The Sea Level Affecting Marshes Model (SLAMM) was developed with EPA
funding in the mid 1980s (Park et al. 1986). Since that time, the model
has continued to be developed and applied resulting
in the release of SLAMM 5 in April 2007.
Within SLAMM, there are four primary processes that affect wetland fate under different scenarios of sea level rise:
· Inundation: The rise of water levels and the salt boundary is tracked by reducing elevations of each cell as sea levels rise, thus keeping MTL constant at zero. The effects on each cell are calculated based on the minimum elevation and slope of that cell.
· Erosion: Erosion is triggered based on a threshold of maximum fetch and the proximity of the marsh to estuarine water or open ocean. When these conditions are met, horizontal erosion occurs at a rate based on site specific parameters.
· Overwash: Barrier islands of under 500 meter width are assumed to undergo overwash during each 25 year time-step due to storms encountered. Beach migration and transport of sediments are calculated.
· Saturation: Coastal swamps and fresh marshes can migrate onto adjacent uplands as a response of the water table to rising sea level close to the coast.
Relative sea level change is computed for each site for each time step; it is the sum of the historic eustatic trend, the site-specific rate of change of elevation due to subsidence and isostatic adjustment, and the accelerated rise depending on the scenario chosen. Sea level rise may be offset by sedimentation and accretion. In the absence of site-specific data, average values are used, depending on the extent of existing marshes (the assumption being that extensive marshes indicate higher accretion rates). Marshes not adjacent to water are assumed to have rates half those adjacent to water. The model is not sensitive to these assumptions for higher sea level rise scenarios, but the predicted changes in wetlands for a 50-cm rise in an area with low subsidence could vary by as much as 50% if the accretion rates were twice as great (the maximum deviation expected). The time step of 5 to 25 years depends on the sea level rise scenario chosen; a shorter step is used for lower scenarios. For each time step the fractional conversion from one class to another is computed on the basis of the relative change in elevation divided by the elevational range of the class in that cell. For that reason, marshes that extend across wide tidal ranges are only slowly converted to unvegetated tidal flats.
If a cell is protected by a dike or levee it is not permitted to change until 2 meters of sea level rise is predicted. For a standard simulation, cells that are largely developed are assumed to be protected by dikes as necessary to prevent inundation. The existence of these dikes can severely affect the ability of wetlands to migrate onto adjacent shorelines.
In addition to the effects of inundation represented by the simple geometric model described above, second-order effects occur due to changes in the spatial relationships among the coastal elements. In particular, the model computes exposure to wave action; if the fetch (the distance across which wind-driven waves can be formed) is greater than 9 km, the model assumes moderate erosion. If a cell is exposed to open ocean, severe erosion of wetlands is assumed. Ocean beach erosion is modeled using a relationship reported by Bruun whereby recession is 100 times the change in sea level. Wetlands on the lee side of coastal barriers are subject to conversion due to overwash as erosion of backshore and dune areas occurs and as other lowlands are drowned. Erosion of sandy areas to maintain equilibrium with adjacent beaches is modeled, but erosion of other dry lands is ignored. This could seriously underestimate the availability of sediment to replenish wetlands where accelerated bluff erosion could be expected to occur. Coastal swamps and fresh marshes migrate as a response of the water table to rising sea level close to the coast.
In 1998 and 1999, Eco Modeling , Stratus Consulting Inc., and Jonathan Clough (now with Warren Pinnacle Consulting, Inc.). performed extensive modifications to SLAMM under contract to the U.S. EPA Office of Planning and Policy Evaluation. These changes included an upgrade into a 32 bit operating system, a user friendly interface, the capability to export to GIS ready data, and a screen display component. Mapping categories were also expanded to twenty categories to comply with existing National Wetlands Inventory data and the cell size was reduced to 30 meters by 30 meters to allow for significantly more data complexity. Below are some screen-captures from SLAMM 4.0.
For more information, please download the technical documentation available on the main SLAMM page.
