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120 lines
7.4 KiB
HTML
120 lines
7.4 KiB
HTML
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<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
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<h1 class='module'>D-Infinity Avalanche Runout</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Identifies an avalanche's affected area and the
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flow path length to each cell in that affacted area. All cells downslope
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from each source area cell, up to the point where the slope from the source
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to the affected area is less than a threshold angle called the Alpha Angle
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can be in the affected area. This tool uses the D-infinity multiple flow
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direction method for determining flow direction. This will likely cause
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very small amounts of flow to be dispersed to some downslope cells that
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might overstate the affected area, so a threshold proportion can be set
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to avoid this excess dispersion. The flow path length is the distance from
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the cell in question to the source cell that has the highest angle.</div>
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<div class='description'>All points downslope from the source area are
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potentially in the affected area, but not beyond a point where the slope
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from the source to the affected area is less than a threshold angle called
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the Alpha Angle.</div>
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<p align="center"><img src="img/arexample.gif"></img></p>
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<div class='description'>Slope is to be measured using the straight line
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distance from source point to evaluation point.</div>
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<div class='description'>It makes more physical sense to me for the angle
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to be measured along the flow path. Nevertheless it is equally easy to
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code straight line angles as angles along the flow path, so an option that
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allows switching will be provided. The most practical way to evaluate
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avalanche runout is to keep track of the source point with the greatest
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angle to each point. Then the recursive upslope flow algebra approach will
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look at a grid cell and all its upslope neighbors that flow to it. Information
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from the upslope neighbors will be used to calculate the angle to the grid
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cell in question and retain it in the runout zone if the angle exceeds
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the alpha angle. This procedure makes the assumption that the maximum
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angle at a grid cell will be from the set of cells that have maximum angles
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to the inflowing neighbors. This will always be true of angle is calculated
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along a flow path, but I can conceive of cases where flow paths bend back
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on themselves where this would not be the case for straight line angles.</div>
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<div class='description'>The D-infinity multiple flow direction field assigns
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flow from each grid cell to multiple downslope neighbors using proportions
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(<tt>P<sub>ik</sub></tt>) that vary between 0 and 1 and sum to 1 for all
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flows out of a grid cell. It may be desirable to specify a threshold <tt>T</tt>
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that this proportion has to exceed before a grid cell is counted as flowing
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to a downslope grid cell, e.g. <tt>P<sub>ik</sub> > T</tt> (=0.2 say) to
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avoid dispersion to grid cells that get very little flow. <tt>T</tt> will
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be specified as a user input. If all upslope grid cells are to be used
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<tt>T</tt> may be input as 0.</div>
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<div class='description'>Avalanche source sites are to be input as a short
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integer grid (name suffix *ass, e.g. demass) comprised of positive values
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where avalanches may be triggered and 0 values elsewhere.</div>
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<div class='description'>The following grids are output:</div>
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<ul>
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<li>rz — A runout zone indicator with value 0 to indicate that
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this grid cell is not in the runout zone and value > 0 to indicate that
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this grid cell is in the runout zone. Since there may be information in
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the angle to the associated source site, this variable will be assigned
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the angle to the source site (in degrees)</li>
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<li>dm — Along flow distance from the source site that has
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the highest angle to the point in question</li>
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</ul>
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<h2>Parameters</h2>
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<dl class='parameters'>
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<dt>Number of Processes <div class='type'>Integer</div></dt>
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<dd>The number of stripes that the domain will be divided into and the
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number of MPI parallel processes that will be spawned to evaluate each
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of the stripes.</dd>
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<dt>D-Infinity Flow Direction Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid giving flow direction by the D-infinity method. Flow direction
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is measured in radians, counter clockwise from east. This can be created
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by the tool "D-Infinity Flow Directions".</dd>
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<dt>Pit Filled Elevation Grid <div class='type'>Raster Grid</div></dt>
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<dd>This input is a grid of elevation values. As a general rule, it is
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recommended that you use a grid of elevation values that have had the
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pits removed for this input. Pits are generally taken to be artifacts
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that interfere with the analysis of flow across them. This grid can be
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obtained as the output of the "Pit Remove" tool, in which case
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it contains elevation values where the pits have been filled to the
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point where they just drain.</dd>
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<dt>Avalanche Source Site Grid <div class='type'>Raster Grid</div></dt>
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<dd>This is a grid of source areas for snow avalanches that are commonly
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identified manually using a mix of experience and visual interpretation
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of maps. Avalanche source sites are to be input as a short integer grid
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(name suffix *ass, e.g. demass) comprised of positive values where
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avalanches may be triggered and 0 values elsewhere.</dd>
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<dt>Proportion Threshold <div class='type'>Double</div></dt>
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<dd>This value is a threshold proportion that is used to limit the disperson
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of flow caused by using the D-infinity multiple flow direction method
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for determining flow direction. The D-infinity multiple flow direction
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method often causes very small amounts of flow to be dispersed to some
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downslope cells that might overstate the affected area, so a threshold
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proportion can be set to avoid this excess dispersion. Default <strong>0.2</strong>.</dd>
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<dt>Alpha Angle Threshold <div class='type'>Double</div></dt>
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<dd>This value is the threshold angle, called the Alpha Angle, that
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is used to determine which of the cells downslope from the source cells
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are in the affected area. Only the cells downslope from each source
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area cell, up to the point where the slope from the source to the
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affected area is less than a threshold angle are in the affected area.
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Default <strong>18</strong>.</dd>
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<dt>Measure distance along flow path <div class='type'>Boolean</div></dt>
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<dd>This option selects the method used to measure the distance used
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to calculate the slope angle. If option is "True" then measure
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it along the flow path, where the "False" option causes the
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slope to be measure along the straight line distance from the source
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cell to the evaluation cell. Default <strong>True</strong>.</dd>
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</dl>
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<h2>Outputs</h2>
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<dl class='parameters'>
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<dt>Runout Zone Grid <div class='type'>Raster Grid</div></dt>
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<dd>This grid Identifies the avalanche's runout zone (affected area)
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using a runout zone indicator with value 0 to indicate that this grid
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cell is not in the runout zone and value > 0 to indicate that this grid
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cell is in the runout zone. Since there may be information in the angle
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to the associated source site, this variable will be assigned the angle
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to the source site (in degrees).</dd>
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<dt>Path Distance Grid <div class='type'>Raster Grid</div></dt>
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<dd>This is a grid of the flow distance from the source site that has
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the highest angle to each cell.</dd>
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</dl>
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</body></html>
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