Attribute:

HREI [-], (node-wise) (3D models only)

Waterworks can operate wells in so-called siphon lines. With this mode of operation it is not possible to control the individual rates of the wells. When operating these wells under extreme conditions, it is possible for individual wells in a siphon row to fall below their permissible drawdown target due to neighbouring, deeper wells and thus cause the total failure of the entire siphon line.

A polder well is a well that meets a defined drawdown target, but under the restriction of not exceeding a defined maximum possible withdrawal rates - usually the performance limit of the pump. If the free groundwater surface in one or more wells in a siphon series falls below the permissible drawdown target, e.g. due to neighbouring cones of depression, the wells in the entire series are switched off and the boundary conditions at the affected element mesh nodes are cancelled ("total failure" principle). The following figure shows the principle of a siphon line:

The creation of such a siphon line in the groundwater model is carried out according to the following scheme: The mesh nodes that make up the siphon line are assigned a unique group number (HREI). The permissible drawdown target is defined for each of these nodes as a reference water level for watercourses (VORF) with a quasi-infinite leakage coefficient (LERA). In addition, there is a restriction on the extraction rate (MXKE), e.g. via the performance limit of the pump, as well as a restriction of the infiltration rate (MXKI) to zero. The latter restriction is therefore absolutely necessary, as a siphon line node is still a watercourse node (Cauchy boundary condition) in terms of its representation in the model, thus the leakage rate can be positive or negative, and it can therefore result in a water influx to the groundwater if the groundwater level is lower than the reference water level for the watercourse.