Identifies an avalanche’s affected area and the flow path length to each cell in that affacted area. All cells downslope from each source area cell, up to the point where the slope from the source to the affected area is less than a threshold angle called the Alpha Angle can be in the affected area. This tool uses the D-infinity multiple flow direction method for determining flow direction. This will likely cause very small amounts of flow to be dispersed to some downslope cells that might overstate the affected area, so a threshold proportion can be set to avoid this excess dispersion. The flow path length is the distance from the cell in question to the source cell that has the highest angle.
All points downslope from the source area are potentially in the affected area, but not beyond a point where the slope from the source to the affected area is less than a threshold angle called the Alpha Angle.
Slope is to be measured using the straight line distance from source point to evaluation point.
It makes more physical sense to me for the angle to be measured along the flow path. Nevertheless it is equally easy to code straight line angles as angles along the flow path, so an option that allows switching will be provided. The most practical way to evaluate avalanche runout is to keep track of the source point with the greatest angle to each point. Then the recursive upslope flow algebra approach will look at a grid cell and all its upslope neighbors that flow to it. Information from the upslope neighbors will be used to calculate the angle to the grid cell in question and retain it in the runout zone if the angle exceeds the alpha angle. This procedure makes the assumption that the maximum angle at a grid cell will be from the set of cells that have maximum angles to the inflowing neighbors. This will always be true of angle is calculated along a flow path, but I can conceive of cases where flow paths bend back on themselves where this would not be the case for straight line angles.
The D-infinity multiple flow direction field assigns flow from each grid cell to multiple downslope neighbors using proportions (Pik) that vary between 0 and 1 and sum to 1 for all flows out of a grid cell. It may be desirable to specify a threshold T that this proportion has to exceed before a grid cell is counted as flowing to a downslope grid cell, e.g. Pik > T (=0.2 say) to avoid dispersion to grid cells that get very little flow. T will be specified as a user input. If all upslope grid cells are to be used T may be input as 0.
Avalanche source sites are to be input as a short integer grid (name suffix *ass, e.g. demass) comprised of positive values where avalanches may be triggered and 0 values elsewhere.
The following grids are output:
This value is a threshold proportion that is used to limit the disperson of flow caused by using the D-infinity multiple flow direction method for determining flow direction. The D-infinity multiple flow direction method often causes very small amounts of flow to be dispersed to some downslope cells that might overstate the affected area, so a threshold proportion can be set to avoid this excess dispersion.
Default: 0.2
This value is the threshold angle, called the Alpha Angle, that is used to determine which of the cells downslope from the source cells are in the affected area. Only the cells downslope from each source area cell, up to the point where the slope from the source to the affected area is less than a threshold angle are in the affected area.
Default: 18
This option selects the method used to measure the distance used to calculate the slope angle. If option is True then measure it along the flow path, where the False option causes the slope to be measure along the straight line distance from the source cell to the evaluation cell.
Default: True
processing.runalg('taudem:dinfinityavalancherunout', -ang, -fel, -ass, -thresh, -alpha, -direct, -rz, -dfs)