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The hazard map shows the areas that are prone to debris flow caused by overland flow on sparsely vegetated slopes. Given the steep slopes of the mountains in the terrain, the high precipitation, one would expect that overland flows must play a role in transporting eroded material down slope (Horton, 1933). As is the case with organized debris flows along streams, one would expect that in general, the steeper the slope, and the less the vegetation, the more likely overland flows will occur. Again, we considered that areas covered by terraces are less likely to be susceptible to overland flow. We used a minimum slope of repose of 40° as a condition for overland flow to occur. This angle was selected based on reported angle of repose for rock units similar to the ones in our study area, such as granites (Barton 1974, Wylie 1992). Another condition that we set for overland flow to occur is sparse vegetation. The lesser the vegetation, the more likely that overland flow will be effective in transporting debris down slope (Horton, 1933; Scott 1971; Wells et al. 1987; Weirich 1989; Florsheim et al. 1991). Only barren or nearly barren slopes were considered as subject to overland flows. Only the steep slopes with NDVI values of less than 0.09 were considered prone to overland flow occurrence. The third condition has to do with the presence or absence of terraces. Overland flow is less likely to occur in areas where terraces were established. Terraces have the effect of increasing infiltration and reducing runoff. |