Tides and Currents Questions and Answers

Answer: A

Correct answer: approximately the lowest astronomical tide at that location

Chart datum is set at or just below the lowest astronomical tide, so that charted depths are the least a mariner should expect and the actual depth is almost always greater. The convention errs on the side of safety. It also means chart datum differs from the national land survey datum by a different amount at every port.

Answer: B

Both are true and the reason gives the safety margin, and negative tidal heights are rare. A significant negative surge can nonetheless reduce the depth below chart datum. Negative surges are a real hazard for deep draught vessels.

Answer: C

Correct answer: A-2, B-4, C-1, D-3

Four datums spanning the tidal range. Chart datum sits at about the lowest astronomical tide, at the bottom. Mean sea level is the long-term average. MHWS, the mean high water springs, is the average of the higher spring high waters. And HAT, the highest astronomical tide, is the top of the range achievable from astronomical causes alone, with no surge.

Answer: D

The first two are correct and are the two datums most often confused. The third is wrong, and the difference between them must always be stated when levels are quoted. The two differ by a different amount at every port.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is why it matters. Chart datum is defined locally, at about the lowest astronomical tide of that port, so the offset between it and the national land datum is different at every port. A level quoted without its datum is therefore ambiguous by a metre or more, which on a sea defence is the difference between adequate and not.

Answer: B

It varies through the spring neap cycle and from place to place. Ranges vary from a few centimetres in parts of the Mediterranean to well over ten metres in the Bay of Fundy. Basin resonance rather than latitude explains the extremes.

Answer: C

Correct answer: 16 metres

The Bay of Fundy reaches a spring range of about 16 metres, and the Severn estuary and Ungava Bay are close behind. Such ranges arise where a converging, shallowing basin has a natural period close to the semidiurnal tidal period, so the tide is amplified by resonance. Most of the open ocean sees a metre or less.

Answer: D

Both are true and the reason gives the cause, and resonance amplifies the ocean tide entering the bay. Basin geometry rather than the local astronomical forcing produces the extreme range. Resonance is the controlling factor at such sites.

Answer: A

Co-tidal lines radiate from it and co-range lines encircle it. The tide rotates about the point, driven by the Coriolis effect in the enclosed basin. Range grows with distance from the point. Co-range lines therefore form closed loops around it.

Answer: B

Both are true and the reason gives the mechanism, and rotation is anticlockwise in the northern hemisphere. Amphidromic systems are the reason tidal range varies so much around a sea basin. Range is near zero at the point and largest at the edges.