Pollution of a drain equals obstructions in the drain. This is why the amount of pollution determines the to be taken action. Pollution has a significant influence on the technical lifespan of the drain as well. The drain must be cleaned regularly. If the drains contain that much dirt in them that it can no longer be removed, the drains are technically unusable. Depending on the requirements set for the use of the plot, drainage will have to be carried out again for proper drainage.


This chapter contains information on the following topics:
The effect of pollution on drainage
Rinsing of soil particles
The allowed amount of flushing
Pollution with iron
Degree of pollution concerning- transpiercing 

The effect of pollution on drainage

An impression of the influence of contamination on the operation of the drainpipe is shown in figure 1. For the drainage of the same amount of water, more pressure is required as the drain becomes longer. The same goes for the level of pollution. As more dirt enters the drain, more pressure is needed to allow the water to flow properly through the drain to obtain the same drainage. Measurements indicate that the required pressure head can increase considerably. Table 1. shows the effect of pollution on the discharge and the required pressure head. The level of pollution is the thickness of the layer of dirt divided by the diameter of the drain pipe. Measurements and drainage checks have shown that with normally maintained drains the drainage capacity is approximately equal to the drainage capacity in the presence of around 13 to 14 mm of dirt. Table 2 shows what this means for the discharge capacity of the various drain diameters. 

[table 1 & 2]

Tables 1 and show the influence of pollution in percentages. Table 3 shows what this means for a drain with a diameter of 60/54 that drains the water from an area of 3000 m². 

When calculating the maximum drain area for a certain hydraulic gradient concerning the drain diameter, a pollution level of 25% will be taken into account. This means that in a tube with an inner diameter of 54 mm there is a presence of 13 to 14 mm of dirt.

[table 3]

The data in table 3 shows that such contamination does not yet constitute a major obstacle to properly functioning drainage.

The length of the drain is important as well. With the same drain length, the additional pressure height that is required depends on the drain diameter. With a longer drain length, the required pressure head also increases. Table 4 shows what this means for the same drain pipe, but with a different length. Table 4 shows that the required pressure head increases with an increasing drain length and a reduction in the effective diameter due to more dirt. Pollution of the drain causes loss of energy due to a higher entry resistance and greater frictional losses. Besides, there will be more losses that are harder to determine. The presence of dirt will not be constant everywhere. This creates “rapids” within the drain at local flushes, which also causes energy loss. After all, tube widening and narrowing cause changes in the flow patterns. Dirt flushing means clogging pores through which water normally flows in. The surface area of the pores in PVC drainage pipes are at least twice as large as that should minimal be for the ingression of water. With the pe and pp tubes, this is three to four times as large due to the greater length of the perforations. The surface of the pores will therefore not become too small to allow enough water to enter.


Rinsing of soil particles

Casing material aims to facilitate the entrance of water by increasing the "wet perimeter." As the soil in the area surrounding the drain is poorly permeable, the function of the encasing material becomes more important. Thus, the casing material not only increases the outer diameter but also lowers the entry resistance. 

However, the influence of the actual drain distance as a result of these factors is manageable, and besides, the encapsulating material must repel the soil particles. Counteracting soil particles increases the risk of entry resistance. These conflicting factors make it difficult to choose the right encasement material in the various soil types and soils. This often results in a compromise between the choices. 

Rinsing soil parts in the drain does not necessarily have to be undesirable. When the fine soil particles are discharged through the drain, the permeability of the total profile can increase, because the larger parts remain behind. If the fine bottom parts remain in the enclosure, it can become clogged, causing the entrance resistance to increase considerably. An important note is that the drain must be able to rinse out permitted soil parts.

[ table 4] 

Drainage through natural drainage

The washed-in parts will also be discharged during the natural drain discharge. The speed of the flowing water in the drains plays an important role in this. The speed of the water and the falling speed of the soil particles are important here.  The flow rate in a drain can vary greatly. The speed at the drainage of 7 mm per day is approximately four centimeters per second (0.04 m per second). The average drainage is approximately one to two mm per day. 

The speed at which the soil particles are falling is shown in table 4.

If a soil part ends up in the pipe while the flow velocity is greater than the falling velocity, this part will be drained. If the flow velocity is less than the falling velocity, the part will remain in the tube. With a discharge of 1 mm per day, the flow velocity is high enough to discharge particles smaller than 85 µm and with a discharge of 2 mm per day, particles smaller than 120 µm.


Rinseable parts

In periods without drainage, the soil particles settle in the drain and attach themselves. Force must be applied to detach these parts. Certainly, after a prolonged dry period, a greater force than usual is required to move soil particles. When flushing the drain a flow velocity of five to six centimeters per second is created.  At this speed, soil parts of approximately 300 µm can be moved. Due to the loosening of soil parts and the water loss through the pores, quite a lot of water loss occurs during flushing.  The transport of the larger bottom parts is therefore disappointing. However, flushing does ‘smooth out’ the local contamination over a greater length of the pipe. Little research data exists regarding the proportion and size of the soil parts that can be washed out. For the time being, we assume that bottom parts smaller than 50 µm can be removed by rinsing. The parts in class 50 to 150 µm will only be rinsed to a limited extent. For the time being, we can assume that approximately half of this can be removed during flushing.

[ table 5]

Composition of washed-in parts

Research regarding the size of washed-in soil particles, the following compositions were found, as seen in table 5. This means that all particles with a grain size of 0 -2, 2 - 16 and 16 - 50 µm and half of the particles between 50 - 105 µm can be rinsed. Table 6 shows the parts that can be rinsed. The percentages are rounded because it is not exactly known which part can be rinsed out. 

[table 6]

Irrigation and digestion of casing material 

It was investigated whether there is a relationship between the degree of digestion of the (coconut) coating materials and the degree of wash-in. The result of this research is shown in table 7. This table shows that there is a clear relationship between the degree of digestion and the number of millimeters of dirt in the drain. Although the number of observations is limited, this confirms the function of the encapsulating material as a filter against washing up soil parts. 

[table 7]

Chance of flushing

The likelihood of flushing increases as the soil becomes ‘lighter’ since the soil contains less sedimentable parts. This applies especially to the soil layer in which the drains are located.  This is because the danger of loosening and flushing is greatest for this soil layer. However, a light layer can also occur elsewhere in the profile, these parts can flush to the drain via the coarser (sand) parts. When mixing the soil, for instance with the construction of a drain in a dug-up drain trench, soil particles can be transported from all layers of the soil. This is something that mainly happens in the period immediately after construction.  After some time, the drain trench will stabilize.  If drainage is constructed in a period with a high groundwater level, the chance of sedimentation is greatest and a high level of transportation of soil parts will take place. In (light) sandy soil the chance of sedimentation is present since the binding through natural forces is small. Humus and iron compounds can, however, be a binding factor. In peat soil, few parts of the soil will move. In a peat soil, which morphs into a sandy soil at drain depth, fine humus particles can compact and smear the drainage. Flushed-in parts, however, are easier to rinse out again, even with a normal discharge from the drain itself. 

The allowed amount of flushing

Margin flushing equipment

Due to the presence of dirt, some margin is needed between the head and the inside diameter of the drain for a smooth passage.  A common drain diameter in agriculture is approximately 54 mm, this means that a flushing head of 30 to 35 mm provides sufficient space. For the hose, an additional margin is needed. This is because a drain is rarely aligned. Tightening the hose creates resistance against the inside of the drain. A minimum clearance of 10 mm is therefore required around the hose. This means that if the hose itself is 20 to 25 mm thick, it will leave some space.  This space may not exceed the amount of dirt present in the drain. Although a larger amount of dirt may be flushable due to the particle size (a high percentage of sludge able) constrictions in the drain will make insertion and flushing more difficult. 

[table 8]

The severity of the flushing is determined by the content of non-flushable parts. The layer is also important here since this plays a role in the infeed and outfeed of the flushing hose. The amount of sand that cannot be rinsed is shown in table 8. With lutum classes 0-4, 5-10 and 11-17, the washed-in dirt can be can be no longer sufficient rinseable. With a clay content higher than 17%, the flushing is low and can always be rinsed. Where the clay content is a derivative of the determined sludge content, with the factor “clay = 2/3 sludge”, it is necessary to apply a safe limit in connection with the presence of the actual clay content. In this context, a limit of more than 25% clay is maintained as a safe limit with a view to movable parts. That is to say, in soil with more than 25% clay in the layer in which the drain will be located, an enclosure is not strictly necessary to prevent soil parts from washing in.



Degree of pollution concerning transpiercing 

Due to the presence of dirt, the insertion of piercing equipment is more difficult. Since it takes quite a bit of force to pierce through. Research into the relationship between pollution and the force required to penetrate shows the following. Comparing the measured piercing pressure and the amount of dirt present after excavation showed that a counter-pressure of 0.5 kg meant that 5 mm of dirt was present. Based on this, the criteria from table 9 were developed.

[table 9]

The need for flushing

Research into the degree of flushing showed that within two years after flushing, there was already so much dirt in various drains that flushing was again necessary.  This was the case in places where the original coconut shell had been digested and the lutum content at the location of the drain was less than 25%. Flushing was considered necessary if more than 1.5 kg of counter-pressure was encountered in piercing. The presence of digested casing material increases the likelihood of entry after flushing. Compete for digestion of the coating material often only occurs after about three years. The exact speed of digestion mainly depends on the carbonic acid lime content of the soil. In soils where there is a risk of digestion of casing materials, organic-based materials are no longer used for the casing. However, the chance of soil parts penetrating soil layers that have less than 25% lutum will always remain. A regular check on the occurrence of contamination in such soil is therefore recommended. This can be done by checking the drain or through the piercing method. Flushing on time can here for be very useful. If you are too late, it will be difficult to clean the drain and an accelerated depreciation will be necessary.


Pollution with iron

Iron is found in many soils around the world. Iron is not clearly visible if it does not come into contact with air. (oxygen). If iron does come into contact with oxygen, it will become rust (iron oxide). This will mainly take place in periods when the drains discharge little water. This is because the air uses the opportunity to react to the ferrous water. Microorganisms can release iron that is bound to organic compounds, which increases the chance of iron deposition. The extent to which iron occurs in the soil and the extent to which it is drained determines the amount that is deposited in the drain. In addition, however, the drain from elsewhere is also important. In seepage situations, this can mean a significant amount, which is permanent. After the construction of the drain, an increased deposition can temporarily take place.  this is because of the drier soil sue to drainage, more air enters the soil, which can lead to more rust formation. Due to the construction of the drain, seepage can increase, which increases the drainage from elsewhere. 

[table 10] 

Determination of iron

Various methods are available for determining the iron content. For example, in addition to the Atomic Absorption Spectrophotometer (AAS), there are various test kits of comparative color strips. In the past, observations were made in Flevoland by using the Ford test kit. With this method, the iron content in the soil and groundwater can be determined in a fairly simple manner. The exact result of this method is not always comparable. However, in connection with the prediction of iron deposition in drainage, a global indication is often sufficient. This can be done based on several characteristics listed in table 10.  By preventing seepage, the drains are almost permanently supplied with water, so that it remains wet around the drains and air entry (and therefore iron deposition) remains limited. Seepage can occur in a plot where it had previously occurred, after the installation of the drainage. This is due to the additional pressure difference compared to areas higher up or areas with a higher water level. If no seepage occurs even after the drainage had been constructed, iron will only be available from that part of the soil that has become drier after the drains have been laid. In that case, the extra iron formation in the drains is only temporary and after some time (a few summer seasons) a new equilibrium situation will arise in which no extra iron is deposited. 


Consequences for drainage

Drainage creates a drier soil. This means that more oxygen will enter the soil. This causes an amount of iron to precipitate.  This does not only happen in the soil and on the soil parts but also in and around the drain pipes. After all, especially in this place, the supply of oxygen is large.  In the first meters, calculated from the power tube, the ‘air exchange’ is greatest. The deposition of iron will therefore be greatest here.  Iron deposits inside the drain can cause a considerable amount of stagnation in the drain. In some cases, the power tubes are more than halfway filled with iron in a short time. Often this is in an iron emulsion-like state.   If the groundwater level rises, the pressure in the drain increases, rusty brown emulsion is forced out and forms a rust layer at the end of the pipe and in the ditches. The deposition of iron in the soil directly around the drain has a much more disruptive effect on the drain and the way it functions. This can cause the filter to clog and the perforations in the drain. The latter has been established in practice in incidental cases and will inevitably lead to accelerated depreciation of the drainage system.


Restriction measures 

The likelihood of iron deposition can be somewhat affected by the choice of casing material. However, tests have shown that the influence is small. Organic material in which organisms feel at home, such as coconut, peat, or straw, should preferably be avoided. When choosing a plastic material, preference is given to the largest possible circumference. The thicker the sleeve around the less chance of blocking the perforations in the wall of the drain. Drains with a large perforation surface per meter are preferred. 

By preventing oxygen from entering the drain, the chance of iron deposition is smaller. This can be done by preventing air from entering the drain. (figure 19 A). Drains can also be installed in such a way that they are constantly filled with water. There are some conditions for this;


  • The use of a special end pipe constructions that prevent the drain from draining. (Figure 2 B and D).
  • Drains must be installed horizontal, this means that they may not rise above the groundwater level elsewhere in the soil, causing them to become dry, with the risk of air entering and thus corrosion. 
  • The water level of the ditch must be easily adjustable, so it cannot happen that there are periods when the drains do break through the water. This could occur when cleaning the ditches and when extra water is pumped out in preparation for a wet period. These problems can also occur during a somewhat longer drier period in the summer. 
  • The use of drain pipes with a slightly larger perforation (width 1.4-2.0 mm) or a larger surface per meter (p.e / p.p pipes) can be considered in order to limit possible harmful effects.
  • The power tubes must be properly marked. This prevents damage when cleaning the locks. 



Cleaning the drains

To risk as little iron deposition as possible, extra attention must be paid to cleaning the drains. Depending on the amount of iron formation, it is sometimes necessary to rinse the drains every year.  In some areas, it is not superfluous to clean twice a year. Cleaning the drains when they are already draining water is preferred. This way iron can easily be made ‘mobile’.  The iron that has been rinsed from the drain will then mainly disappear from the drain through the water that the drain itself drains through the end pipe. This situation is permanently present when trying to prevent a permanent seepage.  But flushing at the highest discharge is still preferred.   If seepage does not occur, additional maintenance may only have a temporary effect. However, regular inspection and cleaning of the power tube remain necessary in all cases. Cleaning the end pipe in combination with piercing the first ten to fifteen meters often results in increased drainage of the drain. It is precisely at this place that the greatest rust formation often occurs because this is where the greatest exchange of oxygen takes place.

If you take extra measures in cleaning the drain it can yield excellent results, also when preventing iron. 

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