Land drainage

From WaterWiki.net

Jump to: navigation, search

Terms & Synonyms

Land drainage

Official WHO Definition

Other Definitions

Land drainage is a set of measures aiming at controlling a high water table and waterlogging in the land. It may concern built-up areas or agricultural lands.

Contents

Interpretations and Explanations

Classification

Land drainage can be distinguished in surface and subsurface drainage.

Classification
Classification

Surface drainage deals in principle with water-logging on the land surface and subsurface drainage deals in principle with water-logging in the soil, i.e. the water table is too shallow for the purpose the land is used. Interaction between the two kinds of drainage may occur: surface drainage may reduce the need of subsurface drainage and vice versa. To accomplish the drainage function one needs drainage systems, which can be discerned into field and main drainage systems.

Field drainage systems do the actual work of water-logging control while main drainage systems are designed to collect the water from the field drainage systems and transport it to the outlet. The outlet may work by gravity or it may be equipped with a pumping station. Some outlets are designed too work part time by gravity and part time by pumping. Pumping stations may also be found in the interior along main drains. When subsurface drainage is accomplished by wells, these also need a water lifting device.

Field drainage systems can be regular or checked systems.
Regular systems work immediately when waterlogging threatens to occur as a result of a considerable recharge, e.g. rainfall, irrigation.
Checked systems are systems designed to operate only occasionally.

Checked/controlled drainage system
Checked/controlled drainage system

Examples of application of checked systems can be found in rice growing areas where drainage is normally not required except during certain periods, e.g. during harvest, during exceptionally intensive rainstorms, or to allow the land to dry between crops.
      Another example is the closure of drains or the reduction of the drainage capacity during dry spells to conserve water and reduce the losses. Subsurface drainage by wells and pumps constitute automatically checked systems because there is always the possibility to switch off the pumps.


Surface field drainage systems are usually made by land forming/shaping. The regular systems consist of graded land or bedded land using shallow open drains to collect the water. Checked systems consist of bunded basins (in flat land) or bunded terraces (in sloping land). Bunded terraces in sloping land are more often used for erosion control and water conservation than for drainage, yet these sloping lands may occasionally need surface drainage.

Regular subsurface field drainage can be done using subsurface drains (mole drains, ditches, tile drains, pipe drains) or subsoiling (deep plowing). Mole drains and deep plowing can only be practiced in clay soils.
      It is a common misunderstanding that a ditch drain invariably would be a surface drain. Although shallow ditches are used in surface drainage to collect and transport water and may be called surface drains, the deeper ditches can have an important function in the control of the water table and thus are subsurface drains by the definition given above. In fact, subsurface drainage by ditches is more effective than by pipe drains, but sometimes the open ditches may be a hindrance and the maintenance may be a nuisance.

Checked subsurface drainage systems consist of ditches and pipe drains equipped with check structures or pumped outlets as well as of pumped wells.

If the water quality permits, drainage water can be reused for irrigation. Especially deep wells are useful for this purpose. Thus the water-table may be lowered by pumping during dry spells while an extra underground storage facility is created to be filled up during wet spells. During wet spells it may be desirable to halt or reduce the pumping from wells as no immediate irrigation water is required and the extra storage capacity created in the underground during dry spells now comes in handy.

In irrigated lands of the (semi)arid regions, the drainage water also carries salts and helps to maintain a favorable salt balance in the soil. All irrigation water, however "sweet", bring salts that remain behind in the soil after evaporation. For example, assuming irrigation water with a low salt concentration of 0.3 g/l (equal to 0.3 kg/m3 corresponding to an electric conductivity of about 0.5 dS/m) and a modest annual supply of irrigation water of 10000 m3/ha (barely 3 mm/day) already brings 3000 kg salt/ha each year. In the absence of sufficient natural drainage (as in waterlogged soils) and without a proper leaching and drainage program, this would lead in the long run to a high soil salinity that may need to be checked by salinity control.

References

drainage WHO-Lexicon page (translations and examples)

See also

Internal links:

External Resources

External links:

  • Article on drainage systems used in the world [1]
  • Article on drainage criteria [2]
  • Information on waterlogging and land drainage : [3]
  • Articles on waterlogging and land drainage : [4]
  • Frequently asked questions on waterlogging and land drainage : [5]
  • Spacing equations for subsurface pipe and ditch drains : [6] , and an explanatory paper : [7] (PDF)
  • Spacing equations for subsurface drainage by wells: [8] , and an explanatory paper : [9] (PDF)

Attachments

907 Rating: 2.7/5 (50 votes cast)