Protection of areas against avalanches, mudslides and rockfalls - Supply, Flow, Demand [WIKIAlps

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Protection of areas against avalanches, mudslides and rockfalls - Supply, Flow, Demand

General description:

The following model is an approach to delineate areas where the biotic ecosystems (forests) contribute to the mitigation of natural hazards and the protection of human assets from hazardous natural processes. At the Alpine-wide scale, this has been done by combining separate regional models for avalanches, rock-falls and water channel relevant processes. The models use topographical information derived from the 25m EU DEM and statistically derived threshold values identified by earlier projects and assessments (performed in all Alpine countries) to model potential avalanche and rock-fall release and transition zones.Figure 1describesindetail the calculation procedure to derive the developed indicators per local administrative units (LAU2) of the Alpine Space.

Input data

DEM (slope, slope-length, flow direction, watershed, plan curvature, contour lines) * Land Cover * River Network

Calculation processes

Avalanche release and Transition zones

(1) Calculate potential avalanche release areas:Select all areas with a slope between 28° and 55°, a plan curvature between -2 and 0.2, a forest land-cover and if there is no better information on potential snow-accumulation available above an altitude threshold (Bauerhansel et al 2009).

(2) Split larger areas:To get more realistic snowfields the result of the previous calculation has to be split into smaller areas. To take flow processes of avalanches into consideration this should be done by using very small watersheds that indicate the same flow direction of an area. The areas can be split into horizontal sections using contour lines (50m) to create “snowfields” with a vertical extent no larger than 120m.

(3) Calculate avalanche cost path:The cost path represents the easiest way down a slope following the highest elevation differences from one raster pixel to the next. In Arc GIS this can be accomplished with the tool (cost path) using the Flow-Direction as Cost Backlink raster and the DEM as Cost Distance Raster.

(4) Calculate energy line angle for every release area:The energy line, orgeneralizedgradient, represents the angle of the connecting line from the release point and the outer edge of the run out zone of a mass movement with the horizontal plan. (Bauerhansel et al 2009)

Figure 1 : Bauerhansl, C., Berger, F., Dorren, others, 2010. Development of harmonized indicators and estimation procedures for forests with protective functions against natural

In Arc GIS the calculation of the Energy Line Angle can be done with the following steps:

Creating an integer raster with the elevation of the release areas.
Running the tool Euclidean Allocation and Distance with the integer raster as Input (to speed up the calculation, a maximum distance of 2000m should be set).
Using the Raster Calculator, subtract the result from the DEM. The value you get is the elevation difference from every pixel to the release zones Δh.
The Euclidean Distance calculated in step (b) represents Δl.
The energy line angle can than simply be calculated with ELA = atan( Δh/Δl) (àArc GIS calculates angles in radians to get a result in degrees you have to multiply it with 57.2958).

( 5) Limit avalanche Paths with energy line angle:Knowing the avalanche path, the energy line angle allows a rough estimation of maximal runout distance. The previously measured events occurred at an ELA of 17° - 47° with the mean 28°. Events with ELA 17° are very improbable. We used this threshold for our approach to consider all possible events. (Bauerhansel et al. 2009, PARAmount Project 2012).

Table of Contents

Protection of areas against avalanches, mudslides and rockfalls - Supply, Flow, Demand

Figure 1 : Bauerhansl, C., Berger, F., Dorren, others, 2010. Development of harmonized indicators and estimation procedures for forests with protective functions against natural