Correctly modelling pressurised pipes, variously described as forcemains or rising mains, can be one of the more difficult aspects of the model build process.
The fundamental problem is that no equations have yet been developed which adequately represent the transition from free surface flow, such as exists in open channel or pipes which are not completely full, to pressurised flow which exists in pipes which are completely full. Pressurised flow can exist either intermittently, for example in storm pipes which become surcharged during rainfall, or permanently, for example in pumped systems or siphons.
Pipes which are predominantly free surface and only rarely surcharged are modelled using the Preissmann Slot approximation. This uses a narrow slot, which runs the length of the conduit and extends vertically up to infinity. The width of this slot can be edited, but by default is 2% of the pipe width.
The Preissmann slot allows a free surface to be maintained and therefore a transition does not occur. The slot makes pipes slightly larger than in reality. This is usually compensated for by the base flow depth, which makes the pipes slightly smaller and by manually decreasing the size of the manholes, using the storage compensation feature, so that the correct volume of the below ground system is represented.
The Preissmann slot is not recommended in pipes which are permanently or very heavily surcharged (i.e. highly pressurised).Therefore a dedicated pressurised pipe model must be used, which does not include the Preissmann Slot or base flow, approximations. There are two options for doing this, the ‘Pressure’ or ‘ForceMain’ solution models, both specified as part of the conduit information.