Shallow Water
Estuarine water bodies, wetlands, and other shallow systems are subject to wetting and drying as surface water elevation changes. To simulate these systems realistically, the model must handle wetting and drying accurately.
The Shallow Water tab (Figure 1 ) lets you configure wetting and drying behavior. Starting in version 10, EFDC+ simplified these options compared to the earlier ISDRY settings and added cell skipping to improve computational speed across all of them. Choose from three settings:
Do not allow cells to wet and dry (ISDRY = 0)
Use wetting/drying with non-linear iterations (ISDRY = 11)
Use wetting/drying with non-linear iterations and face masking (ISDRY = 99)
Dry Depth sets the depth at which a cell is considered dry. When a cell’s center depth drops below this value, its faces are blocked and flow across them is forced to zero. The two non-linear options differ in how they treat a dry cell’s neighbors: with ISDRY = 11, all four faces of a dry cell are blocked, so no flow enters from any neighboring cell. With ISDRY = 99, if a neighboring cell is wet, the shared face stays open, letting water flow in and rewet the dry cell.
When using wetting/drying, set Minimum Height (in the Water Depth ICs (Legacy) frame) lower than Dry Depth. Otherwise, the initial water depth will force every cell in the model to start “wet,” even ones that should start dry.
Number of Time Steps Before Water in Cell Goes Dry applies to isolated or perched wet cells with very small depths. If such a cell gets no inflow for a period equal to (this value) x (model time step), EFDC+ removes its water. The value must be positive. EFDC+ continues tracking the cell and the volume of water removed to maintain the model’s mass balance.
When a non-linear option (ISDRY = 11 or 99) is selected, the Only Reset SUB/SVB Flags When Needed checkbox becomes available. By default, EFDC+ resets the SUB/SVB flags to open/wet every iteration, then the CALPUV/CONGRAD process iterates to identify and close dry cells. Enabling this checkbox speeds up wetting/drying iterations at the cost of slightly different results. It’s available for the 2TL (two-time-level) scheme only.
Introduced in EEMS 12.0, the Moving QSER Inflow option lets flow keep entering the domain through boundary cells even after their water depth drops below the dry depth threshold. EFDC+ handles this by locating a deeper adjacent cell and temporarily routing flow through it. Setting this option’s minimum depth to zero disables it.
