Agriculture Reference
In-Depth Information
is higher than the conductivity (or the supply) of the second layer, thus negative
pressure will exist at the bottom of the second layer.
9.4 Design of Surface Drainage System
9.4.1 Estimation of Design Surface Runoff
The runoff to be used in drain design is termed as “design runoff.” Surface runoff to
be generated from an area (design runoff) can be determined from the equations such
as “Rational method” or “SCS method.” Details of these methods are described in
Chapter 6
(
Land and Watershed Management
), this volume. According to Rational
method, peak surface runoff rate (
Q
)is
Q
=
CIA
where
Q
is the runoff rate (m
3
/h),
A
is the area from where runoff generates
(drainage area) (m
2
),
I
is the peak rainfall intensity (m/h), and
C
is the runoff coeffi-
cient (dimensionless). Runoff coefficient is the fraction of rainfall which contributes
to runoff. For agricultural field, its value ranges from 0.5 to 0.7 depending on ini-
tial soil moisture, rainfall intensity and duration, and soil condition/coverage. For
design purpose, the value of
I
can be taken from long-term (20-50 years) peak
rainfall records.
9.4.2 Design Considerations and Layout of Surface
Drainage System
A network of surface drains is needed to remove the excess water (from rainfall
and/or irrigation runoff) from the agricultural field. Drain layout should be based on
the topography, shape of the farm/catchment, direction of natural slope, position of
farm buildings and roads, and position/existence of natural depression, channel, or
river. Drain layout should be done with consideration of minimum length of run and
minimum crossing of roads. This will minimize wastage of land and minimize cost
for culverts. Sometimes, land grading serves the purpose of surface drainage.
Sample typical layout of a farm/catchment is given in Fig.
9.12
9.4.3 Hydraulic Design of Surface Drain
Hydraulic design of surface drain (also termed as “drainage channel”) is simi-
lar to the design of an open irrigation channel, as described in
Chapter 1
(
Wa t e r
Conveyance Loss and Designing Conveyance System
) of this volume. The capacity
of the drainage channel should be based on the design peak surface runoff from the
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