# D-Infinity Avalanche Runout

(c) 2010 by David G. Tarboton
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:

- rz — A runout zone indicator with value 0 to indicate that this grid cell is not in the runout zone and value > 0 to indicate that this grid cell is in the runout zone. Since there may be information in the angle to the associated source site, this variable will be assigned the angle to the source site (in degrees)
- dm — Along flow distance from the source site that has the highest angle to the point in question

## Parameters

Number of Processes
Integer
  The number of stripes that the domain will be divided into and the number of MPI parallel processes that will be spawned to evaluate each of the stripes.
D-Infinity Flow Direction Grid
Raster Grid
  A grid giving flow direction by the D-infinity method. Flow direction is measured in radians, counter clockwise from east. This can be created by the tool "D-Infinity Flow Directions".
Pit Filled Elevation Grid
Raster Grid
  This input is a grid of elevation values. As a general rule, it is recommended that you use a grid of elevation values that have had the pits removed for this input. Pits are generally taken to be artifacts that interfere with the analysis of flow across them. This grid can be obtained as the output of the "Pit Remove" tool, in which case it contains elevation values where the pits have been filled to the point where they just drain.
Avalanche Source Site Grid
Raster Grid
  This is a grid of source areas for snow avalanches that are commonly identified manually using a mix of experience and visual interpretation of maps. 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.
Proportion Threshold
Double
  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.
Alpha Angle Threshold
Double
  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.
Measure distance along flow path
Boolean
  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.

## Outputs

Runout Zone Grid
Raster Grid
  This grid Identifies the avalanche's runout zone (affected area) using a runout zone indicator with value 0 to indicate that this grid cell is not in the runout zone and value > 0 to indicate that this grid cell is in the runout zone. Since there may be information in the angle to the associated source site, this variable will be assigned the angle to the source site (in degrees).
Path Distance Grid
Raster Grid
  This is a grid of the flow distance from the source site that has the highest angle to each cell.