From the tech doc:
QuoteMaximum fetch is calculated on a cell-by-cell basis at the beginning of each model time-step. Sixteen points of the compass are examined for every cell that borders on water (each 22.5 degrees). The maximum length of open water is calculated after examining all of these vectors. Tidal-flat erosion is assumed to occur at the open-water interface regardless of its calculated fetch.
In other words, SLAMM searches for a land cell that borders on a predicted water cell at the beginning of a time step. When this is found, it searches 16 lines (N,S,E,W,NE,SE,NW,SW,NNE,NNW, etc.) until it finds dry land again. The maximum distance of open water is assigned to the Maximum Fetch. If SLAMM does not find dry land (open water persists to the edge of a study area) it assumes that fetch is > 9km at that location. SLAMM usually predicts cells are open water when their land elevation falls below MLLW.
Hope this is useful -- Jonathan