1. . . . we cannot tell

2. When any given tide on the heart’s shore

3. Comes to the full.

4. . . . Few could endure

5. That knowledge, and not die.

6. It is better to be unsure.

7. Jan Struther, High Tide

The application of sea-level and tidal knowledge to the design and construction of useful marine structures and systems includes:

• harbour design and operation,

• design of coastal defences to resist flooding,

• coastal sediment control, groynes,

• flood warning systems,

• estuary, wetland, lagoon and inlet management,

• offshore structures for gas and oil extraction,

• schemes for generating power,

• cooling water intakes, effluent discharges to the sea,

• climate change forecasts and planning.

Tides offer many invaluable on-going environmental services that are not costed or charged. Ship routing between ports has used tides since historical times. Most of the great ports of the world are situated near the mouths of large rivers and many are a considerable distance inland. London, on the River Thames, and Hamburg, on the River Elbe, are good examples of inland ports. By travelling inward on a flooding tide and outward on an ebbing tide, ships can make considerable savings of fuel and time. The vigorous tidal currents serve to keep channels deep. The tidal flows can also prevent harbours freezing during winter, for example in New York, both by their mixing action and by the introduction of salt water which lowers the freezing point. Pollution, inevitably associated with large industrial developments and centres of population, is also more readily diluted and discharged to sea where there are regular exchanges of tidal water. The conditions for ports where tidal ranges are relatively large may be contrasted favourably with those, for example Marseilles, where tides are small. The pollution problems are much greater in the Mediterranean, and despite their high rates of fresh-water discharge, neither the Rhône nor the Nile have proved navigable for any but the smallest sea-going vessels.

When planning marine engineering works, the design parameters include not only sea-level changes: tides, surges, tsunamis and mean sea level (MSL), but also waves, winds, earthquakes, sediment movement, marine fouling and ice movement. Here we focus only on the sea-level aspects of the design engineer’s considerations.