinary wind-generated wave. Ordinary waves are commonly generated by wind acting on the sea surface, while tsunamis are usually caused by major water displacement, often due to an undersea earthquake or landslide. A strong or prolonged earthquake, unusual sea-level changes and official alerts are important warning signs. Thus option A is correct; colour and fish numbers are not reliable criteria.

At full moon, the Sun, Moon and Earth are approximately aligned, producing a spring tide and a relatively large tidal range. A narrow or funnel-shaped bay can amplify the local water level by concentrating incoming tidal water, depending on its shape and dynamics. The high level can therefore reflect both astronomical forcing and local coastal effects. Option C is correct; neap tides occur near the first and third quarters, not at full moon.

Coastal erosion depends both on how resistant the rock is and on the energy delivered by waves. Weak or fractured rock is more vulnerable, so erosion is possible, but low wave energy may limit how quickly material is worn away or removed. Option A is therefore the balanced conclusion; weak rock alone does not guarantee extreme erosion.

A sediment budget describes the balance of sediment entering, being stored in, transported through and leaving a coastal area. River flow and tidal currents can both transport sediment, and waves may also redistribute it. When these processes operate together, they can change the amount and location of sediment in a bay. Therefore, option B is correct.

The governing concept is the complete geomorphic sequence of erosion, transportation and deposition. Wave impact can loosen or remove particles from coastal rock, which is erosion. The moving water then carries those particles along the shore or offshore; this is transportation. When wave energy or current velocity decreases, the water can no longer carry the same load, so the material settles and forms a deposit. Therefore option A correctly names all three connected stages. Option B refers only to the periodic rise and fall caused mainly by lunar and solar gravity, while options C and D are unrelated to coastal sediment processes. The evidence in the question describes a linked process chain, not a single tidal event.

Tidal range means the vertical difference between high tide and low tide. A very large range indicates that the gravitational effects of the Moon and Sun are reinforcing one another, which is characteristic of a spring tide. Spring tides occur near the new-moon and full-moon phases, although the exact range is also influenced by coastline shape, basin geometry and local conditions. Since the Moon phase is unavailable, the unusually large range is the strongest clue, so option B is the best answer. A neap tide produces a smaller range when the solar and lunar effects partly oppose each other. A normal current is horizontal water movement, and a tsunami is a long wave caused by sudden seafloor or other displacement, not a regular tidal range.

The governing idea is compound coastal risk: tsunami and astronomical tide have different causes, but their water levels can overlap in time and space. A tsunami is generated by a sudden displacement of the sea surface or seabed, whereas high tide is part of the predictable tidal cycle produced mainly by lunar and solar gravity. If a tsunami arrives during high tide, the pre-existing water level is already elevated, so inundation can extend farther inland and currents can become more dangerous. Therefore option C gives the correct risk-based analysis. Option A is false because high tide cannot stop a tsunami, and option B ignores the most urgent warning. Option D incorrectly treats the situation as a neap tide.

Sediment movement depends on the combined forces acting on particles, not on wave energy alone. Lower wave energy may reduce wave-based erosion and transport, allowing some material to settle, but a strong tidal current still exerts drag on the bed and can move suspended or bed-load sediment. Its direction may reverse between flood and ebb tides, so transport can remain active even when the waves are weak. Thus option D is correct: the tidal current can still transport sediment. Option A wrongly assumes that weak waves stop every process. Option B reduces a complex system to the Moon alone, ignoring current speed, seabed material and other controls. Option C is too absolute because deposition depends on changing velocity, sediment size and channel or coastal form.

Safe port entry depends on more than the vertical difference between high and low tide. A small tidal range does not guarantee a deep or unobstructed navigation channel. Strong flood or ebb currents may push a vessel sideways, while deposited silt, sand bars or a shallow channel can reduce under-keel clearance and increase grounding risk. Channel width, charted depth, waves, wind and the timing of current should also be examined in a real navigation assessment. Therefore option A is the only relevant choice because it identifies physical factors that can directly make entry difficult. The Moon’s colour, fish sounds and coastal language do not control channel depth or vessel manoeuvrability. The question tests interpretation of several coastal conditions rather than reliance on one tidal measurement.

Wind-generated waves are disturbances that transfer energy through the water. Once formed over a sufficiently long fetch, waves can leave the area where the wind originally acted and continue propagating as swell, even after the local wind weakens or stops. Individual water particles generally move in orbital paths rather than travelling bodily with the wave, while the wave form and its energy move forward. Therefore option A is correct: the arriving waves may preserve energy from a distant source region. Option B is wrong because wind is a major generator of ordinary surface waves. Option C is unjustified because tides are long-period changes caused mainly by gravitational forces and have predictable timing. Option D contradicts the physical observation that wave energy is still reaching the coast.