Subject Code: WER300S Subject Name: Engineering Hydrology Chapter 5 - Groundwater
Subject Code: WER300S Subject Name: Engineering Hydrology Chapter 5 - Groundwater
Subject Code: WER300S Subject Name: Engineering Hydrology Chapter 5 - Groundwater
Chapter 5 - Groundwater
Groundwater storage
Porosity
Specific retention
Specific yield
Groundwater movement
Permeability (hydraulic conductivity)
Porosity
Specific yield
Well hydraulics
Groundwater Hydrology
Groundwater hydrology deals with the occurrence,
storage, movement and quality of water
beneath the earth’s surface (subsurface water).
Groundwater is defined as that water that is
resident / held by strata beneath the earth’s surface and will
flow to boreholes, wells, springs and other abstraction points.
In strict terms the term refers to water found in the
saturated zone
Sandstone
Water transmitted
between grains
Water transmitted in
bedding planes, cracks
and fissures
Chalk or
limestone
Definition
The importance of groundwater
Primary source of base flow for many (effluent)
rivers and streams.
Source of drinking water supply.
Superior quality to surface water & cheaper to
develop
Provides attenuation of flood peaks. The
management of storm water can be accomplished by
onsite infiltration – encouraging groundwater
recharge.
Informs engineers on choice of foundations and
other structural considerations
Sources of groundwater
Porosit
Specific y
Void ratio - % by
Pressure head hp
Measuring
point in flow
field
Elevation head
Total head h z
Darcy’s law
Current understanding of groundwater movement is based
on the findings made by Darcy in the 1850’s
In 1856, Darcy carried out experiments to examine the factors
governing flow of water in a saturated, homogeneous
medium (sand).
Darcy’s apparatus:
- a steady flow of water forced through at a discharge rate of Q
Findings
v = -K i
where i is the hydraulic gradient = ∆h/∆L or
( h1-h2)/L. Natural hydraulic gradients rarely exceed 0.2 –
0.4%
K is the hydraulic conductivity (L/T) & is a
function of the size & shape of the voids between the
particles making up the porous media as well as the viscosity
of the water.
The negative sign indicates that the direction of flow is in the
direction of decreasing head.
The results of Darcy’s experiments defined the basic
empirical principles of groundwater flow on which current
assessments of groundwater flow rates are premised.
Using the principle of continuity, v = -K i
v = -K (∆h/∆L) or v =-K ( h1-h2/L)
Q = - K A (h1 - h2 /L) or Q = -K A ∆h/∆L
The equations may readily be applied to steady
state one dimensional groundwater flows
Worked example
Given a vertical sand column of length L = 1,20
m and cross-sectional area A = 200 cm2. The
difference between water levels at the inflow and
the outflow reservoirs is h = 120 cm. The
hydraulic conductivity K = 20 m/day.
What is the total discharge Q? (3)
Q = K A (h1 - h2 /L)
Q = [20 x100x 200 x (120/120)]/(24x60x60)
Q= 2000 x 200/ 86400
Q = 4.63cm3/sec more appropriately 4.63x10-6 m3/s
Problem
Q : Given the unconfined aquifer system below, calculate the
flow rate (Q) using Darcy’s law,
The average width is 0.7km
The length is 2.5km
The average thickness is 200m
Hydraulic conductivity is 2m/day
The elevation at point 1 is 350m
The elevation at point 2 is 300m. [5]
SOLUTION:
Q = K A ∆h/∆L
K = 2m/d = 2.315 x 10-5 m/s
L = 2500 m
Thickness of aquifer is 200 m
Average width = 700 m
h2 = 350 m
h1 = 300 m
∆h = 50 m
Area = 700 x 200 = 140 000 m2
Q = 2.315 x 10-5 (140 000)(50/2500)= 6.48 x 10-2 m3/s
Calculate the Darcy flux of water in an aquifer. The
water flows through a sand with permeability of
5m/day. There are 2 piezometers placed 20m
apart and they give the following data:
Piezometer 1 Piezometer 2