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3.3.1. Physical characteristics- Mobility, Buoyancy, Movement of water
Unit 3- Nature of Inland water environment
3.3.1. Physical characteristicsMobility (Viscosity)
Water is an exceedingly mobile liquid. Nevertheless, it has internal friction (viscosity). This viscosity varies with the temperature. Water is distinctly more mobile at ordinary summer temperatures than that are just before it freezes. The viscosity changes with temperature. The response of water to wind of fixed velocity would differ with different temperature of the water. Pressure does not cause any significant change in viscosity.
Buoyancy is the direct outcome of density and varies with the same factors. The law of Archimedes states that the buoyancy of an object is equal to the weight of the water it displaces. The greater the density, the greater the buoyant force; the denser the water, the floating object will ride higher in the water. Thus, ship passing from fresh water into sea water rises little higher, and the same ship with the same load would ride somewhat higher in winter than in summer.
Movement of water
The principal forms of movements of water are waves, currents and seiches.
a) Waves
Waves are mainly produced by wind. They occur on every body of water in forms and magnitudes depending upon various local conditions, such as area of open water; direction, and velocity of winds; shape of shore line and relative amounts of deep and shallow water. The greater the expanse of water over which the wind blows the greater the potential wave height, wave length, and wave velocity. Stevenson (1934) formulated a formula for computing the maximum height of wave in small bodies of water as
h = 1/3 √F
h = Maximum height in water
F = Fetch of the wind in km.
In open water two types of waves are formed namely waves of oscillation and waves of translation.
b) Currents
Currents in lakes are mainly of three kinds, viz, vertical, horizontal and returning. True vertical currents seldom occur in inland lakes, but may be present in large waters such as the Great Lakes. When present in inland lakes, they are the result of some unusual thermal, morphological, or hydrostatic circumstance and upwelling of water from deep water source.
Horizontal currents (undertow currents) are common in lakes. They are usually produced by wind and often modified by the shape of shore line and form of the basin. The ratio of wind velocity to water movement diminishes as the wind velocity increases. Also, water velocity diminishes with the increase in depth.
Returning currents are formed when water is piled up on an exposed shore as a result of an onshore wind. Such action raises the water level at the position, and, as a result, the excess water may return underneath along the bottom. The magnitude and duration of such currents depend upon the velocity and duration of the wind. Steady vigorous, onshore winds may set up return currents which extend to the opposite side of the lake.
c) Tides
In inland lakes, tides are almost imperceptible, even in the Great lakes. Lake Michigan is said to have a tide of about 5 cm. This virtually means that tides in freshwaters are so far as known is negligible phenomena in Limnology.
d) Seiches
In lakes and along the sea coasts, oscillations of the water level occur under certain circumstances which are called seiches (pronounced as Saches). A seiche consists of a local, periodic rise and fall of the water level. It is an example of standing wave in which the water particles do not travel in circular orbits but the advance and return of the particle are in the same path. Any influence which produces a temporary, local depression or elevation of water level may produce a seiche.
Movement of water
The principal forms of movements of water are waves, currents and seiches.
a) Waves
Waves are mainly produced by wind. They occur on every body of water in forms and magnitudes depending upon various local conditions, such as area of open water; direction, and velocity of winds; shape of shore line and relative amounts of deep and shallow water. The greater the expanse of water over which the wind blows the greater the potential wave height, wave length, and wave velocity. Stevenson (1934) formulated a formula for computing the maximum height of wave in small bodies of water as
h = 1/3 √F
h = Maximum height in water
F = Fetch of the wind in km.
In open water two types of waves are formed namely waves of oscillation and waves of translation.
- i.Waves of oscillation: In this type of wave, the water particle moves up and down but no horizontal movement of water.
- ii.Waves of Translation: In this type of wave there is definite forward movement of water
b) Currents
Currents in lakes are mainly of three kinds, viz, vertical, horizontal and returning. True vertical currents seldom occur in inland lakes, but may be present in large waters such as the Great Lakes. When present in inland lakes, they are the result of some unusual thermal, morphological, or hydrostatic circumstance and upwelling of water from deep water source.
Horizontal currents (undertow currents) are common in lakes. They are usually produced by wind and often modified by the shape of shore line and form of the basin. The ratio of wind velocity to water movement diminishes as the wind velocity increases. Also, water velocity diminishes with the increase in depth.
Returning currents are formed when water is piled up on an exposed shore as a result of an onshore wind. Such action raises the water level at the position, and, as a result, the excess water may return underneath along the bottom. The magnitude and duration of such currents depend upon the velocity and duration of the wind. Steady vigorous, onshore winds may set up return currents which extend to the opposite side of the lake.
c) Tides
In inland lakes, tides are almost imperceptible, even in the Great lakes. Lake Michigan is said to have a tide of about 5 cm. This virtually means that tides in freshwaters are so far as known is negligible phenomena in Limnology.
d) Seiches
In lakes and along the sea coasts, oscillations of the water level occur under certain circumstances which are called seiches (pronounced as Saches). A seiche consists of a local, periodic rise and fall of the water level. It is an example of standing wave in which the water particles do not travel in circular orbits but the advance and return of the particle are in the same path. Any influence which produces a temporary, local depression or elevation of water level may produce a seiche.
Last modified: Tuesday, 24 April 2012, 5:54 AM