A storm surge is defined as
Answer: A
Observed level minus predicted level is the operational definition everywhere. Separating the two allows the meteorological contribution to be forecast on its own. Tide and weather are predicted by quite different means.
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Match each contribution to extreme still water level with its cause.
List I
A. Astronomical tide
B. Inverse barometer
C. Wind setup
D. Wave setup
List II
1. wind stress driving water against the coast
2. gravitational forcing by the moon and sun
3. momentum flux from waves breaking inshore
4. reduced atmospheric pressure over the sea
Answer: B
The first three combine into the still water level used in design. Wave setup and runup are then added on top of that level. Keeping the terms distinct avoids double counting. Each is estimated by a quite separate method.
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A depression whose central pressure is 40 hectopascals below normal raises the sea surface by approximately
Answer: C
Correct answer: 40 centimetres
The inverse barometer effect raises the sea surface by about one centimetre for each hectopascal the pressure falls, so 40 hPa gives about 40 cm. It is a real but modest contribution - in a severe tropical cyclone the wind setup is several times larger, and it is the wind rather than the pressure that produces the destructive part of a surge.
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Assertion (A): Wind stress usually contributes more to a surge than pressure does in shallow seas.
Reason (R): Wind setup grows as the depth of water along the fetch decreases.
Answer: D
Both are true and the reason gives the dependence, and the shallowest shelf seas record the largest surges of all. Depth along the fetch is what governs the magnitude, which is why deep ocean margins escape comparatively lightly.
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Wind setup at a coast is greatest where
Answer: A
Correct answer: the water is shallow and the fetch is long and onshore
Wind setup is the water piled against the coast by wind stress on the surface, and it grows with the fetch over which the wind acts and inversely with the water depth - a shallow shelf offers little cross-section for the return flow, so the water heaps up instead. A long onshore fetch over a shallow shelf is therefore the worst combination, and it is why the same storm produces a far larger surge on a broad shelf than on a steep margin.
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Consider the following statements about surge magnitude.
1. Shallow shelf seas experience larger surges than deep margins.
2. A long onshore fetch increases the wind setup at the coast.
3. Surge magnitude is independent of the water depth involved.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: shallow shelf seas produce much larger surges, and a long onshore fetch increases the setup. Statement 3 contradicts the first - depth is one of the principal controls, which is why the North Sea, the Bay of Bengal and the Gulf of Mexico are all surge-prone while steep-margin coasts of similar exposure are not.
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The most severe storm surges recorded in the Bay of Bengal have reached approximately
Answer: C
The funnel shaped shallow bay amplifies the surge from a landfalling cyclone. Combined with a dense low lying population it has produced the worst surge disasters on record. Geography and demography compound one another there.
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Assertion (A): The Bay of Bengal is exceptionally prone to catastrophic storm surge.
Reason (R): It combines a shallow shelf, a funnelling coastline and a dense low lying population.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives all three ingredients. The northern Bay of Bengal has a very shallow shelf, which amplifies the surge; a funnelling coastline that concentrates the water as it is driven north; and a dense population living on low deltaic land with little high ground to reach. Any one would be serious; together they have produced the deadliest surges on record.
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The right forward quadrant of a landfalling tropical cyclone in the northern hemisphere typically experiences
Answer: A
The forward motion of the storm adds to the rotational wind on that side. Identifying the quadrant is central to forecasting which stretch of coast will be worst affected. Track and quadrant together locate the worst surge.
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Assertion (A): Surge from a tropical cyclone is highly asymmetric about the track.
Reason (R): The storm's forward motion adds to the wind on one side and subtracts on the other.
Answer: B
Both are true and the reason gives the asymmetry, and a small shift in track moves the worst surge. Track uncertainty is the dominant source of error in a surge forecast. A small track shift moves the peak many kilometres.
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