Shallow Water

Owned by Bui Ngoc Son (Deactivated)
Last updated: 07 Jun, 2023 by Kester Scandrett
2 min read

Estuarine water bodies, wetlands other systems are subject to wetting and drying processes due to surface water elevation changes. So, the numerical model must be capable of simulating the wetting and drying processes realistically in shallow water.

The Shallow Water tab is shown in Shallow Water#Figure 1 below. These Wetting and Drying Options are simplified compared in EFDC+ from release of version 10 compared to the previous release ISDRY options, and cell skipping is used to improve computational speed for all options. The user may select from three options:

  • Do not allow cells to wet and dry (ISDRY = 0)
  • Use wet/dry with non-linear iterations (ISDRY = 11)
  • Use wet/dry with non-linear iterations and face masking (ISDRY = 99)

The Dry Depth defines at what depth a cell can be considered dry. The general approach is that when the depth in a cell center falls below a defined dry depth, the cell faces will be blocked, and the flow across these cell faces are forced to zero. If we consider one dry cell, for example, the difference between the two options provided in EFDC+ is that in the first option (ISDRY = 11), all the four cell faces of the identified "dry cell" are blocked, so there is no flow coming into from its neighbor cells. However, in the second option (ISDRY = 99), if one of the neighbor cells of the identified dry cell is considered wet, the face between these two cells will not be blocked and will still allow the water flow to cross and wet the "dry cell" through this face.

When using wetting/drying the Minimum Height in the Water Depth ICs (Legacy) frame should be less than the Dry Depth. If this is not the case, the initial water depths everywhere will force all of the model's cells to "wet", even if they should be dry.

The Dry Step is used when there is a wet cell that is isolated or perched and which has a very small depth. In this case, after a certain number of time steps set by this Dry Step value, EFDC will waste/drain the water. The cell is wasted if there is no inflow to the cell for a period of time equal to (number of dry steps) x (model time step). The dry step should be a positive number. EFDC continues to keep track of this cell and the volume of water removed from the system to calculate the mass balance.

If non-linear options are selected, then the Only Reset SUB/SVB Flags When Needed checkbox is available. By default, the SUB/SVB flags are reset to open/wet every iteration. The CALPUV/CONGRAD process then iterates to find the dry cells and turns off those cells. This checkbox option allows speeding up wetting and drying iterations but will cause a slight difference in results. This option to only reset cells that require resetting is available for 2TL only.


Figure 1. Hydrodynamic Options: Wetting and Drying.