Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 368

the proposed envelope material enable the selection of an envelope material, which
best suits the soil. In designing, we have to compare the two particle size distribu-
tion curves and decide by the set of criteria (described earlier) whether the envelope
materiel is satisfactory or not.
In determining maximum size of the perforations or joint openings of the
drainage pipe, the 85% size of the envelope material is used from the particle size
distribution curve.
9.7.6 Drain Excavation and Envelope Placement
Excavating and trenching machines, driven by steam engines, were introduced in
1890. Trenchers dig a trench at the required depth and grade and place the drain
pipe at the bottom of the trench. Several types of trenchers are produced in various
sizes and a wide range of capacities. They can install pipes to a depth of about 3 m
in trenches up to 0.50-0.60 m in width. Trenchers have been developed in different
modified models so that they can also be used to install drains in stony soils, in
orchards, or in soils with high water tables.
Water enters the drain through the sides and bottom of the drain. The hydraulic
gradient that develops at the drain openings is often high enough to cause piping
of soil material at the openings. For this reason an adequate amount of envelope
material is needed under the drain pipe as well as on top.
Where drains are laid manually, a layer of envelope material is placed and lev-
eled to the design grade in the bottom of the trench (before the drain is laid). The
drain pipe is then put into place and covered with envelope material to the required
depth. The trench is then backfield with soil. Trenching machines can be fitted with
two-hoppers for placing envelope material under and over a drain on a continuous
basis.
Care should be taken to protect the drain-envelope system immediately following
installation. No heave loads, mechanical or hydraulic, should be imposed until the
loose back-fill material in the trench is consolidated naturally.
9.8 Models in Drainage Design and Management
Nowadays, models are useful and easy-to-way tool to design surface and subsur-
face drains, and management of drainage system. Some of the existing models are
described below:
9.8.1 DRAINMOD
DRAINMOD is a deterministic hydrologic model developed to simulate a soil-
water regime of surface and subsurface water management systems (Skaggs, 1978).
It predicts surface runoff, infiltration, evapotranspiration, subsurface drainage, and