GROUNDWATER HYDRAULICS H Darcy DEFINITIONS Ground water is a - - PowerPoint PPT Presentation

GROUNDWATER HYDRAULICS

H Darcy

DEFINITIONS

• Ground water is a major source of water supply. It needs very little

treatment

• Vadose zone is a partially saturated zone above the water table. Capillary

forces play a major role in it

• Aquifer is a geological formation that contains and transmits groundwater.

Velocity of groundwater is less than 1m/day

• Aquiclude is an impermeable geological formation (clay, shale etc.)

DEFINITIONS (cont.)

• Confined aquifer is an aquifer confined by two aquicludes. Recharge
• ccurs through outcrops
• Unconfined aquifer is an aquifer with free water surface for an

upper boundary. Recharge takes place through the ground surface above the aquifer. In case a stream crosses an aquifer, either the stream will recharge the aquifer or groundwater will discharge into the stream in the form of springs along the channel

• Volumetric porosity,

• Specific yield is the fractional volume of water that will drain freely by

gravity from a unit volume of the formation. If water table elevation for an unconfined gravel aquifer is lowered by 1m, a yield of 0.22 m3/sq.m is expected

• Safe yield is the amount of groundwater that can be withdrawn without

impairing the aquifer as a water source

DEFINITIONS (cont.)

Types of Aquifers

Creeping motion

• In 1856, H. Darcy found experimentally that the seepage velocity

(averaged over pores and solid region) is given by where, k is the hydraulic conductivity, m/s H is the piezometric head, m vx is the superficial velocity, m/s

• Through the pores, the seepage velocity = .

Creeping motion (cont.)

• For creeping flow in a gap of height b we had found
• For a stream-tube in a porous media, b is proportional to the particle size
• diameter. Thus,
• By comparing this with Darcy’s equation, the hydraulic conductivity,

• Intrinsic permeability depends upon the physical properties of the porous

medium

• Hence,
• r
• Conductivity (k) depends on both physical properties of the porous

medium and the fluid properties.

• Let us consider Darcy’s law in Rectangular Cartesian component form

, and

Creeping motion (cont.)

• For isotropic material, k is the same in all directions. Substituting in the

continuity equation, we get

• H behaves like the potential function
• Darcy’s law is valid for

.

Creeping motion (cont.)

Bore-well

Steady State Equation for Wells

• After a well has been pumped for an extended period, steady state

conditions are approached

• In the simplest possible case, use Darcy’s equation for an isotropic,

homogeneous aquifer

• a) Confined aquifer

Or

Steady State Equation for Wells (cont.)

• Therefore,

where T is known as the transmissivity and is equal to (bk). The above equation is known as Thiem equation

• Drawdown = H – h =
• Where, R is the radius of influence and rw is the radius of well