2D Boundary Conditions
After identifying the flood-prone subset of your project and delineating a computational grid, you will want to define 2D boundary conditions. The boundary conditions (and associated hydrological interactions) for the 2D grid area are set up with a combination of calculation options, grid initial settings, and layout of dedicated elements in the drawing.
Boundary Conditions – Calculation Options
Within the calculation options, the Rainfall Method option allows user to declare watershed boundaries of their model space. The option of the Minimum Depth on Grid is a way to define the wet and dry portions of your 2D space. When the surface depth on grid cells has yet to exceed this minimum, the cells are effectively dry in relation to the grid cells that are computed at that time step of the simulation. To further delineate the boundary between the wet and flooded portions of the grid, use the threshold option Minimum Flood Arrival Depth.
The flow onto and off the grid can be setup to occur at a subset of the boundary grid cells. Boundary grid cells are those around the perimeter of the extents of the computational grid, as well as the cells which share a face with an internal void cell. One way to limit the flow between boundary cells to prevent inflow from off the grid using the Allow Boundary Inflow? calc option. Alternatively, you can declare a project-wide elevation threshold with the Maximum Terrain Elevation for Boundary, so that the higher elevation boundary cells have a closed boundary.
Boundary Conditions - Grid
Several attributes of the Grid element provide the user the flexibility to set some default and grid-wide boundary conditions. A grid element is initialized with a Closed boundary. But one can declare a constant or variable tailwater boundary around the perimeter which could purposefully inhibit ponded water from leaving the 2D modeling space. Again, combined with the calc option setting to not Allow Boundary Inflow you can prevent any back flow onto the grid’s outer boundary.
Boundary Elements
You can establish boundary conditions (either flow or elevation-based conditions) at specified locations on the computational grid using boundary elements.
There are two types of boundary elements:
- Boundary Point - a discrete point source for either flow injection onto or removal from the computational grid.
- Boundary Line - a spatially distributed transfer of flow onto the computational grid, or a tailwater conditions along the edge of a grid.
Hydrologic Grid Connections
In addition to 2D Boundary Points and 2D Boundary Lines distributing catchment runoff onto the computational grid, other hydrologic connections are possible. Rainfall can directly fall atop the grid surface area. An outfall or headwall element that is located within a 2D computational grid area will simulate the terminus of an upstream 1D network draining to a gridded flood zone. Also, the upstream-most node (i.e. Transition node element) of a 1D subnetwork placed within the grid space will provide a flow path from the corresponding 2D grid cell to the 1D model.