3. CLASSIFICATION OF DRAINS
DRAINS
ACCORDING TO
CONSTRUCTION
NATURAL ARTIFICIAL
ACCORDING
TO FUNCTION
OPEN
SURFACE SEEPAGE
SURFACE-
CUM-SEEPAGE
CLOSED/SUB-
SURFACE
TILE
DRAINS
MOLE
DRAINS
VERTICAL
4. 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
6. B. ACCORDING TO FUNCTION
a) Open drains
b) Closed/ Sub-surface drains
c) Vertical drains
7. 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
13. 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
14. 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
15.
16. Mole drains:
Cylindrical channels
below ground surface
Formed at desired depth
with a grade
No lining material
Clay soils are suitable
Constructed using mole ploughs
18. 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
19. d) Bio drainage
Drainage effect produced
by certain plants
Eg: Eucalyptus
Caused by withdrawal of
high rate of water
20. 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-
Forchcheimer
assumptions
21. 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’
22. 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
23. S- Drain spacing
D-Distance from the impermeable
layer to the maximum height of
water between the drains
K- Hydraulic conductivity
R- Replenishment rate
24. 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
25. 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
26. 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
27. 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
28. Equivalent depth, d’ = S/8F
where,
S- Spacing between drains
F-Equivalence factor
In original Hooghout’s equation, d is replaced
by d’
29. 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
30. 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
31. 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
32. Horizontal head,
hh = L2q / 8KhDh
where,
Kh – Horizontal Hydraulic
Conductivity
Dh - Thickness of layer through
which horizontal flow is considered
L - Spacing
33. Radial Head:-
hr = (qL / πKr) ln(Dr /u)
where,
Kr – Radial Hydraulic Conductivity
a - Geometric Factor
u- Wetted Perimeter of the drain
Dr – Thickness of the layer in which
radial flow is considered
34. i.e, Total Head,
h = [qDv / Kv]+[L2q / 8KhDh]+[(qL / πKr) ln(aDr /u)]
This is the Earnst equation in complete form
35. UNSTEADY STATE DRAINAGE EQUATIONS
a) 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
36. Glover-Dumn equation is used to
describe a falling water table after its
sudden rise due to an instantaneous
recharge
37. Drain spacing = π (Kdt /µ)½ (ln 1.16(h0 / ht))-½
where,
d-Equivalent depth of soil layer below the
drain level
K- Hydraulic Conductivity
L- Drain spacing
t- Time after instantaneous rise of water table
µ- Drainage porosity
h0 - Initial height of water table
ht – Height of water table at t=t