Waves and tides

The governing concept is the practical importance of tidal cycles in coastal activities. Tides periodically raise and lower sea level and also influence the movement of water through creeks, estuaries, harbours and shallow coastal areas. Fishermen can use expected high- and low-tide times to decide when boats can safely cross shallow entrances, leave or return to a harbour, reach fishing grounds and handle landing operations. Tide information is not the only safety factor, because weather and waves also matter, but it is an important planning aid. Option A is correct; the other options concern unrelated landforms, clouds or forests.

The governing concept is the distinction between wave dimensions. Wavelength is the horizontal distance from one crest to the next crest, or from one trough to the next trough, measured in the direction in which the wave travels. Wave height is the vertical distance between a crest and the adjacent trough. Amplitude is another vertical measure, extending from the mean water level to a crest or trough, and is approximately half the height for a regular wave. Therefore option A is correct. Option B confuses two different measurements, while C and D do not define physical wave properties.

Tidal range means the vertical difference between the water level at high tide and the water level at low tide. During a neap tide, the Sun and Moon pull at approximately right angles relative to Earth, so their tidal-generating effects partly oppose one another. The resulting high tide is less high and the low tide is less low; therefore, the range is small. A spring tide has the greatest range because the Sun, Earth and Moon are nearly aligned. A tsunami is caused by sudden displacement of seawater, while a storm wave is produced mainly by strong winds, so neither describes a small regular tidal range. Hence option A is correct.

Coastal erosion is the wearing away and removal of beach or cliff material by waves, currents and related processes. High-energy waves carry greater kinetic energy and exert stronger hydraulic pressure when they strike a shore. Their turbulent swash and backwash can loosen, lift and transport sand, gravel and pieces of rock. Repeated impact may undercut a cliff, narrow a beach or move sediment offshore and alongshore. Erosion is also influenced by rock resistance, beach width, tide, sea level and coastal protection, but high wave energy generally increases the erosive capacity. Low-energy waves are more likely to deposit material, while no water or no wave contact cannot produce wave erosion. Therefore C is correct.

When the Sun, Earth and Moon are nearly aligned, the gravitational effects of the Sun and Moon act along nearly the same line. Their tidal-generating forces therefore reinforce one another, producing a greater difference between high-tide and low-tide levels. This condition occurs around the new moon, when the Moon is between the Sun and Earth, and around the full moon, when Earth is between the Sun and Moon. The resulting tide is called a spring tide; it is not named for the season and can occur in any season. A neap tide occurs when the Sun and Moon are approximately at right angles, while a tsunami requires sudden displacement of seawater. Thus A is correct.

The governing wave property is the period, usually represented by T. Wave period is the time required for one complete wave cycle to pass a fixed observation point; operationally, it is the interval between successive crests or successive troughs. Therefore, if the time between two crests increases, the wave period increases, so option A is correct. Frequency is related to period by f = 1/T, meaning that a longer period corresponds to a lower frequency, provided the measurement is made consistently. Wavelength is a spatial distance between successive crests, while wave height is the vertical distance from trough to crest. Tidal range describes the difference between high and low tide and is not the time interval of ordinary wave crests.

The governing concept is wave period, represented by T. It is the time interval between the passage of two successive crests, troughs, or equivalent points at a fixed location. If that interval increases, the period has increased. Frequency is related by f = 1/T, so frequency would decrease if the wave speed and other conditions were considered accordingly. Wavelength is a distance between successive crests, not a time interval, so option A is incorrect. Tidal range is the vertical difference between high and low tide and is not the property described. Salinity measures dissolved salt concentration. Therefore option B gives the precise answer.

The governing wave concept is period. The wave period, represented by T, is the time required for one complete cycle to pass a fixed point; in an ocean observation this can be measured as the interval between two successive crests. Thus, if that interval increases, the period increases directly. Frequency changes inversely because f = 1/T, but frequency is not offered as an option. Wavelength is the distance between successive crests, not the time separating them, and it would require additional information about wave speed before any conclusion could be made. Tidal range and salinity describe different ocean properties. Therefore option B is correct.

The governing concept is the spring tide at new moon. During a new moon, the Moon is positioned approximately between the Sun and Earth. The Sun’s and Moon’s tidal-producing gravitational effects act along nearly the same line and reinforce each other. This increases the tidal range, so high tides can be higher than usual and low tides can be lower than usual. Therefore option D, spring tide, is the best identification. A neap tide occurs near the first or third quarter, when the relevant bodies are approximately at right angles and the range is smaller. A normal wind wave is generated mainly by local wind, while a storm current is not the standard name for the astronomical tide described here.

Wavelength is a distance: it is the distance between successive crests (or troughs) at the same stage of a wave. Wave period is a time: it is the time between successive crests passing a fixed point. Therefore, option B correctly distinguishes the two measurements. Wave height, by contrast, is the vertical distance from crest to trough.