I was wondering if you could give me some background on how the default accretion rate parameters are set up (or arrived at).
We generally do not use "default" accretion rate parameters when possible but prefer to use site-specific or (lacking that) regional data.
To see many examples of how we set these parameters, download some of the reports from the following folders and search on accretion. You may also find sources of regional or site-specific accretion data within these reports.
Note also that SLAMM has two modes of operation, one which does not account for feedbacks between accretion and SLR and one that does. The latter approach is somewhat more complex and requires parameterization of a curve that describes the relationship between cell elevation and accretion rates (as seen in the Excel file that is delivered along with the SLAMM installer-- SLAMM6_Accretion_1-11-2010.xls) Also see the technical documentation for more information about using this accretion feedback model.
If historical accretion rates are used (no feedbacks) there is a danger that accretion rates may be undercounted under conditions of increasing SLR. Because of this, model results may predict more wetland loss than would be predicted with a feedback parameterized. On the other hand, the strength of the feedback between SLR and accretion can be uncertain so the curve can be difficult to parameterize. We have parameterized this curve using elevation-to-accretion data available, historical marsh-loss data to calibrate the strength of the feedback, or mechanistic 0-D sedimentation and accretion models.
Good luck with your project -- Jonathan