# Hydrology

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```					Hydrology

Groundwater

R. Hudson - VFR Research
Groundwater Topics...
• General principles
– Darcy’s Law, saturated groundwater flow
•   Hydraulic conductivity K
•   measurement of K
•   porosity
•   effects of heterogeneity on flow
•   groundwater flow patterns on a slope

R. Hudson - VFR Research
… relevant to Forest Hydrology
– Unsaturated groundwater flow
• hydraulic properties of unsaturated soil
• drainage and infiltration
– Interflow
• Groundwater in relation to Forest
Hydrology
– How does forest harvesting affect groundwater
– significance of those effects
R. Hudson - VFR Research
• Groundwater flows along an energy
– there are two possible energy gradients that
affect groundwater flow: gravity and fluid
pressure
z = z1
p1                      p2

z = z2              flow under fluid pressure
gravity drainage R. Hudson - VFR Research
Groundwater head - energy for flow
is measured using                    above reference
a piezometer.                        elevation (datum)

= P/rg
where
Y
P = fluid pressure
h=z+Y
r = fluid density
g = acceleration
z                                 due to gravity
datum
R. Hudson - VFR Research
Water table well vs. piezometer

R. Hudson - VFR Research
Darcy’s Law
• Groundwater flow is a function of
– total flow Q has units of volume/time
• typically m3/s or litres/sec
– specific discharge q is flow per unit area, units
of length
q  K                          that flow moves in the direction
R. Hudson - VFR Research
Hydraulic conductivity K
• groundwater flow is driven by the
• K is a measure of the resistance to flow, is
a property of the porous medium and the
fluid
• K has units of m/s or cm/s
krg     • k is permeability, is a property of the
K             medium related to diameter, packing,
        shape and roughness of grains (m2, cm2)
•  is the viscosity of the fluid (kg/m.s)

R. Hudson - VFR Research
Range of values of K
Medium                            K in m/s
Gravel                            10-3 to 1
Sand                              3X10-6 to 10-2
Typical BC Forest soil            10-7 to 10-5
Bog soils                         10-9 to 10-7
Marine clay                       10-12 to 10-9
Basal till                        10-12 to 10-10
Igneous rock, shale               10-13 to 10-10
Sandstone                         10-10 to 10-6

R. Hudson - VFR Research
Porosity
• Porosity is another important property of
porous media that governs water flow
– porosity is a measure of the capacity of the
medium to hold water
– a volume VT of soil of rock is divided up into
the volume of voids Vv and volume of solids Vs
– porosity n = Vv / VT
– void ratio e = Vv / Vs

R. Hudson - VFR Research
Range of values of porosity
Medium                                        Porosity (%)
gravel                                           25-40
sand                                             25-50
silt                                             35-50
clay                                             40-70
sandstone                                         5-30
limestone                                         0-20
shale                                             0-10
fractured basalt                                  5-50
fractured crystalline rock                        0-10
dense crystalline rock                            0-5

R. Hudson - VFR Research
Relations between K and n
• for soil , they are inversely proportional
– for well sorted sediments, the finer grained they
are, the lower K is and the higher n is
– for poorly sorted sediments, smaller grains fill
in voids between larger grains reducing K and n
• for rock, K and n are related to structure
– sedimentary rock, both n and K are less than
that of parent sediments due to mineral
deposition in voids
R. Hudson - VFR Research
Relations between K and n...
– metamorphic and igneous rock have very low
primary porosity, but K and secondary porosity
are related to fracture spacing
• porosity affects velocity of flow:
– the lower the porosity, the greater the flow
velocity:
v = q/n
– flow velocity = specific discharge/porosity
R. Hudson - VFR Research
Heterogeneity
• Geologic formations are generally
not homogeneous
– in BC, most forested terrain is characterized by
relatively thin (1-2 metre) coarse grained soils
over basal till or igneous/metamorphic bedrock
– the contact between soil and basal layer
involves a sharp discontinuity in K such that the
till or bedrock interface forms an impermeable
boundary
R. Hudson - VFR Research
Flow in layered heterogeneity
Flow lines are perpendicular to equipotentials

clay

Flow will tend to go along the zone of higher K, and across
the zone of lower K. Thus preferential flow occurs in high K
zones.
R. Hudson - VFR Research
Flow in layered heterogeneity...
– the grain size distribution of soil is generally
not uniform, so there are variations in hydraulic
conductivity
• zones of relatively high K in soil become preferred
flow paths - they carry more flow than zones of
lower K
• the distribution of K zones can be random, or K can
decrease with depth in soil due to increasing clay
content - the latter situation will result in more rapid
groundwater flow as the water table rises
R. Hudson - VFR Research
Effects of slope steepness on flow
• Slope gradients affect both direction and
rate of groundwater flow
– flow perpendicular to equipotentials
– approx. lateral for steep slopes

dh/dl               dh/dl

R. Hudson - VFR Research
Groundwater flow on a slope

R. Hudson - VFR Research
Groundwater Recharge

Groundwater flow
follows hydraulic
decreases with depth,
thus there is a downward
component to the
groundwater flow. This
is groundwater recharge,
and in the abcence of
water input, the water
table will fall.
Groundwater discharge

At riparian sites, ground-
water discharge often occurs.
with depth, resulting in an
upward component to ground-
water flow. In the example
shown, under high flow condi-
tions the water table rises
to the surface near the stream,
groundwater discharges out
of the soil and enters the
stream by overland flow.
Later that year...

...at the same site, under low
flow conditions, the water table
and the stream stage have
dropped. Groundwater is still
discharging to the stream
channel, but not at the soil
independent of depth within the
soil. There is no longer an
upward component to ground-
water flow. Discharge to the
channel is essentially horizontal.
Occurrence of groundwater
• Saturated vs. unsaturated
– Define q as water content of soil
– Saturated: all the void spaces are filled with
water: qs = n
– Unsaturated: void spaces are only partially
filled with water: q < n
– K is reduced because cross sectional area for
flow is less than saturated cross section: K is
now a function of moisture content
R. Hudson - VFR Research
Saturated vs. unsaturated flow
below water table                  above water table

Saturated: void spaces               Unsaturated: voids partially
filled with water: q = n,            occupied by air: q < n,Y < 0.
Y>0                                  K is reduced; K = K(Y) or K(q)
R. Hudson - VFR Research
Soil drainage and infiltration
• If pressure head increases with depth,
then why does soil drain?
– recall, there are two components of head:
– soil drains under gravity when elevation
– drainage will continue until equilibrium is
reached
– equilibrium may never occur
R. Hudson - VFR Research
Infiltration
• Initially, moisture content at the surface is
low, hence K is low
• When water is supplied to a dry soil,
initially the water is absorbed, raising the
moisture content and hence increasing Y
water down towards the water table.
• water moves down under large head
gradient at the wetting front , overcoming
the fact that K is low for dry soil
R. Hudson - VFR Research
Infiltration rates
• Over time, infiltration rate will tend
towards the saturated hydraulic
conductivity of the soil
• Initial infiltration rates for dry soil can be
up to 5 times Ks for very dry soil
• typical infiltration rates for forest soil are
in the range of 50 to 300 mm/hr depending
on the soil and its moisture content

R. Hudson - VFR Research
Macropore flow
• It is generally accepted among forest
hydrologists (if not hydrogeologists) that
macropore flow is a significant component
of runoff from forested catchments
– Darcy’s Law does not describe macropore flow
– difficulty is in defining a representative
dimension for a macropore
– macropores can form a large interconnected
network
R. Hudson - VFR Research
Macropore flow (interflow)
– they are formed from the rotting out of dead
tree roots, aminal burrows, cracks in soil
resulting from blocky structure, etc.
• how to define a representative dimension for such a
feature?
– subsurface flow through macropore networks is
much faster than soil matrix flow
– often called interflow
– we still do not know how to describe it
mathematically
R. Hudson - VFR Research
Forest harvesting and groundwater
• Forest harvesting alters groundwater
levels (thus, groundwater flow) by altering
the water balance
– increase in water available for infiltration due
to decreased interception, increased snowmelt
– decrease in extraction of water from the soil
due to decreased evapotranspiration
• Related activities can also alter soil
structure

R. Hudson - VFR Research
Effects of ground skidding and roads
– ground based yarding can result in soil
compaction, thereby reducing infiltration
capacities
• ground skidding
– these effects would tend to result in increased
runoff, hence reduced infiltration
• Road cuts on steep terrain can
interrupt subsurface flows
R. Hudson - VFR Research
Effect of road cut on groundwater flow

Before: ground-                     After: potentially
water flow on                       increased flow, inter-
treed slope                         ception by road cut,
conversion to
ditch flow

R. Hudson - VFR Research
– intercepted flows can either be routed to the
stream channel thereby altering streamflow
hydrograph, or can be routed back onto slope
below the road by way of culverts or cross
ditches
– in many cases, poorly placed culverts and
inadequate culvert density have resulted in
concentration of ditch flows onto unstable
slopes, resulting in landslides
R. Hudson - VFR Research
Improper culvert placement
A plan view schematic of a road cut showing water flow pattern

cut bank

Too few culverts and poor placement results in flow disruption
Landslides and pore pressure
– Increased pore pressures at the failure plane of
potential instability results in reduced frictional
contact between soil grains
– this results in a reduction in the forces that keep
the soil on the hillside.

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