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CLOSE THIS BOOKSpecial Public Works Programmes - SPWP - Soil Conservation - Project Design and Implementation Using Labour Intensive Techniques (ILO - UNDP, 1982, 220 p.)
CHAPTER C. REHABILITATION TECHNIQUES FOR WATERLOGGED SOILS
VIEW THE DOCUMENTC.1. AIMS OF DRAINAGE
C.2. DIFFERENT TYPES OF DRAINAGE
VIEW THE DOCUMENT2.1. Open ditches
2.2. Underdrains
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT2.2.1. Fascine drains
VIEW THE DOCUMENT2.2.2. Stone slab drains
VIEW THE DOCUMENT2.2.3. Drains with stone backfill
VIEW THE DOCUMENT2.2.4. Wooden box drains
VIEW THE DOCUMENT2.2.5. Peat drains
VIEW THE DOCUMENT2.2.6. Earthenware piping drains
VIEW THE DOCUMENT2.2.7. Concrete drainpipes
VIEW THE DOCUMENT2.2.8. Bituminous fibre drainpipes
VIEW THE DOCUMENT2.2.9. Plastic drainpipes
VIEW THE DOCUMENT2.3. Mole drainage
VIEW THE DOCUMENT2.4. Subsoiling
C.3. DETERMINING THE CHARACTERISTICS OF A DRAINAGE NETWORK
VIEW THE DOCUMENT3.1. Water movement in a drained soil
3.2. Determination of basic data
VIEW THE DOCUMENT3.2.1. Causes of wetness
VIEW THE DOCUMENT3.2.2. Optimal water table level
VIEW THE DOCUMENT3.2.3. Permissible submersion time
VIEW THE DOCUMENT3.2.4. Type of soil - permeability
VIEW THE DOCUMENT3.2.5. Intrinsic flow rate of network
3.3. Calculating the drainage network
VIEW THE DOCUMENT3.3.1. Network design
VIEW THE DOCUMENT3.3.2. Drain depth
VIEW THE DOCUMENT3.3.3. Drain spacing
VIEW THE DOCUMENT3.3.4. Drain diameter and length
VIEW THE DOCUMENT3.3.5. Calculation of collectors

Special Public Works Programmes - SPWP - Soil Conservation - Project Design and Implementation Using Labour Intensive Techniques (ILO - UNDP, 1982, 220 p.)

CHAPTER C. REHABILITATION TECHNIQUES FOR WATERLOGGED SOILS

C.1. AIMS OF DRAINAGE

The aim of drainage is to evacuate water in excess of vegetation and soil requirements. Certain soils have a natural drainage system others do not; these latter require the installation of an artificial drainage system.

Drainage creates in the soil the conditions required for good plant development and promotes soil aeration and root penetration. It improves the soil’s physical qualities and makes it possible to reclaim zones which were previously considered unsuitable for cultivation.

In irrigated zones, drainage is linked to leaching techniques to prevent accumulation of toxic salts in the soil and also to prevent gullying and soil erosion.

Drainage techniques are suitable for labour-intensive work in activities for the good of the community.

C.2. DIFFERENT TYPES OF DRAINAGE

2.1. Open ditches

These are the most simple and most widely used. They are suitable for a wide range of flow rates with very slight or zero slopes provided they are of adequate cross-section.

They are most effective from the drainage point of view since they intercept both ground water and surface run-off. Their construction and maintenance are highly labour-intensive when no machines are available.

Their main disadvantages are their size in view of the large area that they take up on crop land and the hindrance they cause to the movement of machines and animals and men. Where the drainage requirement entails only short distances between ditches, the land loss and the hindrance become excessive and preference will be given to underground drainage. Open ditches are therefore used mainly for low density networks and when they are also required to evacuate surface water.

Channels are a type of small open ditch or gully designed to evacuate surface water. Channel networks are usually installed on pasture land or when the agricultural value of the land does not justify the installation of underdrains. The channels are usually 40-50 cm deep, 0-0.20 m wide at the base and around 1 m wide at the surface. They can be dug by hand or by machine. If they are to remain effective, they require relatively frequent maintenance.


Channel drain

Farming techniques can achieve the same objectives as channels. Beds 8-30 m wide and 0.50 m high are constructed or furrows and ridges cut down the slope with a ditch at the base of the slope to collect the water.

2.2. Underdrains

These are collectors made of drainpipes or other piping material, which are buried in the soil. They can be interconnected between each other and run into a drainage ditch or into a natural collector. They cannot be used to evacuate surface water directly.

This type of drainage is expensive and is suitable only for soils with a high commercial yield. It does however have the advantage of requiring minimum maintenance and to have a long service life if it is correctly installed. Another advantage is that the land over the drain is free for cultivation. A very wide range of materials are used for underdrain construction. Certain are traditional materials and can be of interest in regions where modern drainage materials are difficult to obtain or expensive.

The main types of underdrains are:

2.2.1. Fascine drains

A trench is dug to the required dimensions, the bottom is filled with fascines leaving a space for water drainage. The fascines are covered with the trench backfill.


Fascine drain

2.2.2. Stone slab drains

A cavity is produced using stone slabs placed at the bottom of a trench.

2.2.3. Drains with stone backfill

These drains are made with round stones so that the water can run off through the gaps between them. They clog up easily.

2.2.4. Wooden box drains

These are suitable for soft and marshy land. They are made using a wooden box structure with an internal dimension of 7 cm or more. They are used only rarely.

2.2.5. Peat drains

These are used in peaty soils in which the peat itself is used to form the drain.

2.2.6. Earthenware piping drains

These have been used since the beginning of the nineteenth century. They comprise fired clay pipes between 5 and 15 cm in diameter and usually 30 cm in length. Also, they usually have flat ends and consequently they may be moved out of alignment if the earth settles. To overcome this disadvantage, interlocking drain pipes have been produced in the Netherlands. Fired clay pipes are manufactured in brickworks in the same way as tiles. They should be of very good quality if breakage is to be avoided during laying and if they are not to crumble under the effect of water. All defective drains should be eliminated before laying.

2.2.7. Concrete drainpipes

These are used where fired clay pipes are difficult to produce. They are of similar dimensions.

Concrete drainpipes are manufactured using special machinery and drainpipe quality depends on the method of manufacture and the quality of the aggregates used. These aggregates should be permeable to ensure good drainpipe porosity; however, the pores are rapidly blocked by fine soil particles in the water. In acid and peaty soils, the concrete is attacked by sulphates in the water.

2.2.8. Bituminous fibre drainpipes

These drainpipes are made of fibre impregnated with bitumen and moulded under pressure. Holes or slots are made in the pipe to allow water to enter. They are lighter than clay or concrete pipes and are more resistant.

2.2.9. Plastic drainpipes

The use of plastic drainpipes is becoming widespread in all types of agricultural underdrainage work.

They are made of rigid or flexible non-plasticised polyvinylchloride (PVC). There are slots at regular intervals in the pipe walls to allow the passage of water. The diameters used are between 30 mm and 100 mm. The characteristics of rigid PVC drainpipes are shown in fig. C.1.

Fig. C.1: Characteristics of rigid PVC drainpipes

Perforated PVC drainpipe

Exterior diameter in mm

32

40

50

63

Weight in kg

0.110

0.138

0.195

0.273

Wall thickness in mm

0.8

0.8

0.9

1.0

Rigid PVC drainpipes are produced in 6 m lengths and, in special cases, 9 m lengths. The pipes have slits of 35-40 mm long and 0.5-0.8 mm wide perpendicular to the pipes’ axes and at intervals of 20-50 mm depending on the type of soil. The slits are staggered. The drainpipes are joined together by a sleeve.


Rigid PVC drainpipe

Flexible PVC drainpipes are supplied in 200-250 m lengths on a drum and are intended for mechanical laying. The pipe itself is corrugated, which ensures greater resistance to crushing and improved flexibility. The main characteristics are as follows:

Wall thickness: 0.7-1.0 mm.

Connection by T’s, sleeves, etc.

Perforations - circular: 1.5 mm diameter or rectangular 1 × 1.5 - 3 mm

No. of perforations/m of drain: 600-700

Surface area of perforations/m of drain: 10-25 cm2.

These drainpipes are easily blocked by loam and roots; they are also vulnerable to clogging by ferric hydroxide. During storage before laying they should be protected from sun and weather which causes ageing of the plastic and a deterioration of the mechanical characteristics.

2.3. Mole drainage

Mole drainage is used in plastic and low permeability soils. The technique consists in digging a drain in the soil, without any external support whatever, using a ripper blade fitted with a “mole” - a pointed cylindrical metal tool - which digs a circular drain 0.05-0.12 m in diameter at a depth of 0.60-0.80 m.

These mole holes act as drains but must be closely spaced. They require very special conditions: the soil must be sufficiently plastic, the clay content should not exceed 30-35 per cent and at the moment they are dug the water content should not be greater than the plasticity limit (ATTERBERG’s Plasticity Index). The aggregates should have excellent stability (stability index lower than 1).

The requirements for mole drainage are:

- a drain length of less than 80 m between two outlets;
- a slope between 0.2 and 5 per cent.

Mole drainage costs very little but it has a relatively short service life (from three to ten years).

2.4. Subsoiling

Subsoiling is used to mechanically break up hard pans or ploughing compaction which reduce water penetration into the soil.

Subsoiling rapidly changes hydrodynamic characteristics, in particular the soil’s hydraulic conductivity and makes it suitable for underdraining. It also loosens and aerates the soil which promotes better root penetration.

C.3. DETERMINING THE CHARACTERISTICS OF A DRAINAGE NETWORK

3.1. Water movement in a drained soil

Water movement in soil is shown diagramatically in fig. C.2.

It is presumed that the soil is permeable, isotropic and the deep water table is not drained naturally.

Between two rows of drainpipes, the deep water table is convex in shape, the height of the table being at its maximum halfway between the two rows.

A drop of water which infiltrates the soil’s surface, first descends vertically into the non-saturated soil under the effect of gravity. When it reaches the water table, it is no longer subject only to gravity and it will flow in the direction of the drain.

Fig. C.2 shows diagramatically the route taken by water streams depending on their distance from the drain.


Fig. C.2: Water movement in drained soil

- The water drop 1 which falls directly above the drain is subject only to gravity and descends vertically until it reaches the drain.

- Water drop 2 reaches the water table close to the drain and finally reaches the drain after travelling a short distance in the saturated soil.

- For water drops 3 and 4, the distance travelled in the water table increases. Following predominantly vertical movement in the upper part of the water table, the liquid-flows curve towards the drain and may finally reach it via an upward movement.

- Water drop 5 lands the furthest from the drain and its route first descends and then curves when the impermeable layer is reached; it then moves horizontally and the final section ascends towards the drain.

On the other side of the yy’ axis, the situation is symmetrical.

When the depth of the permeable layer is small in relationship to the distance between the drains, the flow diagram shows that, in the segment of soil above the drain level, movement is predominantly vertical; below the drain level the movement is predominantly horizontal. The flow diagram differs depending on the depth of the impermeable layer; if this is at the drain level or only slightly below it, horizontal movement in the direction of the drain will predominate. If the impermeable layer is at greater depth, the proportion of vertical movement will be greater. The shape of the water table surface between two drains varies depending on the input rate, the distance between the drains, and the permeability and thickness of the permeable layer.

When the input rate is zero, the water table surface will tend towards the horizontal to reach an equilibrium at drain depth. It may also descend below drain level as a result of capillary losses. If the shape of the water table surface is more convex, it will tend to come closer to the soil’s surface the more the distance between the drains is increased or if the thickness or permeability of the draining layer is decreased.

The shape of the water table surface between two drains is, therefore, dependent upon:

- the input rate q;
- the depth of the permeable layer D;
- the permeability of the drained layer K;
- the distance between the drains L.


Fig. C.3.

The optimum level of the water table (p-h) is the basic datum in drainage problems and it is dependent on crop requirements. The other data are input rate or “characteristic flow rate” q, the depth D and the permeability K of the drained layer.

The interval between the drains is determined on the basis of these parameters.

3.2. Determination of basic data

3.2.1. Causes of wetness

Before trying to solve a problem of soil waterlogging, it is necessary to determine the causes of the wetness. These causes may differ and the control measures should be adapted to each case.

The main causes of soil wetness are:

(a) inflow of run-off water from catchment basins above the zone in question. In such a case, it is necessary to intercept this run-off water and divert it into natural or man-made collectors (cf. Chapter B);

(b) river water overflowing into an alluvial plain. In such a case, it is necessary to protect the zone by embankments or to dig drainage ditches to evacuate the flood water in a minimum time compatible with the type of crop;

(c) a high underground water table due to specific topographical or geological conditions;

(d) a high level of rainfall producing excessive rise in the ground water level or the development of saturated horizons in low permeability soils. In an arid zone, the high level of irrigation required for soil leaching may produce the same effects. In this case, conventional drainage techniques are used;

(e) inadequate capacity of natural collectors. This may trap the water and, in such a case, increasing the size of the collector may be sufficient to solve the problem.

3.2.2. Optimal water table level

The ability of the top layer of soil to dry out is essential for good rooting of plants and for good agricultural yield. If the water table is permanently too close to the surface of the soil, good rooting will not be possible; a water table which is too low may also have an unfavourable effect since it will not provide adequate capillary water supply during the dry season.

The optimal depth of the water table varies depending on the plant’s rooting depth and the soil texture. In a light soil, the water table may be higher than in a heavy soil in which capillary supply is greater.

In the case of leguminous crops for which the roots do not penetrate deeply, the water table should be relatively close to the soil surface (30-60 cm) to ensure optimum yield. This is also the case for grasslands.

Cereals require a water table between 60 and 80 cm below the soil’s surface.

Fruit trees require a lower water table of 1-1.50 m or more depending on the species.

3.2.3. Permissible submersion time

Crop submersion covers:

- partial submersion which affects the plant’s root system;

- total submersion when the water level rises above soil level and affects the plant stalks.

Total submersion may cause greater damage than partial submersion.

Crop damage caused by submersion depends on:

- submersion time;
- the point at which submersion occurs in the plant’s growth cycle;
- the type of plant being grown.

In general, submersion of 1-3 days retards development. Submersion of 7-15 days may result in total crop loss.

Grassland may withstand submersion times of 1-2 months prior to the vegetative period. During the vegetative period, submersion for 15 days may reduce yield by 30-50 per cent.

Market garden produce is very sensitive to submersion and even one day’s submersion may result in a reduced yield which will vary depending on the species.

Cereals are particularly sensitive to submersion during flowering and grain formation. Maximum loss may be as high as 20 per cent following a three-day submersion.

Fruit trees are highly sensitive to submersion although the results vary considerably depending on the species.

In project planning, one may use the following permissible submersion times:

Market garden crops

1 day

Cereals

3 days

Grassland

7 days

The figure below gives some figures for the effect of submersion on yield:

Fig. C.4: Effects of submersion on yields (maximum loss in percentage of the optimum harvest)


Submersion time


3 days

7 days

15 days

Grassland

-

20

50

Autumn cereals

20

50

100

Spring cereals

20

40

100

Maize

20

80

100

Perennial fodder

10

40

100

Potatoes

50

100

100

Sugar beet

10

50

100

Sunflower

10

40

100

3.2.4. Type of soil - permeability

Permeability is the ability of water to drain down into the soil. It is a significant parameter for calculating a drainage network. Soil permeability depend on texture, porosity and organic-matter content.

A number of formulae relate permeability to soil texture. The most important of these are:

- the SCHLITCHTER formula

This applies when all the soil particles are the same diameter D; the value of R is related to the porosity;

- the HAZEN formula

D10 is the diameter which permits passage of 10 per cent by weight of the portion of the soil made up of particles less than D10.

Use of the formulae will ensure the correct order of magnitude of the results; however, it will not give a sufficiently accurate result for practical purposes.

Direct measurements of permeability is preferable to the use of formulae. This can be done in a laboratory using whole samples or by the direct measurement or soil in situ. The most simple and reliable method is that of ERNST which is also called the “auger-hole method”; the principle is as follows:

A hole is made in the soil with a 4-5 cm auger down as far as the impermeable layer or a depth of 2 m.

As much water as possible is removed from the hole by means of a ladle so as to reduce the ground water level by at least 40 cm. The speed at which the ground water rises in the hole is measured at 5-10 s intervals.

Where:

H is the depth of the hole under the ground water level in cm
S the depth of the impermeable layer below the bottom of the hole in cm
r the diameter of the hole in cm
Y the mean distance between ground water level and the level of the water

Y over a period of time T

K the hydraulic conductivity in m/day.

The hydraulic conductivity is calculated as follows:



Fig. C.5: ERNST’s method

3.2.5. Intrinsic flow rate of network

This is the flow that the drainage network must collect and evacuate in relation to a unit of the land’s surface area. The intrinsic flow rate is determined by the following equation:

where

qc = the intrinsic flow rate in l/s/ha


e = the coefficient of evaporation (a dimensionless number lower than 1)


i = critical rainfall intensity in mm/h.

The critical rainfall is the rainfall for a time q corresponding to the permissible submersion time for a recurrent time T. The values for time 9 and recurrence time T depend on agricultural and economic factors.

3.3. Calculating the drainage network

3.3.1. Network design

The design and layout of the drainage network depend primarily on topographical factors. The first task is to locate the thalweg (middle line of a river) and the crest lines. The main collector drains should be located along the thalweg. Minor drains should empty into the main collector drains without ever crossing a crest line.

The minor drains should be laid out at right angles to the lines of greatest slope, i.e. more or less parallel to the contour lines. Where the slope is very slight (less than 0.003), they may be laid out parallel to the slope. The minor drains are also placed at right angles to the ploughing direction (see fig. C.6).


Fig. C.6

The collectors are always laid out along the line of greatest slope. The main collectors are situated in the main thalwegs and the secondary collectors in the secondary thalwegs.

The layout of drains and collector/drain combinations may vary depending on the terrain, and should therefore be adapted to each specific case. Some typical layouts are shown in fig. C.7.

Fig. C.7: Layout of a drainage network


Transverse drainage


Intermediate collectors


Belt drains (protection against external run-off water)


Drainage of variable permeability land


Draining of waterlogged pocket

3.3.2. Drain depth

The depth and spacing of drains are two closely linked parameters. As drain depth is increased, spacing between drains may also be increased. In homogeneous land, deep drainage has a certain number of advantages over superficial drainage since:

- the water table is lowered further, giving better soil aeration;

- drains may be more widely spaced, which results in a reduction in drain number and drainage cost;

- drains are in this way protected against root invasion which tends to clog them.

On the other hand, deep drains tend to cause a too rapid fall of the water table in the dry season.

In heterogeneous land, a pedological study should be carried out to determine the most suitable drain depth.

If the permeable soil overlies a very impermeable subsoil, it is preferable not to put the drain into the impermeable layer but to locate it at the borderline of the two layers.

If the impermeable land lies above a permeable subsoil, one should try to locate the drain as deeply as possible in the subsoil.

For practical purposes, drain depth varies between 0.70 m and 1.50 m. A depth of 0.60-0.80 m is considered small; a depth of over 1.20 m is considered as deep drainage.

On very slightly sloping land, the drain depth will depend on drainage slope and the depth of the outlet. It may be necessary to have the drain shallow at the start and deeper as it approaches the outlet to ensure a sufficient slope for the water to drain away effectively.

3.3.3. Drain spacing

3.3.3.1. Selection of methods of calculation

A distinction should be made between two regimes of drain water input and output: the permanent regime and the transitory regime.

A large number of formulae have been developed for calculating drain spacing for the permanent regime. These presuppose constant input. They may be used in Europe and anywhere else where rain is of long duration and low intensity.

In irrigated zones and in regions of high intensity and short duration rainfall, drain water input is not constant and it is therefore necessary to use transitory regime formulae.

Calculating the spacing between drains is easier for the permanent regime than for the transitory regime; moreover, use of the permanent regime formulae is often justifiable even in the transitory regime, especially when precise knowledge about food conditions and hydrological constants is not available.

3.3.3.2. Calculating drain spacing in the permanent regime

(a) The HOOGHOUDT formula

Drain spacing is given by the following formula:

where:

L = the spacing between drains in m

q = the intrinsic flow rate in m/days or m3/m2 of the drained zone

K2 = the hydraulic conductivity of the layer below the drain in m/day

K1 = the hydraulic conductivity of the layer situated above the drain in m/day

h = the height of the water table above the drain level halfway between the two drains (in m)

d = the depth of the equivalent layer. A value which is a function of the spacing between the drains L, the drain radius r and the depth D of the impermeable layer below the drain (see Fig. C.8).

Use of this formula presupposes that the boundary between the two permeable layers is at drain level, which is not always checked. The calculation is carried out by successive approximations, with d not being known accurately until L is already known.


Fig. C.8: Values of d in the HOOGHOUDT formula

(b) The ERNST formula

The general equation is as follows:

where:

h = the height of the water table above the level of the drains halfway between the two drains in m

q = the intrinsic flow rate in m/day

Dv = the thickness of the saturated layer above the drain in m

KD = K1 D1 + K2 D2 = the product of the thickness multiplied by the permeability of the various layers in m2/day

D2 = the thickness of the lower layer in m

D1 = the mean flow section of the upper layer with permeability K1

= Rr = radial resistance which is a function of drain position

D0 = thickness of the layer for which the radial resistance has been calculated

u = wettened perimeter of the drain.

Depending on the position of the drain in relation to the boundaries between the layers of different permeabilities, the formulae to be used are as follows:


Figure

3.3.3.3. Calculating drain spacing in the transitory regime

(a) GLOVER-DUMM formula


Figure




where:

j = reservoir coefficient (in days)
V = the effective porosity in per cent
ho = the load above the drain at times t
ht = the load above the drain at times t
t = the time expressed in days.

The effective porosity V is given approximately by the square root of the permeability expressed in cm

(b) BOUSSINESQ formula (1903)

This applies when the drain is located on the permeable layer and is expressed as follows:

3.3.4. Drain diameter and length

The choice of drain diameter and length is a function of the plot to be drained and the amount of water to be evacuated.

Drain diameter is usually constant over the whole length and the flow in this drain increased with drain length, i.e. the surface area drained.

The flow rate drained is equal to the product of the surface area drained multiplied by the intrinsic flow rate.

Q = S.qc = L.E. .qc

Where the length to be drained becomes too large, drain capacity becomes inadequate. Large diameter drains may therefore be used; in this case the drain length may be longer than for small diameter drains, but the cost is also higher.

(The simplest and the most widely used method is to select the type of drain and its diameter and then calculate the maximum length that can be used.)

The commonly used diameters of underdrains are between 5 and 8 cm. Collectors however may have diameters up to 30 cm.

Fig. C.9 shows the flow rate in drains and collectors depending on gradient and drain diameter.

3.3.5. Calculation of collectors

The flow rate in collectors is equal to the sum of the flow rates in the minor drains which feed into them.

Collector diameter may be variable as a function of flow rate and gradient (see Fig. C.9).


Fig. C.9: Flow rate in drainage collectors (in l/s)

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