I used the overwash model with different percentages of marsh loss (0%, 50%, 75% and 100%) to understand how this process is modelled.
Although I understand that the criteria for the overwash is the barrier island to be less than 500m, I cannot understand which cells are subjected to overwash. For my case for example 75% of the marsh should be lost. Is this random or based on some criteria?
Thank you in advance
Since we started generally working with cell sizes less than 30m, the state of our practice has been to not use the overwash model. It produces streaky unreasonable output at under 30M and we have not had funding to update and refine the model.
That being said, the model is described in the Tech Doc as follows:
As erosion of backshore and dune areas occurs and as other lowlands are submerged, wetlands on the lee side of coastal barriers are subject to conversion due to overwash, the process by which sediments are carried over the crest of the barrier and deposited onto adjacent wetlands. This process is simulated only for areas having a beach and only during the time step in which the lowland is breached.
SLAMM default assumptions are that 50% of the adjacent transition marsh and salt marsh, and 25% of mangrove (if present) in the adjacent 500m to lee is converted to beach and tidal flat; the percentages are professional judgment based on observations of existing overwash areas (Leatherman and Zaremba, 1986). Beach migration occurs as well with estuarine beach within 500m of the ocean beach advancing by 60m. The front edge of the ocean beach will recede by 30m. Dry land adjacent to ocean beach will be converted to ocean beach. These professional judgments may now be edited as part of the sub-site input parameters.
SLAMM 6 assumes that a barrier island or narrow peninsula is present when there are estuarine waters, salt marsh, scrub-shrub, irregularly flooded marsh, or mangroves within 500m to lee of the front edge of an ocean beach. The user may specify the frequency of large-storms during which overwash is predicted to occur. All overwash effects are estimated to be localized to the 500 meters to the lee of the front edge of the ocean beach. The 500 meter cutoff may also be edited as a model parameter.
The parameters are described below along with their defaults:
Max Width Overwash (m) 500
The model needs to interpret which lands are subject to overwash. In this case, a horizontal determination is made. If a spit is found with ocean beach on one side and fresh waters on the other of less than 500 meters then every large storm this spit is assumed to be subject to the same overwash.
Beach to Ocean Overwash (m) 30
This much ocean beach is converted to open ocean from an "overwash event."
Dryland to Beach Overwash (m) 30
If dry land is up against beach, this much dry land is assumed to convert to beach as part of the barrier island migration.
Estuary to Beach Overwash (m) 60
On the back side, this much estuary is assumed converted to beach as part of the migration.
Marsh Pct Loss Overwash (%) 50%
Mang. Pct. Loss Overwash (%) 25%
If marsh and mangrove are present on the horizontal spit being overwashed this much of these categories are assumed lost (converted to beach), distributed evenly across the island.
This model, while very simple, has been applied with site specific data by USFWS in modeling Chincoteague island. These parameters can be varied on the subsite basis. Still, we hope to make improvements in this model some time in the near future.