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52
Geotechnical News • December 2012
www.geotechnicalnews.com
for Hydrogeological Site Assessment
(HSA) [3] for technical assistance of
the Qualified Person (QP), P.Eng. or
P.Geo.
The PTTW’s objective is to imple-
ment the MOE’s water management
policy for fair sharing, conservation
and sustainable use of Ontario water
resources without water allocation but
preventing unacceptable interferences.
The PTTW hydrogeological study and
application are based on six principles
listed on Table 1. Included in these
six principles is incorporation of the
precautionary principles summarized
on Table 2.
Ground water flow principles
and equations
The water in soils (porous media)
exists in three forms of adsorbed
(hydroscopic or pellicular), capil-
lary (or matric) and gravitational (or
free). The free or gravitational water
in soils is usually referred to as the
ground water. The ground water bod-
ies can exist in perched condition in
sand seams/lenses within silty/clayey
soils and in regional condition within
aquifers (in sand and gravel, confined
or unconfined) that may be separated
by aquitards (clayey soils).
The ground water flow varies with
space, time, boundary conditions,
medium (soil or rock) properties and
behaviour as well as the fluid (perme-
ant) temperature, density and viscos-
ity. The ground water flow types
in relation to time can be steady or
transient; in relation to space uniform
or varied and in relation to the driving
force laminar or turbulent. The driving
force or free energy for ground water
flow can be expressed in terms of
“potential” (Ф: free energy per unit
mass of fluid) or “total head” (h: free
energy per unit weight of fluid), or
“pressure” (p = free energy per unit
volume of fluid).
The laminar ground water flow in
saturated soils is usually analyzed by
Darcy’s law which is based on linear
relationship between the rate of flow
and the driving forces. It is important
to note that Darcy’s law is valid as
long as the Reynold’s number based
on an average grain size does not
exceed some value between 1 and 10
which is indicative of linear laminar
flow. The ground water flow in frac-
tured rock mass is usually nonlinear.
The Reynold’s number for flow
through porous media is:
Where
r
and
m
are the fluid density
and viscosity,
v
is kinematic viscos-
ity, v is specific discharge, and d is a
representative length dimension for
the porous medium, variously taken
as a mean pore dimension, a mean
grain diameter, or some function of the
square foot of the permeability K.
The most commonly encountered Dar-
cian ground water flow equation is
Q =
KiA
(2)
Where Q is the flow rate L
3
/T); i is the
hydraulic gradient (dimensionless); A
is the total cross-sectional area of flow
(L
2
); and K is the constant of propor-
tionality (L/T), which is termed the
hydraulic conductivity; or
q =
Ki
(3)
v =
Ki/n
(4)
Where q is the specific discharge or
flux and v is the average linear veloc-
ity of the flow in a porous medium
with a volumetric porosity of n.
The “specific yield” (Y
s
) of a porous
medium is the drainable water and the
“specific retention” (R
s
) is the undrain-
able water. The porosity of a saturated
soil n, which is a function of void ratio
Table 1. Permit-To-Take Water Principles
1
Ecological approach
2
Prevention and resolution of unacceptable interferences
3
Adaptive management for better response to the evolving
environmental conditions
4
Addressing cumulative impacts of water takings
5
Incorporation of precautionary principles (Table 2); and
6
Promotion of the public and local agency involvement
Table 2. Precautionary Principles
1
Demonstrate that all aspects of the project have been examined and planned
in a careful and precautionary manner in order to ensure that they do not
cause serious or irreversible damage to the environment and/or the health of
current or future human generations
2
Outline and justify the assumptions made about the effects of all aspects of
the project and the approaches to minimize these effects
3
Evaluate and compare alternative means of carrying out the Project in
light of risk avoidance, adaptive management capacity and preparation for
surprise
4
Demonstrate that in designing and operating the project, priority has been
and will be given to strategies that avoid the creation of adverse effects
5
Provide that contingency plans explicitly address worst-case scenarios and
include risk assessments and evaluations of the degree of uncertainty
6
Identify and propose follow-up and monitoring activities, particularly in
areas where scientific uncertainty exists in the prediction of effects; and
7
Present public views on the acceptability of all of the above
Re
=
pvd
u or vd
v
(1)