State whether the following is true or false.
The tide propagates as a shallow water wave even across the deepest ocean basins.
Answer: A
Its wavelength is thousands of kilometres against an ocean depth of only a few. Depth therefore controls the celerity throughout its propagation, and the whole water column takes part in the motion. Nothing about the tide resembles a deep water wave.
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All of the following affect the water level at a coastal site EXCEPT
Answer: B
Correct answer: how saline the water in the nearshore happens to be
Salinity affects density by a fraction of a per cent and has no appreciable effect on the water level at a site. The other three all contribute directly: the astronomical tide and the storm surge make up the still water level, and wave setup raises the mean level inside the surf zone by a further amount that can reach several tenths of a metre.
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The still water level used in coastal design normally comprises
Answer: C
Wave setup and runup are then added on top of the still water level. Keeping the terms distinct avoids double counting in the design calculation. Each term is estimated by a different method. Adding runup twice inflates the design level badly.
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Consider the following statements about extreme water level assessment.
1. Still water level combines the tide and the surge.
2. Wave setup and runup are added on top of that level.
3. Wave runup is part of the still water level itself.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 describe the standard build-up: the still water level is tide plus surge, and wave setup and runup are added on top of it. Statement 3 confuses the two - runup is the wave-driven excursion above the still water level, not part of it, and adding it twice would overstate the design level substantially.
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Assertion (A): Coastal flood assessments use joint probability rather than adding extremes.
Reason (R): The extremes of tide, surge and waves rarely coincide at their individual maxima.
Answer: A
Both are true and the reason gives the justification, and simple addition is unrealistically conservative. Joint probability gives a defensible return period for the combined level. Correlation between the terms must be respected.
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Tidal streams in a narrow strait are typically
Answer: B
Correct answer: stronger than in the adjacent open water on either side
Continuity requires the same volume to pass through a smaller cross-section, so the flow accelerates through the constriction. This is why the strongest tidal streams in the world are found in narrow straits - the Pentland Firth and the Strait of Messina among them - and why those are the sites tidal stream turbines are proposed for.
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State whether the following is true or false.
Tidal stream energy extraction can itself reduce the tidal range in the basin behind.
Answer: C
Extracting energy adds resistance to the flow passing through the channel. Large scale deployment therefore has a natural limit set by the resource itself rather than by the technology. Extraction changes the resource being exploited.
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All of the following are consequences of building a tidal barrage EXCEPT
Answer: D
Correct answer: an increase in the astronomical forcing of the tide
The astronomical forcing comes from the sun and the moon and no engineering work can alter it; what a barrage changes is the local response to that unchanged forcing. The other three are all genuine consequences: the range within the basin falls, intertidal habitat is lost or altered, and the sediment regime of the estuary changes with the flow.
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