Saturated Flow:Boundary conditions
Solution of the 2-D, lateral flow, saturated groundwater equations requires that boundary conditions be assigned to every cell in the grid. A boundary condition map must be created that assigns an integer value representing the boundary condition to every cell.
Boundary conditions are assigned from the Map Module. The coverage is set to GSSHA Boundary Condition from the Coverages dialog box under the Feature Objects menu. Boundary conditions are established from feature points or arcs. The points or arcs are created over the grid cells to assign a boundary, and the type of boundary condition is selected from Attributes ... under Feature Objects menu. The option of which type of boundary conditions can be assigned as Attributes... depends on whether a feature node or arc is selected.
The following boundary conditions can be assigned:
- Regular infiltration cell (no special boundary condition)
- Specified head
- Dynamic flux, well (not currently active in GSSHA)
- Stream cell with calculated flux between stream node and groundwater cell
- Stream cell with a specified head
- Static flux (well)
This boundary type is typically assigned along the watershed boundary or along portions of the watershed boundary. This boundary condition represents a region where no lateral groundwater flow is permitted. When no-flow arcs are located along the watershed boundary, this is referred to as a groundwater divide. All cells not within the active watershed are no-flow boundary cells. No-flow boundaries are assigned with feature arcs.
The general type specifies that the cell is a regular infiltration cell, indicating no special boundary is desired. The general boundary condition is the default option. General boundaries can be assigned with either feature nodes or arcs.
A constant head boundary indicates that the groundwater level in that cell remains constant for the duration of the simulation. The value for the head boundary is taken from the initial values assigned with the water table map (See above). Specified heads are normally applied along the watershed boundary where known values of head exist. They may be assigned with either a feature node or arc.
This boundary indicates that a dynamic, temporally varying, pumping well exists in the cell. They may be assigned with feature nodes. This option is not currently supported by GSSHA.
This boundary condition indicates that the cell contains a stream node and the flux between the stream node and saturated groundwater below the stream node be calculated based on Darcy’s law, as described by McDonald and Harbaugh (1988), allowing an exchange of water between the stream and groundwater during every groundwater update. These boundaries are assigned with feature arcs.
When assigning flux river boundaries to a cell, the thickness of the bed material, M river (cm), and the hydraulic conductivity of the bed material, K river (cm/hr), must be assigned to the stream network as Attributes... to the GSSHA coverage. Only trapezoidal stream sections may be specified as flux river boundaries.
Another possible boundary condition for cells containing stream nodes is a head boundary condition, head river. This boundary condition indicates that for the purposes of the saturated groundwater simulations, the cell will behave as a constant head cell and will not be updated during the saturated groundwater calculations. The value of the head in the cell is set to be the elevation of the water surface in the stream node above the cell. In this case, the stream cell influences the groundwater calculations, but the groundwater calculation does not influence the stream calculations.
A static well boundary means that the cell will have a source/sink term of constant rate, m3×d-1. These boundaries are assigned with feature nodes. Select, or create and select, the node where the static pumping well is to be located. Select Feature Objects -> Attributes. Select static well from the drop down list and type in the pumping rate in the box labeled Pumping rate. Groundwater extractions have a positive sign; injections have a negative sign.