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Structural components of vertical drainage system: calculation and designing

Vertical drainage is the drainage type that allows controlling soil water and salt regimes by means of drainage wells. It is one of new land reclamation methods.

Use of vertical drainage will allow automatizing the soil water regime control process, which will provide more stable and heavier yield, fully mechanizing construction works, raising labor productivity by 3-5 times, and more economically consuming water resources.

Vertical drainage was first used in the USA in 1923-1925. In Central Asian countries, it is widely used since 1950.

Vertical drainage is divided into systematic drainage (uniform spacing of water intake wells on a given area, at the corners of its square or rectangular pattern), random drainage (wells are placed only on particular overdamped sites), bank drainage (linear system of wells protecting the territory from flooding from the river, reservoir, or lake side), and combined drainage (combination of well with horizontal drainage).

Vertical drainage is used for controlling the soil water regime by creating:

  • irrigation and drainage systems by using groundwater captured by wells for overhead irrigation;
  • for controlling groundwater level;
  • for protecting reclaimed lands from groundwater inflow from outside, from flooding from the rivers, reservoirs, or lakes;
  • for lowering the groundwater pressure and reducing (controlling) water inflow to the drainage layer from deep confined aquifers;
  • for using unconfined and confined ground waters from a drainage land for watering neighboring dry valleys, supplying water to residential settlements, farms, etc.;

The irrigation and drainage system of vertical drainage represents the complex of hydraulic facilities (wells, contour canals and catch-water drains, gates, water-accumulation tanks etc.), sprinkling units, surface and subsurface pipelines, control stations and automation equipment.

Vertical drainage is often supplemented with vacuum systems siphon pipes in the form of subsurface pipelines. In the drainage system of vertical drainage, there are elements providing irrigation of fields.

In spring and after steady rain, vertical drainage operates in the drainage mode: pumps of wells are turned on and the groundwater captured by them is supplied to filling water bodies or tail-race canals.

By regulating the discharge of wells and their operating period, a required drainage rate can be ensured. In dry periods, the soil root zone moisture can be controlled by sprinkling as follows: the wells are brought into operation and the groundwater captured by them is supplied through closed pipelines to sprinkling units. The groundwater resources used for irrigation are replenished in autumn-winter and spring seasons.

Certain hydrogeological, geomorphological, geomorphological, and soil conditions need to be created for application of vertical drainage on reclaimed lands, and first of all it is necessary the mantle to be represented by sufficiently permeable soil under which thick layer of water-saturated sandy deposits lay.

Vertical drainage is a deep bore-hole well reaching a thick water-bearing stratum (aquifer) and cutting through it in part or in full. The well depth is determined depending on the geological feature, hydrogeological conditions, and depth of the aquifer occurrence.

Generally, vertical wells (drains) are made to a depth of 30-80 m and more, 0.7-1 m in diameter, with fixation of the well walls with pipe casing.

When pumping out water by a vertical well, the groundwater level around the drain lowers, forming a cone of depression. This cone may be symmetrical (when pumping out water from a groundwater basin) and asymmetrical (when pumping out water from an underflow).

Depending on drain arrangement, they distinguish between: areal (systematic) arrangement, when it is necessary to lower the groundwater level on the irrigated area; and linear, when a line of wells catches the underflow coming to the irrigated area from adjacent lands.

Accordingly, drainage is broken down into types: systematic; linear; and random. The latter type is confined to particular land plots where random lowering of groundwater level is required.

According to their arrangement in the layout, vertical wells can be of single and group type. If a group of vertical wells is arranged in the layout at a space between each other that is less than the radii of their influence, these well are called interacting wells (multiple well system).

The water-intake part of the well is equipped with a filter. As a rule, the filters are made from round metal slotted pipes (also rod-shaped or other designs as well as from other metal can be used). A transformer from power transmission facilities and a cabinet with the equipment for automatic control of the drain pump operation are installed beside every well. To organize remote control and maintenance, vertical wells are united into systems of 20-100 wells.

Vertical drainage is advisable to organize with such a geological feature of soil layer where there are thick coarse-grained or pebbly aquifers with confined water and without solid waterproof interlayer, and where aquifer water transmissibility T is over 100 m2/day:

= k * m

where:

k is the aquifer soil permeability coefficient, m/day;

m is the thickness of this layer, m.

Designing of vertical drainage boils down to the formation of descending water flow estimated based on the water and salt balance analysis as well as to making provision for required decline of groundwater surface on a given irrigated land. Proceeding from this situation, they first define the system parameters, i.e. drainage type, its capacity etc., then calculate well parameters, their quantity, space between each other, their discharges and, in the final analysis, the well design and pumping equipment for it.

The expediency of the construction of vertical drains with different water transmissibility is to be decided in the result of technical and economic evaluation.

Vertical drainage is used both on irrigated and reclaimed lands, in the areas of insufficient, unsustainable and excessive watering.

Specific discharge of the vertical well, i.e. discharge per meter of pumpdown depth, should be at least 5 l/s. Installation of wells with low specific discharge is inefficient.

In comparison with horizontal drainage, vertical one has a number of advantages such as:

  • contributes to fast lowering of groundwater level and soil desalination with expansion of this process to a great depth;
  • when it is applied, the water pumped out (if it is brackish) for irrigation and leaching of saline soils;
  • operation of vertical drainage provides sufficient capacity of the zone of suspended water, which will allow efficiently carrying out autumn-winter and spring leaching of saline soils; enables controlling the groundwater level, which will allow creating optimum moisture of the soil of the suspended water zone.

Keeping optimum groundwater regime during the growing season along with proper agrotechnical complex will prevent restoration of salinization and create favorable conditions for gaining high crop yield.

Despite it has many benefits, vertical drainage has also a few drawbacks:

  • to organize its operation, it needs to be equipped with submerged electric pumps, which will considerably raise the cost of the construction of drains and increase its operating costs;
  • during the operation of a drain, a groundwater surface depression cone will form around it and, as a result, the soil is drained unevenly;
  • under extended and intensive operation of vertical drains, especially in case of a large group of such drains, substantial loss and decrease of the water pressure of the aquifer from which water is pumped out may take place; this in some cases may result in the inflow of the underlying highly saline artesian water to the soils water-bearing and top covering layer.

There are cases when washout of nutrients from the soil and, consequently, decline in its fertility takes place because of deep drop of groundwater level.

Vertical drainage should be designed for the pumpout of mainly dynamic shallow and deep groundwater resources. This requires materials obtained in the result a great package of explorations and investigations of the natural conditions of a given area. Based on these materials and data of the water and salt balance analysis data, they determine the load on drainage, quantity of the excessive water needs to be diverted outside a given territory.

During its operation, vertical drainage is expected to provide descending water flow with lowering groundwater level to a given depth (at least 2.5 m) and maintain this regime during the entire period of the operation of the irrigation system.

When designing and carrying out seepage estimation of the vertical drainage, they determine its parameters as well as drainage type and construction, performance in terms of water discharge (well discharge), minimum positive decline of groundwater level, well influence radius (area of the territory drained by one well and group of wells), and space between the wells.

Vertical drainage parameters are calculated based on annual average load of the operating period when the drains operate both in stable and unstable regimes.

There are different vertical drainage system operation regimes depending on the period of the development of saline irrigated lands. During a reclamation period, the operation regime provides diversion of drainage water at desalination of the soil; during an operating period, it provides optimum water and salt regime.

Well operation regime is subject to natural and economic conditions. It may be constant in terms of discharge, constant in terms of time, constant in terms of the number of operating wells, and constant in terms of period of year. When using pumped-out water for irrigation, the well operation regime is coordinated with the crop water-consumption diagram.

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