Hydraulic Gradient in Confined/Unconfined Aquifers
The concept of coefficient of permeability or hydraulic conductivity (K) – introduced by Hendry Darcy (1856) – relates the capacity of a permeable porous medium to allow the passage of water (volumetric flow of water per unit cross sectional area per unit time, Q/A) under a unit hydraulic gradient (I) @ STP. [Q = KIA; or, V = KI].
In this case, the rate of change in hydraulic head per unit length (dh/dL); or, the hydraulic gradient remains associated with the measure of resistance of a permeable medium to the passage of water.
For single-phase fluid flow, the concept of pressure gradient (dp/dL) can be introduced as the product of hydraulic gradient (dh/dL) and the unit weight of water (γ=𝝆g); and thus, the concept of hydraulic gradient remains to be the driving force that causes water to flow.
On the other hand, if hydraulic gradient can be defined as hydraulic head loss per unit length of passage of a moving fluid through a permeable medium, then, it can be associated with the loss of water (fluid) pressure that explicitly depends on the resistance that the permeable medium (soil/rock) opposes to the water flow; and in turn, hydraulic gradient depends on the soil/rock property explicitly; and thus, by bringing in the concept of pressure gradient, the associated hydraulic gradient is considered as a measure of the resistance to fluid flow.
Whether the concept of hydraulic gradient (dictated as a measure of the resistance of the permeable medium to the passage of water) would remain to be varying for both confined as well as unconfined aquifers?
If it varies, would the hydraulic gradient remain to vary linearly with the external load (with hydraulic resistivity as the proportionality constant) – in both confined as well as unconfined aquifers?
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