CLASSIFICATION OF DRAINSDRAINS
ACCORDING TO CONSTRUCTION
NATURAL ARTIFICIAL
ACCORDING TO FUNCTION
OPENSURFACE
SEEPAGE
SURFACE-CUM-SEEPAGE
CLOSED/
SUB-SURFACE
TILE DRAINS
MOLE DRAINS
VERTICAL
A. ACCORDING TO CONSTRUCTION
a) Natural drains: Lowest valley line between 2
ridges Naturally occurring Eg: Drainage lines, Nallahs etc.
b) Artificial drains: Man made structures Constructed along drainage line
a) Open drains
1-1.5m deep Caters the storm water Lowers water table Reduces sloughing of sides Removes large quantities of
surface as well as sub-surface water
b) Subsurface drains/ Closed drains
Drains laid deep in the ground and then covered
Used to lower the capillary surface and water table below ground
Provides aeration in the root zone Two types: Tile drains and Mole
drains
Tile drains:
Most efficient and permanent drains
Short length pipes called tiles are
laid with a grade 1-1.5m below ground surface Tiles:- Concrete or Burnt clay Pipes are held end to end without
joining
Mole drains:
Cylindrical channels
below ground surfaceFormed at desired depth
with a gradeNo lining materialClay soils are suitableConstructed using mole ploughs
c) Vertical Drains
Water table is controlled by pumping from a network of wells
Number of pumping points over a small area provides lasting effect of pumping in ground water decline
d) Bio drainage
Drainage effect produced by certain plants
Eg: Eucalyptus Caused by
withdrawal of high rate of water
STEADY STATE DRAINAGE EQUATIONS
a) Hooghout’s equation Assumptions:-
1. Soil profile is homogeneous
2. dy/dx=i
3.Darcy’s law is valid
Dupuit-Forchcheime
r assumptions
4. Drains are spaced evenly
5. An impermeable layer underlain the drain
6. Origin of co-ordinates is on the
impermeable layer below the
centre of one drain
7. Rate of replenishment of water table by irrigation rainfall is ‘R’
Hooghout’s equation for drain spacing:-
S2 = 4K/R [H2-2hd+2Hd-h2]where,
d- Depth to the impermeable layer from the
drain bottom
h- Height of water in the drain
H-Height of water in midway between 2 drains
S- Drain spacing
D-Distance from the impermeable layer to the maximum height of water between the drains
K- Hydraulic conductivity
R- Replenishment rate
When drain is considered as empty:-
S2 = 4KH/R [H+2d] {h= 0}
This equation is similar to ellipse equation – Luthin(1973)
Luthin has transformed the origin of coordinate system to the midpoint between the drains
Ellipse equation:-
y2/ (RS2/ 4K) + x2/ (S2/4) = 1
where,
S/2 is the semi-major axis and
S/2 √(R/K) is the semi-minor axis
Hooghout’s equivalent depth:-
Hooghout’s equation considers totally horizontal movement of water towards the drains
But, when ‘d’ increases beyond a certain level, horizontal flow transforms into vertical flow
This limits the application of Hooghout’s equation
Equivalent depth: Depth below the drain level which can transform the radial flow component into an equivalent horizontal flow component
Equivalent depth, d’ = S/8F
where,
S- Spacing between drains
F-Equivalence factor In original Hooghout’s equation,
d is replaced by d’
b) Earnst equation
Applicable to 2-layered soil Advantage over Hooghout’s
equation:The interspace between 2 drains can be either above or below the drain
Earnst equation:-
Total available head, h = hv + hh + hr
where, hv = Head due to
vertical flow hh = Head due to
vertical flow hr = Head due to radial
flow
Vertical head,
hv = qDv / Kv where,
q - Discharge per unit area
Dv -Thickness of the layer through which vertical flow is considered
Kv - Vertical Hydraulic Conductivity
Horizontal head,
hh = L2q / 8KhDh
where,
Kh – Horizontal Hydraulic Conductivity
Dh - Thickness of layer through
which horizontal flow is considered
L - Spacing
Radial Head:-
hr = (qL / πKr) ln(Dr /u)
where, Kr – Radial Hydraulic
Conductivity a - Geometric Factoru- Wetted Perimeter of
the drainDr – Thickness of the layer
in which radial flow is considered
i.e, Total Head,
h = [qDv / Kv]+[L2q / 8KhDh]+[(qL / πKr) ln(aDr /u)]
This is the Earnst equation in complete form
UNSTEADY STATE DRAINAGE EQUATIONSa) Glover Dumn equation
Assumptions: Flow pattern is unsteady Darcy’s law is applicable All velocity vectors are horizontal , v = -K
dy/dx The vertical column of water bounded
above by the phreatic surface and below by an impermeable layer
Glover-Dumn equation is used to describe a falling water table after its sudden rise due to an instantaneous recharge
Drain spacing = π (Kdt /µ)½ (ln 1.16(h0 / ht))-½
where,
d-Equivalent depth of soil layer below the drain levelK- Hydraulic ConductivityL- Drain spacingt- Time after instantaneous rise of water table µ- Drainage porosityh0 - Initial height of water table ht – Height of water table at t=t