Page 231 - Biosystems Engineering

Design of Sustainable W ater Management Systems 209

However, steeper slopes and increased velocities of water will cause
soil erosion. Therefore, the maximum slope should not exceed
0.6 percent in agricultural soils to prevent soil erosion. When the land
surface profile is generally flat, a minimum slope needs to be main-
tained to allow water flow to take place. As the ditch slope decreases,
the velocity of water decreases, allowing suspended sediments to
settle. This will cause frequent silting up of ditches and reduction in
water conveyance capacity, which imposes additional ditch cleanup
work. To avoid this problem, a minimum slope of 0.1 percent is rec-
ommended. Depending on the land surface profile, the slope might
change along the length of the ditch. The slope change should be
gradual to avoid the scouring action of the change in velocity of water
flowing in the drainage ditch.
Sometimes, it would appear that a ditch design would not be pos-
sible without violating the above criteria. If the land profile is too
steep, the ditches could be constructed in sections, at the required
maximum slope, with the different elevations connected by drop
structures consisting of rocks or wood to break the energy of the fall-
ing water. The ease of machinery operation should be taken into
account when such designs are proposed.
Subsurface Drainage
Unlike surface drainage ditches, subsurface drainage tiles are buried
below the ground surface and do not interfere with machinery opera-
tion. The tiles are relatively maintenance free. The word tiles originates
from the use of 30-cm-long clay tiles that were tapered to facilitate
insertion in series during installation. At present, corrugated plastic
pipes are more popular because of their ease of installation, low cost,
and durability. However, the phrase “tile drainage” is still used to
refer to such systems as well. Corrugated plastic pipes are installed at
depths ranging from 1.0 to 1.3 m below the ground surface. Specially
designed drainage installation machinery is used for field installation
(Fouss and Fausey 2007). The depth of installation is controlled by a
laser plane set at the desired slope. A laser beam detector attached to
the installation arm will move up and down to compensate for the
undulating land surface while maintaining a uniform slope along the
length of the subsurface drain. All subsurface drains are connected to
a large collector drain, which diverts the water to the drainage outlet.
Biosystems engineers have developed drainage design methods
and computer models to help determine optimum spacing of depth
of installation for a given soil (Skaggs 2007).

6.7 Salinity Control
The consequences of salinization of agricultural land have been
recorded in history (Hillel 2000). Wherever irrigation and drainage
is practiced, careful precautions should be taken to monitor salt