Drainage types
If saline groundwater occurs at a depth that is shallower than the critical one, its natural outflow is insufficient and the package of agrotechnical and irrigation & drainage measures aimed at preventing salinization and waterlogging of soils does not provide required fall in groundwater level, then its outflow is increased artificially by drainage excavation.
The drainage in irrigation systems implies the complex of hydraulic facilities (collectors, wells, pumping stations) by means of which they collect and divert groundwater from irrigated lands.
On irrigated lands, drainage is used for lowering of groundwater level, for desalination of saline soils, and prevention of their repeated salinization.
Drainage in irrigation systems is broken down into horizontal, vertical, and combined types.
Horizontal drainage types are distinguished by: design – open and closed; purpose and location – systematic, random, and linear; operating time – permanent and temporary; grooving depth of the water-bearing stratum – complete and incomplete.
Open drainage consists of deep channels in a groove, and closed drainage consists of pipes laid in the ground and receiving groundwater from soil and delivering to the collectors that diver water to catch-waters (sumps).
Systematic drainage means the system of horizontal drains located more or less evenly over the drained area. Primary drains connected by a single collector are arranged as a rule parallel to each other.
Random drainage implies the system of drains located relatively rarely in lowered topographic features.
Linear drainage (head or river-side or sectionalized) is arranged in a drained territory or outside it along the recharge front of the groundwater of the irrigated area. It catches external groundwater effluent to the irrigated area and diverts it outside the irrigated area. Head drainage is constructed along the upper boundary of the area; riverside drainage, along the river or reservoir bank.
Permanent drainage works on a continuous basis, while temporary one operates as a rule only during substantial soil leaching operations (for 1-1.5 month).
Complete drains cut through the full thickness of the water-bearing stratum, while incomplete (suspended) ones only partly, i.e. not to the full depth.
Vertical drains are wells that are cut down to unconfined (nonartesian) groundwater or drill in pressure (artesian) groundwater; water is pumped from wells.
Combined drainage means the combination of horizontal drains and vertical relief wells working in water discharge (wells-boosters) without pumps.
A relevant drainage type is selected depending on hydrogeological, topographic, soil, and economic conditions of a given site based on technical and economic assessment.
Horizontal drainage is generally constructed at gravity (free-flow) recharge of groundwater, and seldom at pressured recharge.
Vertical drainage is constructed at pressured recharge and availability of thick coarse-grained sand and pebbly water-bearing strata with confined water and transmissibility of the water-bearing strata of more than 100 m2/day. Gravity-flow is to form at drainage operation.
Combined drainage is built when a well-permeable confined-water bedding rock of a thickness of at most 10-15 m underlays and feeds the upper low-permeable layer of a thickness of up to 1.5 m.
Source: Mse-Online.Ru
