Geology Reference
In-Depth Information
first proposed by Lumb (1962), is still used
for estimating the likely depth of ground that might be affected by a
rainstorm. Given a rainstorm of a certain duration, and knowledge of
the original saturation of the ground and porosity, the thickness of the
surface zone of saturation can be estimated. It is assumed that the
saturated layer will then descend until it meets the groundwater table,
resulting in a rise in groundwater table, equal to the thickness of the
wetting band. This provides a tool for assessing the design ground-
water condition, albeit rather crude. Some of the geological conditions
that will conspire to make such simple approaches unrealistic are
illustrated in
Figure 3.56.
More sophisticated attempts have been made to model in
The wetting band theory,
ltration
and pressure diffusion processes in pressure head response and the
triggering of landslides, mathematically (see Iverson, 2000). Such
methods are useful
in visualising mechanisms but again rely on
rock
exposure
water ingress into
fractures, root holes
etc.
100%
runoff
recharge down fault
zone (days / weeks)
Flow
overland
shallow saturation
and erosion: small
failures during
rainstorm
dyke
local
perching
clay-infilled discontinuity
weathered dyke
acting as aquiclude
lowering of water table by
cutting induces high
hydraulic gradient
fault
Upward flow from
bedrock
cut
seepage pressures
induce pipingâflow
through system (hours
/ days)
=
spring or seepage point
=
flow path
=
water table (variable)
Figure 3.56 Schematic model of water runoff, in
ow and through
ow in weathered pro
le. Note the
importance of minor geological features such as dykes, clay-in
lled joints or simple permeability
contrasts in the pro
le and the development of natural pipes. In rock, water
flowwill be controlled by
joints and speci
c channels along those joints. There is often a zone of more highly fractured rock just
below rockhead with preferential
flow and sometimes upward
flow into overlying soil pro
le
(Hencher, 2010).
