Size and diameter of the drain pipe and surface per drain

Drainage pipes can have various sizes and diameters. Drains with a larger diameter are of course more expensive. The use of unnecessarily large drains, therefore, means unnecessary costs. Small drains, however, transport less water at the same time, which means that it takes longer for the water to drain. This creates high groundwater levels over a longer period. The choice of the right diameter is, therefore, an important choice. Several factors play a role in this choice. The size of the drain is closely related to the amount of water that a pipe has to drain in a given time. The more water, the larger the diameter must be to prevent the groundwater from rising unnecessarily over a long period. Many drain pipes also contain a certain amount of dirt. This dirt impedes the drainage of water. When calculating the required size of the drain, we assume that there may be ten to twelve millimetres of dirt in the drain. Something that also occurs quite often with well-maintained drains. It is also assumed that a low-pressure head is required for the water to flow through the pipe. Together with the amount of water that a drain pipe must drain in a certain time (for example, 7 millimetres per day for grassland), we can calculate the connection between the drain diameter and the maximum surface area to be drained per drain.


Important information for calculating the diameter and the area per drain is the slope of the drain. With a horizontal drain, the water discharge stops when the groundwater level falls below the drain depth. The groundwater level will then be the same depth over the entire length of the drain. (Figure 17 A). If the groundwater level rises as a result of precipitation, flow is created in the drain, and water is drained away. The groundwater level hardly rises near the end pipe, because the water can easily flow away. The greater the distance to this power tube, the higher the water will be because it encounters more resistance. This creates a difference in pressure height between the start and the end of the drainpipe (figure 17 b.) This pressure height is also called ‘the hydraulic slope’. The slope increases if;

  • There is more precipitation
  • The drain distance is bigger because more water has to go through the same linear meters of drainage pipe
  • The length of the drain increases
  • The drains are more narrow


In practice, drains are often installed under a specific slope. This slope is derived from the hydraulic slope. The drain will be placed under the slope of the groundwater, this is created by horizontal drainage. A horizontal drain also stops the discharge of water when the groundwater level falls below the drain depth. Figure 17c shows that the groundwater level is not the same depth over the entire length of the drain. On a large part of the plot, the groundwater level is higher than intended. For example, if the drain has a slope of 10 cm per 100 meters, the groundwater level after 100 meters is 10 cm higher than at the location of the end pipe. If it is 300 meters long, the groundwater level will be 30 cm higher at the end. This means that a worse dewatering condition is accepted as the drains get longer. Due to the bulging of the groundwater between the drains, the groundwater here will reach ground level. In periods with precipitation and therefore drainage, the situation becomes worse, because even now there is a difference in pressure height between the beginning and the end of the drain, which creates a hydraulic slope (fig. 17 D). 

A deeper groundwater level can be achieved by laying the drains horizontally. During drainage, a difference can occur in the groundwater level at the beginning and the end of the drain, but this will disappear as soon as the drainage stops.

Maximum surface per drain

Determining the maximum surface to be dewatered per drain depends on the slope of the groundwater level. This is accepted as soon as the drains are discharged according to the standard. For grassland, for example, this is 7 mm per day. A choice is therefore made regarding the permissible difference in groundwater level between the start and the end of a drain. This so-called hydraulic slope is shown in table 8. The associated area that can be discharged through a pipe of a certain diameter can be gathered from this.

Example 1

A drain distance of 12 meters, with a drain length of 200 meters and a permitted hydraulic slope of 6 centimetres per 100 meters. The area that drains per drain is 12 x 200 = 2400 m² or 0.24 hectares. The table shows the number 0.26 for a drain diameter of 60 mm for a slope of 6 cm. This drain can therefore discharge 0.26 ha of water at a slope of 6 cm. The given area is smaller and therefore acceptable. The drain distance could also have been 13 meters, or the drain could have been 217 meters long for the same distance (2600: 12 =).


Example 2

The drain distance is 10 meters, the drain length 350 meters and the permitted drain slope is 10 cm per 100 meters.

The surface area per drain is 3500 m² or 0.35 ha. The table shows a hydraulic slope of 10 cm, the number 0.36, with a drain diameter of 60 mm.

This pipe can therefore discharge water of 0.36 ha, which is just enough. If the drain distance becomes 20 meters, the area will be 20 x 350 = 7000 m² or 0.70 ha. A larger diameter drain is required for the same hydraulic slope. A drain of 80 mm is sufficient in this example. This can discharge 0.97 ha of water. This drain is more expensive per meter than a drain with a diameter of 60 mm. But because the distance is much greater, fewer meters are enough. To be able to use a cheaper drain, for example, a 65 mm pipe, the slope should be 20 cm. This means that the groundwater rises by 20 cm per 100 meters in length during periods of discharge. This is 70 cm over a length of 350 meters. With a drain depth of 80 cm, the water above the drains is only 10 cm below ground level. It will therefore be necessary to opt for a narrower drain distance or a drain with a larger diameter.

Example 3 

A drain length of 100 meters, a permissible hydraulic slope of 10 cm per 100 meters, and a plot width of 100 meters. Whit the use of a drainpipe with a diameter of 60 mm, the maximum surface area to be drained is 0.36 ha = 3600 m². This means a drain distance of 3600: 350 = 10.3 meters, or more than 10 meters. On the 100-meter-wide plot, there are 10 distances of 10 meters and therefore 9 drain lengths are required. This means 9 times 350 meters = 3150 meters of drains. At the start of the drain, the groundwater level is 35 cm higher than at the end pipe. If a smaller difference is preferred, for example, 20 cm, then the hydraulic slope is; 20: 350 x 100 = almost 6 cm. This slope has a drain diameter of 60 mm, a maximum surface area of ​​0.26 ha, = 2600 m². The maximum drain distance is 2600: 350 = approximately 7.5 meters. With a plot width of 100 meters, 100: 7.5 = 13, and therefore 12 drains are required. This means 350 x 12 = 4200 meters of drainage pipe for the entire plot. In this case, the 15 cm less slope at a length of 350 meters means 4200 - 3150 = 1050 meters, and more drainage is needed.

Example 4 

Based on the same data of example 3, a drain pipe with a diameter of 65 mm is now used. The table shows that with a hydraulic slope of 10 cm, this pipe can discharge water from an area of 0.44 ha, or 4400 m². This means a drain distance of 4400: 350 = 12.5 meters. For the 100-meter-wide plot, 8 drain distances and thus 7 drains are now required. The total drain length is then 2450 meters. With a hydraulic slope of 6 cm, the drain of 0.31 ha or 3100 m² of water can be drained. The drain distance is now 3100: 350 + approximately 9 meters. The plot now requires 100: 9 = 11, and you will therefore need 10 drains. The total drain length is now 3500 meters.

Cost accounting

In examples 3 and 4, the same plot was drained with two different hydraulic slopes and with two different drain diameters. If choosing one of these options, costs will also play a role. Assuming, in this example, a fixed price of 1 euro per meter for a drain pipe with a diameter of 60 mm, the total cost for a hydraulic slope of 10 cm is €3150,00. With a hydraulic slope of 6 cm, the costs will be €4200,00.

With this information, a calculation can be made what a larger diameter may cost. 2450 meters is required for a slope of 10 cm. Per meter this may cost 3150:2450 = €1,29.

3500 meters are required for a slope of 6 cm. Per meter this may cost 4200:3500 = 1,20. In this case, there is a fixed price per meter, but in reality, the price per meter will vary according to the number of meters to be drained. Oftentimes the more meters that are being installed, the lower the price will be per meter. 

The main factors influencing the price per meter are;

  • The diameter
  • Whether or not you use a casing
  • Type of casing
  • The number of meters to be installed at once

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