Transcript No Slide Title

Introduction to drainage theory and practice
Introduction
Problem is mainly the reduction in the respiration
rate caused by reduced oxygen supply.
Groundwater drainage refers to the artificial
removal of water by lowering the water table.
References
Hillel, D. 1998. Environmental soil physics. Academic
Luthin, J.N. 1957. Drainage of agricultural lands. Amer.
Soc. Agron.
Marshall, T.J.; Holmes, J.W. 1979. Soil physics. CUP
Ochs, W.J.; Bishay, G.B. 1992. Drainage guidelines.
World Bank Technical Paper no. 195
Rycroft, D.; Amer, M.H. 1995. Prospects for drainage of
clay soils. FAO Irrigation & Drainage Paper 51
US Dept. of Interior, Bureau of Reclamation. 1978.
Drainage manual. USDI
Web site containing list of computer models for
drainage, irrigation, hydrology
http://www.wiz.uni-kassel.de/kww/irrisoft/all/all_i.html
Water-logged soils
 gas exchange only near the surface.
 within the profile, O2 may be absent & CO2 may
accumulate
 reduction  toxic concentrations of ferrous,
sulphide, and manganous ions
 OM  methane
 nitrification prevented
 plant, especially fungal/root diseases are more
prevalent
 plants may suffer from moisture stress if grown
in waterlogged soils and water table drops
 soils more susceptible to compaction by animals &
traffic
 clogging of machinery
 greater heat capacity so more difficult to warm
up in spring
 heat loss by evaporation greater
 germination and early growth retarded
 plant sensitivity to restricted drainage affected
by temperature since rise in temperature is
accompanied by decline in O2 solubility
high evaporation in a warm climate from
waterlogged soils leads to concentration of salts
at the surface - can only be removed by lowering
the water table through drainage (or growing salt
tolerant crops - only a temporary solution);
Causes of waterlogging
 shallow ground water - e.g. riparian zones
 “perched” water table - clay parent material or
impervious rock
 hydraulic properties of the soil
 over-irrigation
Clay soils
 vertisols - shrinking and swelling
 infiltration and structure - bypass flow through
shrinkage cracks, root channels, worm holes and
horizontally along ped faces;
 infiltration approximately linear :
Vi = Vc + Ist
Vc is the crack volume (m3/m2)
Vi is infiltrated volume
 hydraulic conductivity typically 0.1 to 1 mm hr-1
 non-Darcy flow
 if K sat were used without modifications in usual
drainage equations, spacings would be too close
and so uneconomic - influence of sub-soiling
 drainage changes the actual values K
(because of structural changes):
 Changes in K follow the sequence:
K(tiles + surface)
> K(tiles)
> K(surface drains)
> undrained
 various models exist which relate clay content,
crack dimensions and bypass flow to
conductivity (e.g. LEACHW (Wangenet &
Hutson, 1989; Booltink, 1993))- Booltink modified
model to calculate bypass flow based on physical
and morphological factors
Surface
 beds /furrows
 ditches
•
open drains - interfere with operations,
weeds, pests, but easy to monitor
Subsurface involving modification to structure
 Mole drainage
 Gravel tunnel
 Subsoiling
These methods are suitable only for soils with
high clay content – if clay content is too low, the
unlined drainage lines would collapse.
Design is very much done by rule of thumb and
local experience.
Little in the way of determinative design.
Drainage by means of ditches or underground
pipes to control the height of the water table
Introduction
• examples of the application of Darcy’s law to
the development of models for drainage
• ditches
• underground pipes
• control height of water table or remove
excess water from soil with low hydraulic
conductivities.
• drainage improves hydraulic conductivity and
vice versa.
Flow rate to drains depends on:
 hydraulic conductivity of the soil:
 anisotropy;
 texture;
 soil profile
 configuration of water table : localised (alluvial),
 regional,
 perched,
 artesian or sub-artesian
 depth of drain;
 outlet condition - no flow if submerged
 slope and diameter of subsurface drains
(or cross-sectional area of ditches) must be
sufficient to lead water away;
 ochre deposits due to reduced iron and
manganese or salt deposits such as gypsum
reduce the diameter - need oversizing as well
as flushing out;
 spacing
 type of drains - clay tiles or slotted plastic
 use of envelope - gravel reduces clogging and
increases seepage surface;
 rate at which excess water reaches
groundwater - often taken as difference
between rainfall and evapotranspiration but
there will also be a natural drainage rate which
occurs without artificial drainage
Steady flow v. transient flow solutions
•
steady flow solutions assume constant
infiltration rate (even when it is not raining!)
•
transient flow solutions try to allow for the
fluctuation in the water table due to
intermittent rainfall or irrigation - much more
difficult to solve mathematically
Adjustment of K, x and y values to allow for
anisotropy
Conductivity,. especially in clay soils, will usually be
different in the vertical & horizontal directions the conductivity in this case is said to be isotropic.
A simple method is used to derive a single value for
use in drainage calculations:
K  KxKy
/
As well as using this composite value for K,
modified values for depth (y) are used:
x/ = x
(i.e. x is unchanged)
y/ = yA
in which A is the anisotropic ratio, Kx/Ky
Depuit-Forcheimer (DF) soils
A DF soil assumes:
(a) for small inclinations of a water table in a
gravity flow system, the streamlines can be taken
as horizontal, i.e. the water has no vertical
component of flow
(b) the velocities along the streamlines are
proportional to the slope of the water table
The assumption is widely used and often produces
solutions which are comparable to more rigorous
treatments
In effect, the DF solution imagines the soil to be
divided into a lot of narrow vertical slabs ---
Hooghoudt method for ditch drainage
Like many others, Hooghoudt’s method oversimplifies the field situation but even so gives
useful solutions.
The method assumes:
 isotropic and constant K
 parallel and equally spaced drains
 hydraulic gradient is equal to the slope of
the water table
 Darcy’s law applies
 impervious layer exists
 constant flux
 the soil is a Depuit-Forcheimer (DF) soil
q
x
• DF solution assumes that there are no streamlines
below the bottom of the impermeable layer.
• solution seems to be a good enough approximation
for water table surface (though not for the route
the water takes to the drain)
Consider length of drain (i.e. into the paper) of 1
metre.
Assume water passing horizontally through an
arbitrary plane is product of downward flux, q
(normally mm/day but here we use metres/sec so
that units are consistent) and the distance between
the plane and the mid-point between the drains
Q = -q (S/2 - x) x 1 metre
From Darcy’s Law, horizontal flux at plane is:
Flux per unit area = K
dh
dx
Total flux through the section:
Q = h x 1 metrex K dh
dx
The hydraulic potential is taken as the height of
water table above the impervious layer, i.e. a DF
soil.
Equating the two expressions for Q:
q(  x) = Kh
S
2
dh
dx
So:
1
2
qSdx  qxdx  Khdh
The water table varies from D to (H + D) above the
impermeable layer, so integrating:

1
2
qSdx   qxdx 
HD
Khdh

D
which gives:
 qSx     
1
2
qx
2
2
S
0
2
Kh
2
2
HD
D
Which simplifies to:
qS2
4
qS2
4

qS2
8

K
2
H  D  D 
2
 K(H  2HD)
2
2D  H
S  4KH
q
2
q
8 KHD
S2

4 KH 2
S2
2
Referring to the diagram, since b = H-D,
this can be rewritten as
S 
2
4K
q
(b  D )
2
2
which is the equation of an ellipse.
The above equation can also be written as
S 
2
4K
q
(b  D)(b  D)
Since b-D = H and (b+D)/2 is the average depth
that the water can flow through, this can in turn
be written as :-
S 8
2
KH
q
T
where T is the average depth that the water
flows through
This idea has been extended to the situation
where the bottom of the ditch does not rest on
an impermeable layer though it pushes the theory
even further beyond its proper limits - but still
seems to work
Note in practice an impermeable layer is one that
has a K of say less than <10% of the overlying soil
Web sites for Hooghoudt’s solution and a
calculator for drain spacing :http://www.geocities.com/CapeCanaveral/Hall/5606/calculat/ground1.htm
http://www.sedlab.olemiss.edu/java/Hooghoudt_java.html
Hooghoudt’s method for tile drainage
Hooghoudt envisaged a “virtual” drain and
modified the ditch equation to :
q
where
1
d

8 KHd
S2


4 KH 2
S2
8 ( S D 2 )
S
8 DS
2
 ln
1

 
D
r0 2