Explain flow of water in saturated and unsaturated soil.
Answers
Saturated flow decreases as the pore size decreases. Generally the rate of flow in soils of various textures is in the following sequence. ... Water flow when larger pores in soil are filled with air is said to be unsaturated
Flow of Water in Saturated Soils:
We know that groundwater is largely hidden and cannot be seen and understood as surface water. However, groundwater storage in saturated zone or groundwater reservoir forms a significant part of the total water resources. In some respects, groundwater reservoirs are similar to surface water reservoirs. They also have a particular capacity and must have discharge and recharge. The flow of water in saturated zone of soil is primarily governed by Darcy’s law which states that the velocity of flow through a porous media is proportional to the first power of hydraulic gradient.
It can be expressed as:
V = K i
or Q = K A i
Where Q is the quantity of water flowing in the soil.
if is a constant, it depends upon porosity and permeability of the soil and is called coefficient of permeability or hydraulic conductivity of the material.
i is the loss of head per unit length or hydraulic gradient,
Mathematically i = h1 – h2/L
V is velocity of water in soil
and A is cross-sectional area through which the water flows.
In deriving this equation Darcy assumed that there are number of capillary tubes in soils through which water passes in the similar way as it passes through pipes. Since the velocity in the laminar flow is also proportional to first power of hydraulic gradient, the flow obeying Darcy’s law is essentially laminar. Applying same analogy as that of flow through pipes the Reynold’s number can be used as an index to determine the range of applicability of Darcy’s law.
The Reynold’s number is expressed as
R= Vd/γ
where V is velocity of flow
d is average grain diameter
γ is Kinematic viscosity of the fluid
γ = μ/ρ = Dynamic viscosity of fluid/Density of fluid
It has been seen that in natural and artificial sandy medium of uniform grain size deviation of flow from laminar state to turbulent state starts for values of R between 1 and 10 depending on the shape and range of grain size. Almost in all cases the groundwater flow can be seen to be laminar and therefore obey Darcy’s law.
In tube wells also where pumping rates are quite high ordinarily the flow obeys Darcy’s law upto the well face. But at the well face. It may tend to become turbulent owing to high rates of pumping. The observation of the waters table can be very easily and quite accurately done by noting the levels in the rivers, streams and existing wells.
Flow of Water in Unsaturated Soils:
Darcy’s law for flow of water through saturated soils also holds good for unsaturated soils Water always flows in the direction of decreasing energy. Thus the equation V = k i is also true tor unsaturated soils. Unlike in saturated soil, cross-sectional area of flow is decreased since the moisture content is less.
It also lowers permeability rates. Another complications m the unsaturated soil is about measuring negative pressure. For saturated soils energy can be very conveniently measured by inserting piezometer tubes in the soil. In unsaturated soil mass in addition to water flow there exists vapour flow also. It is very difficult to measure vapour flow through the soil. The flow of vapour increases with decrease in moisture content of the soil. Hence, no perfect analytical treatment of flow of water through the unsaturated zone of soil is still feasible.