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climate:beaches

beaches, waves and currents

Water currents:

  • water flows from areas of high energy to lower energy, this is a combination of gravitational, potential or kinetic energy
  • thus in a river the water flows from a higher point to a lower point due to the difference in gravitational potential energy.
  • to see if it is likely to be possible for a person to be able to stand up in a water current, one can apply the rule of one:
    • current strength = depth of water (metres) x speed of the current (metres/second)
    • if the current strength is > 1, an adult is not likely to be able to stand up in it
    • if the current strength is > 0.5, a child is not likely to be able to stand up in it
    • NB. can measure speed of current by timing how long a floating object such as an orange takes to travel 10m

Rip currents:

  • a rip current is a narrow, powerful current of water that runs at right angles to a beach
  • it is due to waves created by external forces, usually wind which push the water against the beach, but instead of this water dispersing back to sea evenly, the receding flow becomes trapped (eg. by a sandbar) which forces the water to run parallel to the beach until it reaches a low point in the sandbar, from where it will rush out to sea forming a deep channel
  • the location of a rip current is often marked by an area of deceptive calm patch of water or perhaps surface turbulence where there are few if any waves
  • the strength of the current tends to be greater if there is high surf or rapid tide changes
  • if a swimmer is caught in a rip current, he should attempt to swim sideways (ie. parallel to the beach) and he will soon be out of the rip current and then able to swim back to shore.
  • swimming directly against the rip current is unlikely to achieve results apart from exhaustion and drowning.

Undertow:

  • though less dangerous than a rip current, undertows can knock people, especially children, off their feet
  • the undertow is a receding water current that passes back out to sea under the incoming waves, particularly strongest along the sea floor

Waves:

  • the size and type of wave depends upon:
    • wind intensity
    • length of time wind has been blowing
    • wind direction
    • shape of the beach

the main types of waves:

  • seas or sea waves:
    • waves generated by local wind - if this wind is persistent over deep water then these sea waves may become swell waves
    • newly formed seas are very chaotic & disorganised, resulting in choppy conditions
    • as the waves begin to grow, the windward wave surface becomes steeper & steeper until the wave height approaches approx. 1/7th of the wavelength, once the steepness reaches 1:7, the wave usually breaks, forming whitecaps & spray.
    • waves merge, overtake each other, cancel each other out, until equilibrium occurs when energy received approximates energy lost resulting in “a fully developed sea” - the time required and the fetch required (distance wind is exposed to the water) to achieve this state increases with wind speed.
    • sea waves usually travel slower than the wind generating them due to “slippage” between the wind and water - usually 1:20 sloping chop moves at ~20% of the wind speed increasing to 50% of wind speed in a fully developed sea with 1:10 steepness.
    • as the sea waves continue to develop, they tend to fan out from the storm area, thus their energy dissipates &  gradually forming lower, longer and more rounded swell waves with perhaps 1:50 slope (see below) from distances in tens of km and more (and usually more than 1 day's travel)  from the storm centre. 
    • indicative fully developed seas for various winds over deep water:
      • Beaufort 3 “gentle breeze” of 19kph or 10knots will produce a 0.5m wave in about 6hrs, reaching peak average wave height of 0.8m by 24hrs
      • Beaufort 4 wind averaging 20-28kph or 11-16knots blowing over a fetch of at least 24km for at least 4.8hrs will result in a peak average wave height of 0.6m with wave period of 3.9sec and wavelength of 16m.
      • Beaufort 6 “strong wind” averaging 39-49kph or 22-27knots blowing over a fetch of at least 140km for at least 15hrs will result in a peak average wave height of 2.5m with wave period of 7sec and wavelength of 51m.
      • a near gale of 30knots will produce a 2m high wave in less than 4hrs, building to 3m by 8hrs, 4m by 16hrs, 5m by 35hrs & reaching peak average height of 6.5m by about 200hrs or more.
      • a gale (Beaufort 8) will produce 2m high waves in less than 2hrs, 3m by 4hrs, 5m by 10hrs, 8m in 35hrs and potentially up to 12m
    • shallow water:
      • when waves reach shallower waters where depth is less than half the wavelength of the wave, friction against the sea bottom can cause them to grow in height & become steep-sided resulting in “short seas”. More worrying for boaters is if the water depth is less than 80% of the height of the wave, the wave will tend to break, thus a 2m groundswell could become a 3-4m high steep-sided 'breaker'. Bass Strait is generally less than 60m deep & some of the waves generated by storms in the Southern Ocean can grow to a menacing height as they encounter these waters.
      • shallow water waves are also influenced by:
        • reflection - waves reflecting back from rocky cliff or sea wall resulting in steep sided bumpy waves that don't go anywhere - “standing waves” which can fool sailboarders into thinking there is more wind than there really is.
        • diffraction - waves diffract around a barrier such as an island or breakwater
        • refraction - waves in shallowest water move the slowest resulting in the wave front bending to become approx. parallel with the underwater contours.
      • waves generated in shallow waters:
        • for water depths > 1.5m, depth does not significantly effect wave height if fetch is < 1km and wind speed < 35knots, but the greater the fetch, the more the effect water depth has on maximum possible wave height produced for a given wind speed, such that for 15knot winds with fetch of 500km, wave height will rise from 0.3m in 1.5m deep water to 1.1m in 15m deep water.
        • likewise for 35knot wind over 500m fetch, wave height will rise from 0.5m in 1.5m water to 2.6m in 15m water depth. 
        • NB. tides behave as shallow water waves even in deep oceans as their wavelength (half of earth's circumference) is greater than 20x ocean depth.
  • groundswell:
    • this is the general swell caused by weather patterns such as highs & lows, ocean currents and continental barriers
    • the swell is often 5-6m in oceans at the centre of a low pressure system, decreasing the further away from its centre
      • these are responsible for “high surf” conditions which surfers love, but the large powerful waves produced whilst often being 4-5m, may have an odd wave reaching even up to 10m high thus:
        • NEVER turn your back on the sea in such conditions
        • always watch the surf for at least 15mins before entering the water
        • never attempt to swim near the water's edge during big surf
        • never surf or bodyboard in big waves unless you are an expert and an excellent swimmer
        • always check with lifeguards 1st before entering water
        • the probability of a “rogue wave” or “freak wave” at a high 1.85x the average wave height is 1 in 1000 waves, and thus one can expect one of these every 3 hours on average assuming a typical wave period is 10-11secs! 
    • the swell is often 2-3m in oceans at the centre of a high pressure system, decreasing the further away from its centre
    • land barriers such as within Indonesian waters, the swell is moderated and usually only 1m
    • swells temporarily increase in size when moving over submerged reefs where water is suddenly shallower
    • swell waves generally move faster than the wind generating them due to complex interactions such as “leapfrogging”.
    • when fully mature, swell wave velocity in mid-latitudes = 1.25 x square root(wavelength) = 1.56 x waveperiod as wavelength = 1.56 x (periodicity in seconds)2   
    • ocean waters south of Canarvon in WA and south of Sydney tend to have higher swells such that the swell is nearly always at least 1m and at least 50% of the time is greater than 2m and at least 20% of the time it is greater than 3m.
    • average swell tends to be highest on the following parts of the Australian coastline:
      • south-west coastline of Tasmania
      • south-west corner of WA
      • south-west coastline of Victoria, west of Moonlight Heads - the “ship-wreck coast”
    • to see swell charts check out Australian WaveCam website
  • surging:
    •  
  • spilling:
    •  
  • plunging or dumping:
    • these are especially dangerous to swimmers as they break with tremendous force and can throw swimmers to the sea floor, potentially causing spinal injuries and drownings
    • they are usually found where the beach falls away steeply & often occur at low tides when sandbanks are shallow
    • whereas a flat beach allows the energy within a wave to disperse, a steep beach does not allow this and results in dumping waves

Holes and Channels:

  • when sandbars are visible in the water, swimmers need to be wary of holes and channels which may find poor swimmers suddenly out of their depth
  • sandbars form a short distance out to sea when waves dump sand & sediment from the ocean floor in one place
  • sandbars act as a barrier, forcing water returning from the beach to find another way out to sea, this results in a rip current which forms deep channels  parallel to the beach and also perpendicular to the beach between the sandbars
  • in addition, the rip currents scour the sea bed, picking up sand and sediment and dumping it in areas that were once channels and creating holes instead

Why is sea water salty?

  • rivers flowing into the sea bring salts from soils, etc into the sea and as the only way for water to escape from the sea is via evaporation, the sea water becomes increasingly saltier as the salt is always left behind.
  • rivers in contrast are always being replenished by fresh rain water and the salts picked up en route are deposited in the sea so there is no salt build up.
  • the oceans hold some 50 million billion tons of salt & if dried & spread out on land, it would cover the whole earth to a depth of 500 feet. Rivers dump some 4 billion tons per year into the oceans. So what added effect will human sewage outflows be?
  • the salinity of ocean water varies, averaging about 3.5% salt by weight, about 220x saltier than freshwater, and consists primarily of sodium and chloride which make up 85%
  • the most saline are in regions with high evaporation rates and low fresh water influx such as:
    • the Dead Sea
    • the Red Sea & the Persian Gulf with 4% salinity
  • the least salty are in polar regions where both melting polar ice & a lot of rain dilute the salinity.
    • the Baltic Sea has 0.5-1.5% salinity
climate/beaches.txt · Last modified: 2013/02/02 00:46 by gary1