Limnology (2+1)

Unit 3- Nature of Inland water environment

3.3.2.Seiches
Most commonly produced seiches in lakes are due to :
1. Winds, temporarily strong, which pile up water on the exposed margin of the lake
2. Sudden change in barometric pressure over a portion of the lake area
3. Earthquakes
4. Land slides
5. Sudden, very heavy rainfall at one end of lake.
The amplitude depending upon the dimensions of the lake and the intensity of the initial cause may vary from a fraction of a centimeter in small lakes to 1 m or more in large ones. In lake Geneva, Switzerland it is reported that the amplitude of a seiche may reach about 2 m.
Forel (1895) used the following formula for computing the period of oscillation of a seiche in a lake whose basin has definite regularity of bottom.
t = l/√gh
where, t = time of one half oscillation in sec
l = length of axis of seiches in meters
g = acceleration of gravity (9,809 m/sec2)
h = depth of water in meters
More complicated formulas were worked out for lakes having irregular basins. Whipple (1927) presents the following formula.
t =2 l / 3,600√dg
where, t = time of oscillations in hours
l = length of lake (or length of axis of seiche) in feet
d = mean depth in feet along axis of seiche
g = acceleration of gravity (32.66 ft/sec2)
Seiche condition in lake Erie, the calculated period is 14.4 hr.
Forms of Seiches
Forel (1895) showed that seiches are of different forms as follows
•Longitudinal seiches - whose axis corresponds with the direction of the long axis of the lake.
•Transverse seiches - whose axis lies in the direction of one of the shorter axes of the lake.
Both longitudinal and transverse seiches are of three different forms :
a) Uninodal - having one node

b) Binodal – having tow nodes

c) Dicrotic seiche – having two beats (show as two peaks on a limnograph) due to interference of unimodal and bimodal seiches.

d) Plurinodal – having several nodes
Lesser forms of water motion are sometimes called seiches as for example the short–period, back and forth flow of water though narrow channels in certain localities in very large lakes and the subsurface seiches, a type which has been postulated as the cause of certain submerged currents in lake Erie.
Subsurface waves, sometimes produced in large bodies of water, occur where subsurface water is denser than the overlying water. A strong localized wind starts an impulse (wave) in the underlying layer of water which moves forward in the direction of the wind. As this wave moves along the warmer lighter water just passes over the crest of the wave but in the opposite direction, thus producing a surface current opposite to the direction of wind.

Subsurface seiches usually arise from a temporary displacement of the thermocline by the weight of piled up surface water on one side of a lake due to strong wind action.