The principal movements in enclosed waters are waves, currents and seiches. The tides are of less import in inland waters, but in the case of coastal aquaculture systems the tides, which flush the ponds, are of maximum importance and would be dealt with separately. From the needs of small farm ponds perhaps these water movements are of lesser significance, but some may be of interest in aquaculture of larger water bodies and floating and deep water aquacultural installations.
These are wind produced. The greater the expanse of water height, wave length and wave velocity. Whipple and Welch give the following equation to calculate the wave height.
h = 0.025 √ f where
h = height in feet of wave, and f = “fetch” or clear distance,
in feet, over which the wind blows. In a wave which is formed in
open water each water particle moves in a vertical circular path and
returns to the same position from which it started - therefore the
wave moves along producing rise and fall at successive positions
without any horizontal movement of water - such a wave is called
a ‘wave of oscillation’. Such a wave when pushed by wind velocity
does move horizontally causing it fall forward, producing a white cap.
Likewise when the wave approaches shallow water the wave height
increases and the top pitches forward in a forward rolling motion,
forming surf. Waves in which there is a definite forward movement
of water are known as waves of translation.
In the sea wave action is said to exert an influence down to 100 metres depth.
Currents occur only in large water bodies and of course in the coastal areas; the latter should be a separate discussion and is indeed of interest to coastal aquaculture. We have already referred to the moving up of thermocline due to upwelling in the coastal areas. In large lakes there are currents - namely, vertical, due to upwelling, horizontal and returning (or undertow). In man-made reservoirs over the rivers it is an entirely different situation. During rains and floods there is considerable flow of water through the reservoirs, though after a temporary stagnation of reduction of current speed at the collection basin. At the upper reaches of reservoirs the conditions may be more like the river with swifter currents. Inside the collection basin the current pattern would be different and would be largely determined by the outflow of water as demanded by overflow and water release for hydro-electric or irrigation purposes. There will be in addition considerable level changes also in the reservoir water, even though one may find occasional stagnation of water (thermocline formation has been described in some tropical reservoirs) depending on the season and the other human demands, i.e. besides fish culture, on the reservoir water. The man-made reservoir though young is a mixture between the lotic (riverine) and lentic (lake) systems.
The aquaculturist using the reservoir for stocking fish and also the reservoir margins for fish farm and nursery (of floating hapas) and also cage and other aquacultural installations must make himself familiar with the current patterns and level differences of the reservoir so as to modify the culture installations (More under “Stocking Open waters” - in the present series).
In larger lakes, besides the vertical current caused by upwelling horizontal currents, occur mostly caused by wind action.
The wind pushing the water horizontally in its direction causes an upper horizontal current in the surface which returns as it strikes the shore and flows in opposite direction above the themocline and sets up a recirculation, thereby forming a shearing plane where no water movement occurs (Fig. 8.3). Below the themocline the water is still, as it appears, in the hypolimnion.
Information on the currents is important for planning erosion control measures (‘soil conservation’) to protect the bunds (dikes) and main gate and also to determine the probability of sediment deposition in water control structures. Shifting mud or sand can block water supply canals and sluice gates. Again one should get information on shifting sand or mud at the site by local enquiry, if a survey is difficult. Here one should consider changing wind and current patterns over the year as well. This aspect though more important in the coastal and brackish water region should be studied with reference to river beds in the freshwater region as well. Seasonal flood patterns should be also studied if not by direct observation, from earlier records.
In the case of coastal and tide affected inland waters, the tidal characteristics in relation to land elevation at the proposed farm site should be determined. Places where tidal fluctuation is moderate, between 2 and 3 metres, are most suitable for farms using tidal flow to fill the ponds. If tidal fluctuations are larger than 4m the place is unsuitable because very high and expensive dikes (bunds) will have to be then made to prevent flooding during high tide. Also it would be difficult to hold water during the low tide, since due to high pressure of the high water column water loss and seepage, crab holes etc. would be greater. Areas with a narrow tidal range of 1 metre or less is also unsuitable, as it would be difficult to fill the pond properly, without recourse to artificial pumping. Use of pumps are advised in areas where tide is less than 2m or more than 3m.
Before siting farms actual measurements of tides consulting Tide tables, especially revieweing the maximum and minimum fluctuations over the year, should be made, to determine the high and low tide bench marks.
Also the highest tides during past floods and storms and wave action during normal tides, storms and floods should be known, perhaps by enquiring with local residents, if no other information access is possible. (See also “Ground elevation and Tides” discussed separately).
