seep2d - geotechnical model - geotechnical modelling software


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seep2d boundary conditions...

Two general types of boundary conditions can be defined for SEEP2D simulations: nodal boundary conditions and flux boundary conditions.

Nodal Boundary Conditions

The most common type of SEEP2D boundary conditions is nodal boundary conditions. Nodal boundary conditions are assigned by selecting the nodes and selecting the Node BC command in the SEEP2D menu. When selecting the nodes, either the Select Node tool or the Select Node String tool may be used. The Node BC command brings up the Node BC dialog. Three types of nodal boundary conditions can be assigned: head, exit face, and flow rate.

Head BC
Specified head boundary conditions represent boundaries where the head is known. They typically are found where water is ponding or at the boundary of a region where the water table is known to remain constant. Since the head along such boundaries cannot change, they represent regions of the model where flow enters or exits the system (flow lines are always orthogonal to constant head boundaries).

Exit Face BC
Exit face boundary conditions imply that the head is equal to the elevation (assuming that the datum is 0). They are used when modeling unconfined flow problems and should be placed along the face where the free surface is likely to exit the model. This boundary condition must be used if the option for deforming the mesh to the phreatic surface has been selected in the analysis options. It may also be used with a saturated/unsaturated flow model. In this case, if the head at a node on the boundary becomes greater than the node elevation during the iteration process, the head at the node is fixed at the nodal elevation and the node acts as a specified head boundary. Thus, water is allowed to exit the boundary above the tailwater. If an exit face boundary is not used with a saturated/unsaturated flow model, all of the flow will be forced through the tailwater.

Flow Rate BC
Flow rate boundary conditions are used to specify nodes at which a certain flow rate is known to exist. They are used primarily when modeling wells and the flow specified represents the pumping rate. Negative values represent extraction of fluid from the system whereas positive values represent injection.

Flux Boundary Conditions

Flux boundary conditions are used to specify a known flux rate [L/T] along a sequence of element edges on the perimeter of the mesh. They are often used to simulate infiltration. Flux into the system is positive and flux out of the system is negative.

Flux BC are assigned by selecting a sequence of nodes along the mesh boundary using the Select Node String tool and selecting the Flux BC command in the SEEP2D menu. This command brings up a simple prompt for the flow rate.

Editing Existing Boundary Conditions

The type or value assigned to a previously defined boundary condition can be edited by selecting the node or node string and selecting either the Node BC or Flux BC commands.

Deleting Boundary Conditions

A boundary condition can be deleted by selecting the boundary condition using either the Select Node or Select Node String tools and selecting the Delete BC command in the SEEP2D menu.


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