Why Does the Sea Boil Off That Headland? Water Forced Through a Gap

Tidal flow moves an enormous volume of water twice daily, and where the passage narrows the same volume must pass through a smaller cross-section, which requires it to move faster. A headland projecting into a tidal stream forces water around it, a strait between islands concentrates flow from a wide area into a narrow gap, and a shallow ridge or sill forces water that was spread through depth to pass over a restricted section. Speeds in such places routinely reach several knots and in the most extreme exceed ten, which is faster than most small craft can travel. The acceleration also produces changes in water level across the flow, since fast-moving water sits lower than slow-moving water beside it.

The resulting water conditions are distinctive and dangerous:

• •Overfalls, where fast water passing over a rise produces steep breaking waves that stay in one place
• •Standing waves, which do not travel and which a vessel meets repeatedly rather than passing through
• •Whirlpools and eddies where flow separates from a headland
• •Upwellings and boils where water forced upward reaches the surface
• •Violent confused water where the stream meets wind blowing against it
• •Smooth fast water beside violently disturbed water within a few metres

The whole phenomenon runs on the tide, which makes it forecastable with precision that most marine hazards do not allow. Tidal stream atlases and charts state the direction and speed of flow at each hour relative to high water at a reference port, so a navigator can determine what a given place will be doing at a given time days or years in advance. Slack water, the interval when the flow reverses and speeds fall to near zero, occurs twice in each tidal cycle and lasts from minutes to an hour depending on the place, and it is when such passages are attempted. The consequence is that these places are safe at certain times and lethal at others, with the difference being a matter of an hour.

Navigating such a place safely follows a small set of rules that are consistent worldwide. Arrive at slack water, which means planning the whole passage backwards from that moment rather than setting out when convenient, and accept that missing it means waiting six hours. Check the wind direction against the tidal stream, since wind against tide produces far steeper and more dangerous water than either alone and a passage safe in one combination is not in the other. Keep to the recommended inshore or offshore route, which local knowledge and charts specify and which frequently avoids the worst water entirely. Carry enough power or sail to make progress against the stream if the timing goes wrong. And treat the published timings as approximate, since weather shifts them.

Fast tidal streams shaped navigation and settlement well beyond their immediate danger. Passages that could only be transited at particular states of tide controlled the timing of voyages and made certain harbours valuable as places to wait. Local pilots who knew the timing were essential and were paid accordingly, which is the origin of compulsory pilotage in several waters. Several such places carry names from folklore and appear in literature as monsters or as supernatural hazards, with a famous Mediterranean pair generally identified as a whirlpool and a rock. Tidal mills exploited the same flows for power. And several of the fastest streams are now being examined for tidal energy generation, which is attractive because the resource is entirely predictable.

The same volume of tidal water forced through a narrower passage must move faster, reaching several knots and occasionally more than ten. The result is standing waves that do not travel, whirlpools and violently confused water beside smooth water. The whole thing runs on the tide, so it is forecastable years ahead, and such places are safe at slack water and lethal an hour later.