B.2 Interactions between oceans and the coastal places

The question asks why two sites with identical wind speed and duration produce waves of different power. The only differing variable is the fetch — the uninterrupted distance of open water over which the wind blows.

Fetch determines how far the wind can transfer kinetic energy into the water. A 3,000 km fetch allows energy to accumulate continuously over that distance, producing waves with greater height, longer wavelength, and more energy than an 80 km fetch under the same wind conditions.

The question explicitly states wind speed and duration are identical at both sites, so neither can explain the difference. Fetch is the only independent variable that differs.

The correct answer is that the open ocean site has a much longer fetch, enabling greater energy transfer from wind to water over a longer distance, which produces larger, more powerful waves.

The question asks which statement correctly explains greater wave power at the open-ocean site, given that wind speed and duration are equal at both sites.

'The open ocean site has stronger prevailing winds because it lacks surrounding land to shelter it from the atmosphere' is incorrect — the question states wind speeds are identical at both sites, so this option contradicts the premise.

'The enclosed sea site has shorter wavelengths, which means its waves carry more energy per unit length than those at the open site' is incorrect — shorter wavelength is associated with destructive, lower-energy waves, not higher energy per unit length.

'The open ocean site receives swell from distant storms, which always has higher frequency than locally generated sea waves' is incorrect — swell actually has lower frequency (fewer waves per minute) than locally generated choppy sea waves.

The remaining option — that the open ocean site has a much longer fetch, allowing greater energy transfer over distance — correctly identifies fetch as the controlling variable and is the correct answer.

The question describes beach building — the net accumulation of sediment on the beach face. This is the hallmark of constructive waves.

Constructive waves have long wavelengths (up to ~100 m), low wave height, low frequency (6–8 per minute), and a stronger swash than backwash. The dominant swash carries sediment up the beach; the weaker backwash cannot return all of it, producing a net gain of sediment on the beach face.

The correct option states 'low-frequency, long-wavelength waves with a stronger swash than backwash' — this precisely describes constructive wave characteristics responsible for beach building.

Beach building in summer is driven by constructive waves, confirming the correct answer is the option describing low-frequency, long-wavelength waves with dominant swash.

The question tests knowledge of constructive wave characteristics and their role in depositing sediment to build a beach berm during calmer seasonal conditions.

'High-frequency waves with steep fronts and a dominant backwash pulling sediment up the beach' is incorrect on two counts: dominant backwash removes sediment from the beach rather than building it, and high-frequency, steep waves are destructive, not constructive.

'Short-wavelength waves generated by strong local winds, whose plunging breakers deposit sediment at the cliff base' describes destructive waves — these erode beaches rather than building berms, and their energy is directed at the cliff base.

'High-energy waves with a wavelength of around 20 m and a frequency of 10–14 per minute' describes destructive wave characteristics (short wavelength, high frequency) — these erode beaches, not build them.

The only option consistent with beach building is 'low-frequency, long-wavelength waves with a stronger swash than backwash' — these are the defining features of constructive waves.