Slope
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.