1. Lecture 20 Ground Water (3) Ground water movement
Darcy’s law
Hydraulic head
Flow nets
Dimensions of water flow

2. Ground Water Movement
Ground water moves more or less continuously from areas of recharge to areas of discharge, i.e., forced by hydraulic gradients
It also moves by chemical gradients causing spatial variation in osmotic water potential

3. Ground Water Movement Darcy’s Law:
Where ν is the macroscopic velocity of water;
K is the saturated hydraulic conductivity;
h/l is the hydraulic gradient comprising the change in hydraulic head ( h) with a distance along the direction of flow ( l ).

4. Hydraulic Head h = ψ + z h = hydraulic head ψ = pressure head
z = elevation head

5. Porosity vs. soil/rock type
Table 5.8, WR Ground Water

6. Terms to Remember
Pressure head: water pressure at a given point, which can be measured by a piezometer
Elevation head: height above a selected reference height
Total head: the sum of pressure and elevation head
Potential energy: product of the total head and the gravitational constant
Hydraulic gradient: change in the total head per unit distance
Hydraulic conductivity: water flux density per unit volume of water and per unit hydraulic gradient
Macroscopic velocity: the speed of water flow through the cross-sectional area of solid matrix and interstices

7. Ground water movement A simple case: a horizontally uniform and extensive surface1
Quantity of water flow per unit time: 2 3 w L 4 5 6 7 2
- 1 = 3
- 2
= = 4
- 3
= …….
Ground Water Movement

8. Ground water movement: flow nets
Figure 5.9, WR Ground Water Movement

9. Mathematical dimension of ground water flow
One-dimensional flow: water potential changes in only one direction, e.g. vertical, applicable to homogeneous, extensive horizontal surfaces
Two-dimensional flow: water potential changes in two directions, e.g., vertical and one of the horizontal directions, applicable to systems like mountain ridges and valleys of infinite length where the variation along the length can be ignored
Three-dimensional flow: water potential changes in all three directions, e.g., vertical and two horizontal directions, applicable to systems like mountain crests
Ground Water

10. Self reading Unconfined groundwater flow (WR, Chapter 5.5.5)