The purpose of this design rule is to identify locations for placement of air-valves and wash-outs. These chambers are an important cost constituent and early indication of their preferred location might help identify risks early in the project. This design rule only provides high-level information and any detail produced requires further refinement by experts.

This design rule is coupled another design rule which ensures minimum and maximum gradients. The key difference is that this design rule identifies changes of gradient and based on these, places appropriate chambers.

Air-valves have to be placed at high-points of the pipeline where air tends to accumulate naturally. Washouts have to be placed at low points of the pipe where the water would flow naturally.

These high and low points aren’t global high- or low-points - they are found locally due to the ‘sawtooth’ pattern of the pipe. These local high- and low-points are found by observing gradient changes.

When the pipeline goes deeper, its gradient is negative. When the gradient switches to positive, a local low point is created. Washout should be placed in such a spot - otherwise the pipeline would be at risk of developing sections where water couldn’t be drained properly. In short: washouts are placed where pipeline gradient changes its sign from negative to positive.

Conversely, if a pipeline is going up (positive gradient) and changes its gradient to negative, a local high-point is created. Air-valve should be placed in such a spot - otherwise the pipeline would be at risk of developing sections where air will accumulate. In short: air-valves are placed where pipeline gradient changes its sign from positive to negative.

The figure above shows a simple example of a pipeline section with correct locations of wash-outs (downward pointing triangle) and air-valves (upward pointing triangle).

In some special cases, additional air-valves might be required when gradient remains positive, but decreases abruptly. Such as situation is depicted above. This situation doesn’t occur frequently but might play a role in critical crossings, where the pipeline is making abrupt gradient changes as it is crossing under a river or a road. This constitutes an exception that can be summarised as: although air-valves are usually placed where gradient changes from positive to negative, there is a risk that an additional air-valve might be required where there is an abrupt decrease of the gradient, even though it remains positive. Since the additional air-valve is not always required, this is calculated as risk and highlighted to the designer as a case that requires further investigation.

In some cases, users might want to restrict the maximum spacing between subsequent air-valves, with a common maximum distance usually in the order of a few hundred meters. The design rule first places air-valves due to gradient changes (as described above) and then checks if there are any gaps between air-valves that are larger than the required limit.

If that is the case, additional air-valves are then placed in the ‘gaps’, at equal distances, to make sure that after the additional air-valves are added, the distance between two adjacent air-valves never exceeds the maximum distance. This part of the design rule ensures the spacing but doesn’t check if the additional air-valves are placed optimally, with the assumption that users will do adjustments manually.

The objectives are calculated by finding the total number of air-valves and wash-outs required and assigning the cost to each of these instances. Moreover, the cost of risk associated with additional air-valves is added. The positions and exact numbers are exported in the metadata while the total cost is used in optimisation.

This design rule doesn't require a dedicated dataset and draws terrain data from the 'elevation' dataset.

N.B.: If the 'Maximum distance between two subsequent air-valves is set to 0 meters, the functionality which places additional air-valves is turned off. Minimum sensible value for this parameter is around 100m as otherwise air-valves would be placed very densely and in incorrect settings.