What causes dra­matic sea states?

Yachting Monthly - - PRACTICAL SEAMANSHIP -

W ith winds of Force 5 or so, a lot of the wave crests top­ple over and break, and this of course be­comes more fre­quent and heav­ier as the wind gets stronger. But it’s not the only fac­tor at work. Wave mo­tion in­volves a dis­tur­bance in the wa­ter that ex­tends down to about half the wave­length be­low the sur­face. A diver hov­er­ing un­der­wa­ter moves in a ver­ti­cal cir­cle as each wave passes. Ex­cept in break­ers, there’s no net on­ward move­ment of wa­ter. But when the wa­ter is mov­ing bod­ily in the op­po­site di­rec­tion, the wind-driven waves are ef­fec­tively slowed so the en­ergy trans­forms into shorter, steeper, higher waves – this is the wind-over-tide sit­u­a­tion. When the wave height to wave­length ra­tio is around 1:7, the waves break, form­ing over­falls. The con­verse is also true, but we tend not to no­tice.

Shoals af­fect sea state

In wa­ter shal­lower than half the wave­length, the seabed starts to in­ter­fere. It slows the waves down, and again they pile up, get­ting shorter, steeper and higher. An un­der­wa­ter reef off­shore can cause a big­ger wave than usual to rear up, ap­par­ently out of nowhere, and break, some­times with tremen­dous vi­o­lence. In the North At­lantic, the swell’s wave­length can be 500 me­tres or more, and be­cause it’s the wave­length that in­flu­ences the ef­fect, it can hap­pen in sur­pris­ingly deep wa­ter and with lit­tle warn­ing. Even the edge of the con­ti­nen­tal shelf, 100-200 me­tres down, is less than half the wave­length so it af­fects the sea state (200 me­tres hor­i­zon­tally is only a good golf shot, af­ter all) and this is one rea­son for the rep­u­ta­tion of the Bay of Bis­cay.

A river mouth bar pro­vides the set­ting for the worst of both worlds – out­go­ing stream meets on­shore waves over a shal­low patch – while the com­bi­na­tion of an ir­reg­u­lar bot­tom, strong tide and ex­po­sure to heavy seas can be spec­tac­u­lar.

In places like Cor­ryvreckan, the Pent­land Firth and Port­land Bill, even in the ab­sence of any wind or swell, the tide by it­self cre­ates a dis­turbed sea – a race – and there may be stand­ing waves, which rear up con­tin­u­ously in the same place and can be al­most wall-like.

Waves im­ping­ing on cliffs with deep wa­ter at their foot tend to bounce back, and the re­sult is a jum­bled and chaotic sea state of danc­ing peaks and hol­lows. The French have a word for it: clapo­tis. The term is fa­mil­iar to kayak­ers, who

fre­quent places like that, but the clapotic sea state is strangely ab­sent from the sailor’s vo­cab­u­lary.

Waves ra­di­ate out­wards in all di­rec­tions over long dis­tances from a storm cen­tre, and as they travel, the com­po­nent wave trains sort them­selves out. The smaller, shorter waves quickly lose their en­ergy and dis­ap­pear, leav­ing the longer-pe­riod waves to reach coasts up to per­haps 1,000 miles away, in the form of swell. This long, reg­u­lar roll from a dis­tant storm may be quite un­re­lated to the wind-driven sea con­di­tions lo­cally, but it can have a big im­pact on pas­sage plan­ning, safety and com­fort. If there is also a big lo­cal sea run­ning in a dif­fer­ent di­rec­tion, a cross sea re­sults, which can pro­duce steep and dan­ger­ous waves. Be­cause of their length, swell waves are also par­tic­u­larly prone to rear­ing up in shal­low wa­ter. Surfers love them. They call them prowlers.

There are sev­eral ex­cel­lent fore­cast web­sites and, on the ones listed on the pre­vi­ous page, sea state re­ports are avail­able from met buoys. Check pres­sure charts for the whole ocean, to see where the storm cen­tres are and how they’re mov­ing, and you’ll get a grasp of the in­flu­ences at work gen­er­at­ing swell.

How does sea state af­fect your pas­sage plan­ning?

Un­less you’re a real glut­ton for pun­ish­ment, you’ll pre­fer not to sail in steep and break­ing seas. So do your home­work and give an un­avoid­able wind-over-tide head­land an ex­tra-wide berth. Check the re­ports from met buoys, and look at sea state fore­casts to see what swell con­di­tions will be like. In a big sea, stay away from shal­lows and shoals.

At har­bour and river mouth bars, check the swell di­rec­tion, and if things are mar­ginal, try to time your en­try for a high and ris­ing tide. Some places and pas­sages may have to be avoided al­to­gether. A long (even barely per­cep­ti­ble) swell can make for a rolly and sleep­less night at an­chor, and swell waves are apt to be re­fracted round head­lands, so bear in mind that an ap­par­ently shel­tered bay may not be as snug as it looks on the chart.

But when all’s said and done, there’s some­thing very pleas­ant about the steady mo­tion of a good boat in an ocean swell. A life on the ocean wave!

When the depth is less than half the wave­length, the seabed be­gins to slow the wave by fric­tion and the wa­ter par­ti­cles start to move el­lip­ti­cally. As the wave slows, wave­length short­ens and wave height in­creases un­til the ra­tio reaches 7:1, when the wave breaks

As the River Arun flows out of Lit­tle­hamp­ton against a gen­tle southerly on­shore breeze, the wind-over­tide ef­fect is clear

This yacht has opted to take a fair tide through The Swinge off Alder­ney, but even a rel­a­tively gen­tle wind against a rac­ing tide kicks up steep seas

In the fore­ground we can see a clapotic wave, which has re­flected off the wall of Dawlish train sta­tion and col­lided with an in­com­ing wave

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.