Penrhyn Mawr

Swell, Wind Waves, Tides and the Effects of Landforms

Presentation by Tom Bergh for


Sea kayaking places us directly within two primary and unusual marine energy systems. Tides and their resulting currents, and ocean swell with their long periods.

Swells are moving hills of heavy, dense energy with often dramatic effects on the surface topography of the sea. Swells can intensify forces in the coastal zone, and often push Paddlers into heightened Fun or Fear dimensions.

Wind, above all environmental variables, has the greatest daily impact on most sea paddlers. So predicting wind speed, its direction, and understanding its impact on the texture of the sea, is essential to your seamanship skills. Winds are the movement of air caused by differences in temperature and pressure.

Wind Direction: where the wind is blowing FROM. (Current direction is where it is headed TO.)

Summary of Environmental Factors On Waves

Opposing winds or currents will STEEPEN WAVES.

Underwater ledges, bottom changes can CONFUSE WAVES.

Narrow funneling passages can HASTEN WAVES.

Points of land BEND WAVES.

Steep shorelines or bulkheads BOUNCE WAVES.

Shallows, exposed rocks, beaches BREAK WAVES.


Waves and swells travel for long distances, with their energy intact … until they run into or hit another force or thing. So we need visualize, understand the impacts and effects of other waves, wind, shallows, shoreline structures, tidal currents on wave and swell shape.

Waves are usually generated by wind’s three primary variables:

  1. Wind velocity. Usually expressed in knots, can be meters per second.
  2. Duration. How long the wind is blowing from a fixed direction
  3. Fetch. The surface distance over water that a wind blows.

So waves will be bigger with increased wind speed, blowing a longer time, over a greater distance on the water.

Swells are usually created by the adding and subtracting of waves into often bigger, usually longer structures. They are sine curves. Know that earthquakes, landslides, volcanic intrusions, even calving icebergs can create large swells or tsunamis.

Two Real World Swell ‘Rules’ that I have found useful, as aids to effective on-water decision-making:

  1. Swells begin to ‘feel’ the bottom, and hence begin to stand up (get taller/steeper), when the ocean depth is half the swell length. So review changes in the ocean depth along your course, and have a way to gauge the varying swell length.
  2. In deep water, the ‘celerity’, the apparent speed of the wave (actually the energy moving through the water) is three times the swell’s period in seconds. So a 3’ swell moving with a 15 second period is moving at 45 knots in deep water. That’s a lot of kinetic energy in a dense, heavy wave structure. Along our Gulf of Maine, we suggest attention at 8+ second periods, and a helmet if in the rock gardens. A useful marker for a paddler in the coastal white-water zone.


Spilling waves. When the white water slides down the front of the surf wave.

Dumping waves. The ‘Hawaii Five O’ type with a curling, plunging, powerful ‘dumping’ of the leading edge of the wave.

Most of us can learn to handle or manage Spillers up into the 6+ foot size; but Dumping surf can be a dynamic, violent experience to a kayaker once over 3 foot tall.

Wind Weather Warnings

0 – 17 knots: Be aware of wind against current and local conditions.

18 – 33 knots: Small Craft Advisory. Beginners best stay home. Novice/Intermediates may have their hands full.

34 – 47 knots: Gale Warning

48+ knots: Storm Warning

72 knots: Hurricane

Special Marine Warning

Lightning and kayaking don’t mix well. Know, understand and apply the 5 seconds per mile Rule, the time between the flash and the blast. Understand the differences between a frontal versus thermal lightning storms. Thunderstorms and squalls are often associated with approaching cold fronts. They form along the front up to a line 100 miles ahead of the front. Squall lines are usually moving at 25 knots, so gusts of 40-60 knots are not uncommon. Keep an eye on the sky, and listen to weather radio frequently. In Florida and hot, more humid environments, thermal lightning more common.

Local conditions can vary significantly from the weather forecast. Keep an eye on the sky; understand cloud formations.

Near-Shore Conditions and Coastal wind direction and speed can vary from a forecast. Winds blowing offshore usually veer and pick up speed. Due to Coastal Convergence, stronger winds are created.

Sea and Land Breezes. Differential heating and cooling of the land and the sea.  Afternoon onshore breezes are common in the summer along the New England Coast.  Onshore clouds forming, onshore breeze beginning. Offshore clouds dissolve, sea breeze beginning.

Funneling and Channeling. Winds take the path of least resistance. Funnel between and wrap around , landforms headlands.

Describing Waves

Wave length – distance between crests or troughs.

Wave height – distance from base to the crest.

Wave period – Time for a wave’s peak to pass a fixed point.  Gives some sense for the amount of energy in a particular wave or swell. A wave's energy is proportional to wave height squared. A four foot wave has 4 times the energy of a two foot wave. A long WL and large WH = lots of water involved.

Wave Generators/Types

Wind Waves. Generated by local winds. Irregular and shorter-wavelength.

Swell Waves. Waves get organized into larger and longer forms. Can travel thousands of miles without loosing energy.

Waves caused by Currents, like on a river.

Seismic Activity & Glacial Calving

Tidal waves have very long wavelength periods. Waves from icebergs breaking off can capsize even big boats.

Boat Wakes. Some can be surprisingly big and fast

Waves are either stationary or move slowly upstream. Wind against current = steeper breaking waves.

Shallow Water Waves. When depth of water < half the WL (wavelength), swells begin to feel the bottom and the leading front slows down. As the wave length shortens, the wave height increases; the period remains the same, so the wave must get steeper.

Breaking Waves. Generally said that waves will steepen until water depth is 1.3 times wave height, then will begin to break. I think this is a useless measure. Winds & Current can vary this from 2x to .5x wave height.

Spillers and Dumpers. The way the wave breaks is determined by the shape of the sea bed. Boomers require a watchful eye, and may have an undertow. Spillers generally OK for play.

Refraction. The part of a Wave traveling over shoal or shallowing water depth slows down faster while deeper wave section continues on. Can refract up to 180 degrees plus. Can result in Clapotis on sheltered side of a rock or island. Zones of convergence (building waves) and divergence (reduced waves) on beaches due to bars.

Reflection. Like a ball bouncing off a wall. Reflecting wave can then interact with incoming waves, instantly increasing wave height (add incoming and reflecting waves together.)

Clapotis. Wave pattern often created by reflected waves combining with the incoming waves, causing an area of disturbed water where wave heights can explosively spike.

Wave-Current Interaction. Waves moving against a current receive energy from the current. WL decreases, WH increases. Rapid steepening can lead to breaking. Example: a wind wave will nearly double on a 5 knot opposing current.

The ocean’s bathymetry, its points, narrows, constrictions can create tidal races and overfalls. Study the geography, visualize the ocean bottom.


Guiding Principle. Tides contribute to the variations in coastal ocean depths, to the “texture of the sea” and the biological productivity of intertidal zones. Since the sea level is going up or down, generally over a fixed time of approximately 6 hours, currents form on the ‘flood’ and ‘ebb’. Understanding the effects of Earth’s tides is essential to good seamanship.

The periodic variation in the surface level of the sea caused by gravitational attraction, the pull of the moon and the sun.

Explanation of a Tidal Cycle. The gravitational force of the moon raises a bulge in the ocean on the moon side of the earth and a corresponding bulge, due to centrifugal force, on the opposite side of the earth. High water corresponds roughly with those bulges.

It takes 24 hours and 52 minutes for the moon to orbit the earth. Therefore, times of high and low water are approximately an hour (52 minutes} later each day.

The sun generally has half the gravitational effect on the tides as our moon.

Spring tides are times of greater tidal range, when the gravitational pull of the moon and the sun are in line, generally lagging one or two days behind full and new moons.

Neap tides are when the gravitational pull of the moon and the sun are at right angles, in the half-moon phases, and thus result in a lower tidal range. Not all  tides are equal. The moon’s elliptical track around the earth and the earth’s elliptical track around the sun mean that during certain times their gravitational pull on the ocean is greater. Further away from the earth, less gravitational pull.

Perigee corresponds to times of unusually great tidal ranges;

Apogee corresponds to times of small tidal ranges.

The general Rule of 12ths allows us to estimate the depth and the rate of tidal change in a theoretical coastal tide zone, from low to high tide or vice versa, per hour for each of the six hours.

For example, given a known total rise or fall of 12 feet during the six hours between Low and High water, the depth will change in each of the hours as follows:

1st hour - 1/12 of total range - 1 foot.

2nd hour - 2/12 of total range - 2 feet.

3rd hour - 3/12 of total range - 3 feet.

4th hour - 3/12 of total range - 3 feet.

5th hour - 2/12 of total range - 2 feet.

6th hour - 1/12 of total range - 1 foot.

Please notice that ½, One Half of the vertical movement of water ( 6’ of the given 12’ feet of tidal range) occurs in the middle two hours of the tide (3rd and 4th hours). So the tidal stream flow during that two hour time period, must then be must faster/stronger.

Some useful tidal terminology

Chart datum - The numbers on charts correspond to the depth of the sea at mean lower low water (MLLW or low tide).

Spring tides - tides of increasing range, occurring twice a month, around times of the new and the full moons.

Neap tides - tides of decreasing range, occurring twice a month, around times of the waning and the waxing half moons.

Diurnal inequality - The difference in height of the two daily low waters or of the two daily high waters.

Mean High Water, Spring - the mean of water heights for spring tides.

High Water, Neap - the mean of high water heights at neap tides.

Where to find tidal information for your area

Weather radio marine forecasts (times but not heights)

Calendars at marine stores


Eldridge Tide and Pilot Book (current year)

Reed’s Nautical Almanac (current year)

Tidal information is generally given in relation to a standard port, such as Portland Harbor. Many other locations are given as corrections in relation to the standard port.

For example:

Portland, Maine tidal information for January 1, 2002 as given in Reed’s Nautical Almanac on p. T24: LW 0549  0.0 ft; HW 1203 11.0 ft; LW 1831 –1.3 ft; Tidal range from 1203 – 1831 is 12.3 ft.

Bath, Maine tidal information is based off of Portland and is given as:

HW in Portland +1:01. HW in Bath, ME occurs at 1304; LW in Portland +1:17  à LW in Bath, ME occurs at 1948.

Actual tidal ranges may vary significantly from predicted values due to weather patterns. Strong storm systems (low pressure) centered off the coast may cause significantly greater tidal ranges. Strong high pressure like a reading of 1030 mb (a heavier air mass) depresses tidal ranges. A low barometric pressure of 980 mb is a lighter air mass which may allow tide levels to rise a foot.

Significance of tides to kayakers

Local topography – A sandy beach at high water could have a challenging vertical rock wall at low water. Several islands at high water may be one island at low water.

Mud flats – Imagine leaving from a nice launch site, only to return from a paddle five or six hours later with a slog through a mile or more of mud flats.

Intertidal zones – Are areas of amazing biological productivity and diversity.  Kayakers can have a destructive impact on an intertidal zone when traversing them at low water.

Ledges – Cover and uncover depending on tide. Ledges can make great surf spots but be aware of a changing tide making the ledge unsafe – remember the Rule of Twelfths. Perhaps you can use them to break up or lessen a swell enough to paddle inside them.


  • Determine times of high and low water for a given location, and visualize the beach or shore topography for a specific location from chart and tidal information.
  • Understand the periods of spring and neap tides from a tidal atlas.
  • Where to find/access tidal information.
  • Predict periods of spring and neap tides.
  • Impacts of tidal ranges on kayakers.
  • What local landmarks might you use to figure the state and range of your tides.
  • Locate on a chart areas that are only passable or accessible at high water.
  • Marine Forecasts for winds, swells, pressures, dew point
  • Weather radio/VHF, Television, Internet, Newspaper
  • Local knowledge – experienced mariners, local paddlers
  • Make your own predictions – watch the sky, barometer, critters
  • Barometer’s rate of change in pressure is highly relevant, and can be a key data point for what may happen in the next hour. Understand the rate of change info.
  • NOAA Marine Weather Forecast, local and offshore. National Weather Service Advisories and Warnings; General meanings for kayakers.

Featured photo is the tidal race at Penrhyn Mawr, north Anglesey, Wales.

Posted in Environment, Seamanship.