Tidal Channel a river reach, in which periodic changes in water level occur due to tides. The alternate rising and falling of water level of seas, oceans, gulfs, bays, rivers and other water bodies caused by the gravitational pull of the moon, sun and earth in combination with the earth's rotation is known as 'astronomical tide' or tide. A tide is mainly generated in the deep sea from where it travels into coastal waters.
A tide produces bi-directional flow in channels. When a tide propagates upstream through tidal channels, it is called flood tide, when it flows downstream into the sea it is known as ebb tide. The periodic rise and fall of water level results in temporary storage of water during high tide and drainage at low tide. During flood tide, seawater intrudes upto the tidal limit of the channel. Salt is introduced into the channel, and the mixing of saline and fresh water takes place. The distance upto, which the tidal effect is felt, depends on the slope of the river or channel, the tidal range, volume of fresh water discharge, configuration of the river, etc. During 'neap-tide' or high river discharge conditions, the interface of fresh and saline water shifts seaward direction while at 'spring tide' and under low river discharge the interface shifts upstream. Tidal channels experience both erosion and accretion. Channel platform may vary from meandering to braided.
The coastal zone of Bangladesh is criss-crossed by large tidal rivers which discharge into the bay of bengal. The tides in the Bay are predominantly semi-diurnal (ie, two high and two low waters everyday; the tidal period is 12 hours 25 minutes). The speed of the tidal wave that propagates through the tidal channels depends on the cross-sectional area of the channel as well as on the water levels of the adjacent rivers. Thus flow from one channel to another may occur. This is a common phenomenon in the coastal region of Bangladesh. The tidal wave from one river meets the wave from another river. This meeting point is characterised by periods of low or stagnant flow and consequently siltation. Where there is no tidal meeting point the flow is continuous and there is less siltation.
In the southwest, tidal action is experienced upto 225 km inland in the wet season and 325 km in the dry season. The average tidal range in the area varies from about 3m on the coast of Hiron Point (pasur river) to 0.5m 275 km inland. The major tidal channels of Bangladesh are: Lower Meghna river, Shahbazpur Channel, Hatia Channel, Sandwip Channel, raymangal, Maloncha, shibsa, Pasur, Baleshwar, karnafuli, sangu, matamuhuri and bakkhali.
Bangladesh Inland Water Transport Authority (BIWTA) collects data of tidal rivers and publishes Tide Tables. bangladesh water development board also collects data of tidal rivers at selected stations. [HS Mozaddad Faruque]