A seepage boundary condition can be applied to any node, but they are usually restricted to those on the surface of the domain. If the hydraulic head at a seepage node rises above its elevation, it becomes a specified head node, and the pressure head is set to zero and water is allowed to flow out. If, subsequently, the hydraulic head drops below the node elevation, then it reverts to an unconstrained state.
The following instructions can be used as input to the Type instruction inside the Boundary condition...End instruction group to assign seepage boundary conditions:
Sets the input type to be a seepage boundary condition.
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boundary condition type seepage node set outflow end ! new specified head
This example shows that a time-value table is not mandatory, since a pressure head of zero is applied if the nodal head is above the nodal elevation. However, if you wish to turn the seepage boundary condition on or off, then this can be accomplished by including a time value table instruction:
boundary condition type seepage node set outflow time value table 0.0 1.0 10.0 -99999. end end ! new specified head
The value 1.0 at time zero is ignored, but the NODATA value -99999 at time 10.0 causes the boundary condition to be turned off and the nodes become unconstrained.
rgm Old text read: The following two instructions can be used to set up a seepage face. If the node was assigned a specified head or fluid flux value by a previous instruction then it will not be set as a seepage node. However, unlike the other flow boundary condition instructions, the seepage face condition will override a previous specified fluid flux condition.
The overall rate of fluid outflow for all seepage face nodes is given in the mass balance output sections of the prefixo.lst file.
Details of the seepage node calculations, including which nodes are currently acting as seepage nodes and the fluid flux exiting the domain at each active seepage node can be written to the .lst file by using the run-time debug utility instruction Write seepage face output to .lst file described in Section C.