Environmental Engineering Reference
In-Depth Information
Drainage systems usually consist of small subsurface channels, called
laterals
, arranged in parallel lines connected to larger main channels called
collectors
. Laterals are usually 8-10 cm in diameter. The proper diameter of
the collector depends upon the number of laterals connected to it and the
size of the area from which water must be removed. To provide unre-
strictedwater flow, lines should slope by at least 1%. Collector lines should
be installed at depths beginning at 100 cm below the soil surface and
generally should not go deeper than 150 cm. Smaller laterals should start
at the depth of 60-70 cm, and the depth should gradually increase towards
the collector line. Because a lateral's depthmust never exceed that at which
it meets the collector, the length of the laterals should not exceed 40 m at
the top of the collector and 90 m at the bottom. The distance between
lateral drains usually ranges from 5 to 15 m, depending on soil permea-
bility and the other conditions mentioned above. Lines are typically
designed in a herring-bone pattern or sometimes in a parallel arrangement
(Fig. 4.5). Three critical rules govern design and installation of drainage
systems. The primary rule is that water flows only downhill. Second, the
terrain and other topographic conditions must allow for adequate outlet of
the collected water. Finally, the subsurface drainage must be installed
before installation of the irrigation system (if an underground irrigation
system is planned). The second rule is the most often forgotten, and the
resulting error is the most expensive to correct.
Parallel
Herring-bone
Combination in
target areas
Fig. 4.5. Alternative drainage system layouts. (Drawing by R. Castro.)
