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The hazard map shows the areas that are prone to debris flows. The larger the slope and the lesser the vegetation, the greater the chances for debris flows to occur. We extracted a relationship for the study area that adequately describes the interplay between these two factors, the intensity of vegetation and the steepness of the slope. Using known locations of debris flows (from field and satellite observations), we extracted the slopes and NDVI values for picture elements (pixels) that we identified as representing the onset points for each of these debris flows. These values were plotted on an X-Y plot, where the slope is plotted on the X-axis and the NDVI value on the Y axis. A linear regression was then used to identify the equation of a straight line that best fitted the points with the steepest slope and the smallest NDVI values. Knowing the equation of the straight line, we can then substitute all pixels of all streams in the equation to test whether the examined stream pixel is on the line (value = 0), above the line (value: +ve), or below the line (value: -ve). A coverage was constructed from points that are on or above the line; this coverage represent locations along mapped stream lines that are susceptible to debris flow. An additional condition was enforced, the positive points had to be outside the areas mapped as terraces. |