Discussion: Ground water movement

Ground water is the water found in the pores,avities,cracks & other spaces in the crustal rocks.it is synonymous to phreatic water & is the most important source in any water supply.

In it's natural state it is invariably moving which is governed by established hydraulic principles.The flow through the aquifers most of which are natural porous medium are expressed by Darcy's law.

Theaore said, law states that the flowrate through the porous media is proportional to the head loss & inversely proportional to the length of the flow path.If the ground water table is assumed to be flowing through a cylindrical aquifer then ground water velocity or Darcy's velocity is defined as the discharge "Q" divided by the total cross sectional area of a cylinder "A".

However,the aforesaid calculation ignores the fact that aquifer cross section "A" contains both solid materials & pores.therefore,round water velocity is an artificial velocity which has no direct physical meaning exept having mathematical convenience.

Darcy velocity(V)=Q/A

Knowledge of hydraulic conductivity is acquired where water is the prevailing fluid.a medium is said to have uniform hydraulic conductivity if it will transit in unit time a unit volume of ground water at prevailing kinematic viscosity through a cross-section of the unit area,measured at right angles to the direction of flow,unde aunit hydraulic graidient.this depends on porosity,particle size & distribution,shape of particles,particle arrangement Physical factors of soil & rocks.

Example:-clayey materials have low hydraulic conductivity while sand or gravel has higher values.From darcy's law it follows that the rate of ground water movement is governed by hydraulic conductivity of an aquifer & the hydraulic graidient.

Ground water velocities vary widely depending on local hydro-geologic conditions where velocities tend to decease with the depth as porosities & permeabilities also decrease.

Velocities can range from negligible (2m/yr to 2m/day are normal velocities) to those of turbulent streams in underground wells & drains act to accelerate flow.Since,no flow crosses an impermeable boundary therefore flow lines must be parallel to it.Also if no flow crosses the water table of an unconfined aquifer it becomes a boundary flow surface.Under steady state conditions,the elevation at any point on the water table equals the energy head & a a consequence ,flow lines lie perpendicular to water table contours & within a confined aquifer flow lines are orthogonal to contours of the potentiometric surface.

With the only 3 known ground water elevations from wells estimates of local ground water contours & flow directions can be determined.Ground water flows in response to differences in mechanical energy to points of low mechanical energy & the difference between the 2 is divided by the distance between the aforesaid 2 points & the result is called the Energy gradient.

In general round water flows in the direction of steepest energy gradient except in case of anisotropic aquifer.It must be pointed aot Darcy's law was developed for laminar flow conditions which characterize slow moving ground water in porous media.In this type of flow water molecules move steadily through poe throats without forming eddies.

Eddies characterize faster,turbulent flow regimes ,common in rivers.Laminar flow is more common than turbulent flow in aquifer.However,turbulent flow could be encountered in ground water moving through cobble deposits,fractures or solution channels.

Reynold's number quantifies a flow regime,with the values less than (approx.) one indicating laminar flow & values greater than 1,turbulent flow.

 

For a confined aquifer under the steady conditions,radial flow to a well is described by the thiem(1906) equation.

Flow nets graphically illustrate the movement of ground water in 2D space & consists of equipotential lines & flow lines.At a ground water recharge area water flows downwards & away from a high area in the water table.thus,at a hill water level in a peizometer is below water table indicating a downward gradient.