Tidal Bore

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Tidal Bore a high tidal wave experienced in a narrow river or estuary, caused by the rushing of water, advancing upstream like a wall of water. The inflowing water rises to a considerable height presenting a very abrupt wall. It is formed by the sudden retardation of the normal tidal wave, as it reaches shallow water and meets the river current; this produces the water to pile up across the river, the crest falls over and the water moves along like a broken wave. Bore is observed at the mouths of the coastal rivers like Amazon, Hooghly, meghna, Indus, Severn, Yangtse-Kiang etc. The bores may cause shipping hazard. Tidal bore is also known as Eagre or Eger and also as Ban in Bangladesh.

In Bangladesh, the terms tidal bore, bore, tidal-surge, cyclonic-surge and storm-surge are often used synonymously. The great surge of seawater, accompanying a cyclone, which moves as an advancing wave is known as cyclonic surge. Two separate types of sea disturbances associated with storms may cause the cyclonic surge. The first type is the cyclonic wave. This develops in the calm centre of the storm. Due to some combination of wind and pressure, a large mass of water at and around the storm centre is lifted up enmasse, often to a height of 6m and progresses with the storm as a wind driven storm surge. When it hits the coastline, this great mass of water breaks with great force and causes heavy flooding of the coastal regions. The second type is caused by the cyclonic tide. This is quite a different phenomenon and covers a wider area. It consists of a general rise in sea level along the coast at the front of the storm winds.

The most destructive element of cyclone is its accompanying surge. There is little that can withstand a great mass of onrushing water, occasionally as much as 6m high. In Bangladesh, cyclones occur in April or May and also between September and December. On an average, 1-5 severe cyclonic storms hit Bangladesh each year and the accompanying storm-surges can reach as far as 200 km inland. Cyclonic surge heights, are directly related to the cyclonic intensity. With the increase of wind speed, surge-height increases. Astronomical tides in combination with cyclonic surges lead to higher water levels and hence severe flooding.

Storm surges accompanying cyclones hitting Bangladesh have been noted to be of magnitude of 3-6m high. In the Meghna estuary of Bangladesh the 1970 Cyclone (Nov 12-13) with cyclonic surge of 3.05-10.6m high with wind speed of 222 km/h occurred during high tide causing most appalling natural disaster claiming 0.3 million human lives. On 29 April 1991 a devastating cyclone hit Chittagong, Cox's Bazar, Barisal, Noakhali, Patuakhali, Barguna and Khulna along with tidal bore of 5-8m high with wind speed of 240 km/h killing 150,000 human being, 70,000 cattlehead, and the total loss was about Tk 60 billion. [HS Mozaddad Faruque]

See also cyclone; natural hazard.

See map in cyclone.