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Introduction to drainage theory and practice

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Title: Introduction to drainage theory and practice


1
Introduction to drainage theory and practice
2
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.
3
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
4
Web site containing list of computer models
for drainage, irrigation, hydrology
http//www.wiz.uni-kassel.de/kww/irrisoft/all/all_
i.html
5
  • 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

6
  • 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)

7
  • 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

8
  • 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

9
  • 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)
  • gt K(tiles)
  • gt K(surface drains)
  • gt undrained

10
  • 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

11
  • Surface
  • beds /furrows
  • ditches

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14
  • open drains - interfere with operations,
    weeds, pests, but easy to monitor

15
  • 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.

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22
Drainage by means of ditches or underground pipes
to control the height of the water
table Introduction
  • examples of the application of Darcys 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.

23
  • 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

24
  • 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

25
  • 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

26
  • 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

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28
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
29
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
30
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
31
In effect, the DF solution imagines the soil to
be divided into a lot of narrow vertical slabs ---
32
  • Hooghoudt method for ditch drainage
  • Like many others, Hooghoudts method over-
  • simplifies 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
  • Darcys law applies
  • impervious layer exists
  • constant flux
  • the soil is a Depuit-Forcheimer (DF) soil

33
q
x
34
  • 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
35
From Darcys Law, horizontal flux at plane is
Total flux through the section
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
36
So
The water table varies from D to (H D) above
the impermeable layer, so integrating
37
which gives
38
Which simplifies to
39
Referring to the diagram, since b H-D, this
can be rewritten as
which is the equation of an ellipse. The above
equation can also be written as
Since b-D H and (bD)/2 is the average depth
that the water can flow through, this can in turn
be written as -
40
is the average depth that the water
where
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 lt10 of the overlying soil
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42
Web sites for Hooghoudts solution and a
calculator for drain spacing -
http//www.geocities.com/CapeCanaveral/Hall/5606/c
alculat/ground1.htm
http//www.sedlab.olemiss.edu/java/Hooghoudt_java.
html
43
Hooghoudts method for tile drainage
44
Hooghoudt envisaged a virtual drain and
modified the ditch equation to
where
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