Depth of drainage

The criteria for the height of the ditch level and the drainage are primarily intended to prevent high groundwater levels. However, this will occasionally be the case. You determine the probability of how often this occurs. This is determined, for example, by placing the drains closer together or at a greater distance from each other. Various choices are also possible for the length and diameter. Soil type and soil structure also play an important role in this. When choosing the diameter of the drainpipe, especially the surface that drains into a drainage pipe is important. In addition, it is important which slope is accepted from the groundwater towards the end pipe, the so-called hydraulic slope.




We express the discharge of a drain pipe in millimeters per 24-hour period. This means that the drain must discharge a certain amount of water per 24-hour period. For grassland and maize land, a discharge standard of 7 millimeters per 24-hour apply. This means that after a rain shower of 14 milters it may take two days before the groundwater level reaches its normal level again. It can take longer even if the drains meet the 7 mm standard. This can be the result of seepage (see figure 2). Seepage is the supply of groundwater from elsewhere. This seepage is sometimes of little significance, but it can also be a number of mm per day. In extreme cases, the number of mm is greater than the applied standard of 7 mm. In the case of seepage, the drain distance will have to be smaller and sometimes the drain diameter has to be larger. 



The drains have a discharge of 3 mm per day on 23 November. It starts to rain at noon and within one hour 18 mm of water falls. There is also seepage of 1 mm per day. It may take until November one o’clock before the drains give a discharge of 3 mm per day again. The drainage of the drains during these three days will not always be the same. It is possible that half an hour after it started to rain, the drains had a considerable discharge, for example, converted to 15 per day. A few hours after it has started to rain, the discharge varies from 10 to 15 mm per day. The measured discharge is therefore always a snapshot that we convert into a situation per 24-hour period. 


Surface per drain

Several starting points apply to the surface per drain. The surface is determined by the length of the drain times the distance between the drains and each other. The length of the drain is limited to 300 meters. With longer drains, there is a chance that the groundwater at the point furthest from the power pipe will regularly rise to ground level. This is because it cannot flow away as easily at this location as near the power tube that flows into a ditch. This creates a slope in the groundwater level, the so-called hydraulic slope. Drains longer than 300 meters are also harder to clean. The distance between the drains is closely related to the permeability of the soil. The drain length times the drain distance gives the maximum surface with the desired minimum drainage of 7 millimeters per 24 hours. This surface is indicated per drain diameter in table 2. A variation in the hydraulic slope is also indicated.

Table 1. Maximum surface (in Ha) for a ribbed drain pipe with a discharge of 7 mm per day
(in mm)
Maximum surface to be dewatered in ha at a hydraulic slope in cm per 100 meters of:
5 8 10 15 20 25 30 40
50 0,11 0,15 0,18 0,23 0,28 0,32 0,36 0,44
60 0,23 0,31 0,36 0,48 0,58 0,67 0,75 0,91
65 0,28 0,38 0,44 0,57 0,69 0,81 0,91 1,10
80 0,61 0,84 0,97 1,27 1,54 1,79 2,02 2,45
100 1,38 1,88 2,19 2,87 3,47 4,03 4,55 5,51

  • The number of the drain diameter refers to the outside diameter

  • The drain slope may be smaller, but not greater than the hydraulic slope.


A parcel is 300 meters long, the planned drain distance is 10 meters. The calculated area per drain is therefore 300 x 10 = 3000 m2. This corresponds to 0.30 ha. If the 50 mm pipe from table 2 is chosen, the hydraulic slope with a discharge of 7 mm per day will be between 20 and 25. This means that with this drain and a drain diameter of 50 mm, the groundwater at a distance of 100 meters from the end pipe is 20 to 25 cm higher than at the end pipe. At a distance of 200 meters, this is double that and with the maximum length of 300 meters, the groundwater at the location of the drain at this outlet is even 60 to 75 cm higher than the end pipe. With a drain depth of 90 cm, this means that the soil is too wet, whereby the groundwater between the drains even reaches ground level due to bulging. It is therefore recommended to assume an external drain diameter of 60 mm for this example. In this case, the discharge is achieved at a hydraulic slope of 8 cm per 100 meters. At the maximum length of 300 meters, in this case, the groundwater level is 25 (3x8 cm) higher than the power pipe. 

The slope of the drain

In practice, the drain is often installed under a slope in the soil. A commonly used slope is 10 cm per 100 meters. This is converted 1 mm per meter. With a drain pipe of 300 meters in length, this means that the drain at the maximum distance from the end pipe is 30 cm higher than at the end pipe. As soon as a drain stops discharging, the groundwater will be at a distance of 300 meters from the end pipe, so 30 cm higher, than at the end pipe. With a flat drain, meaning without a slope, the groundwater level is the same everywhere when the drainage stops. As a result, the groundwater at the maximum distance from the end pipe is 30 cm lower than in the first example. A flat drain pipe, therefore, runs longer and therefore ensures a lower groundwater level over a great length of the drain. A flat drain also has the advantage of a lower groundwater level during a drainage situation. Figure 3 shows the difference in groundwater levels between a flat and a sloping drain, in a situation with and without drainage. In this case, the drain is 100 meters long. When constructing a drain pipe, a maximum length of 300 meters applies. Of course, this also applies to a flat drain. This length has to do with the difference in drainage depth at the beginning and at the end of a drain and with the rate of drainage.


Groundwater levels with and between the drain (bulging) 

The groundwater level at the place where the drain is located differs from that between the two drains. This is because so-called “bulging” occurs during a period of discharge. The maximum bulging that occurs is expressed in the average highest groundwater level (AHG). The standards we use for this are listed in the chapter “Purpose of drainage”. The construction of drainage is nothing more than influencing the average highest groundwater level and is therefore directly related to the bulging. The degree of bulging is determined by the;

  • Soil type and soil structure
  • Drain distance
  • Amount of rainfall


Soil type and soil structure

The flow possibilities of the water can differ per soil type. This is expressed in a measure of permeability. The worse the permeability, the less quickly the water will flow through the soil. The bulging will become bigger in this case. The bulging is much stronger on heavy clay soil than on coarse sandy soil. This is visible in the example of figure 4. The permeability can differ in a horizontal and vertical direction. This is especially the case in soils that have been deposited under the influence of water. It is also possible that the permeability changes over time. Due to swelling and shrinkage and natural ripening, permeability can improve. Compaction can deteriorate permeability. The temperature of the soil also plays a role. We can express the permeability in the so-called transmission factor, expressed in K. The rating is stated in table 3. The permeability factor is expressed in meters per 24-hour period. In this case, it concerns the saturated permeability. This means that all pores are filled with water. Table 4 shows the permeability factors for several soil types. Changes in the flow factor have a major influence on the drainage condition of a field. For example, compaction of the top layer can cause puddles to form on a plot, which previously did not occur. On a drained plot there is a risk of complaints about the functioning of the drainage, while the cause lies elsewhere. 

Drain distance

The drain distance is determined to a large extent by the degree of bulging that we tolerate. In addition, the possible drainage depth present plays a role. This is indicated in figure 5. Due to the greater drain depth (1.5 times), the drain distance may also be one and a half times greater, this way the same drainage depth is still obtained. The drain depth is often determined by the ditch depth and height of the ditch water level. However, a larger drain distance also means a larger surface per drain with the same length of the drain. A larger amount of water will have to be drained through it at the same time. The drain diameter will therefore also have to be larger to prevent a greater hydraulic slope.


Amount of rainfall

The amount of precipitation determines the extent to which the drains discharge water. Since there is more rainfall in our country than can evaporate, water will have to be discharged. This surplus of precipitation largely determines the possible use of the parcels. Apart from the amount of surplus, the frequency with which it occurs is particularly important for use. In an average year, there is a precipitation deficit during the growing season. But outside the growing season, there is a surplus. As a result, drains mainly perform their task outside the growing season. Bulging of groundwater level therefore mainly occurs during that period. The speed at which rainfalls also influences the groundwater level. For example, a shower of 14 mm, with a calculated discharge capacity of 7 mm per day, will need two days to regain equilibrium. If this rain falls within a time frame of three hours, the bulging will be stronger than if the rainfall within a time frame of 6 hours. If this amount of precipitation is spread over three days, there will be hardly any increase in the groundwater level. In the first case, the chance of puddling is high. In the latter case, hardly anything will change in the soil. Puddles can therefore mainly occur when a lot of precipitation falls in a short time. Figure 6 shows how often there is a chance of a large amount of precipitation in a short time. The data is taken from data over a long series of years and refers only to the winter period. The precipitation situation is very important in assessing whether or not the drainage is functioning properly. In addition, it is necessary to have an impression of the seepage that may play a role as a result of supplies from higher areas.



Drainage of water through drains

If the water can flow smoothly into the drains and if it is not hindered by the casing, smooth drainage is possible. But if the drain diameter is such that the surface area per drain is almost maximum, then the chance of a periodic increase in the groundwater is greater. When the drain is contaminated, the drainage can also stagnate. Also, damage or disruption of the end tube may cause this. The discharge we want has a lot to do with the package of requirements we set. For grassland, a discharge capacity of 7 mm per day is sufficient. 10 mm or more is desirable for arable farming and maize. A larger drainage capacity requires a larger drain pipe diameter and/or a smaller drain distance for a comparable length.

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