Windthrow Hazard Mapping using GIS, Canadian Forest Products ...

More documents

Recommendations

Info

Relative Semivariance10.90.80.70.60.50.40.30.20.101 2 3 4 5 6 7 8 9 10 11 12# of 25m segments apartwtcnstrmaxselv100heightmwspeedbrgcnFigure 1. A plot of relative semivariance (semivariance value for a given inter-segmentdistance divided by the semivariance value at 12 segments apart) for: WTCN,windthrow at segment centroid; STRMAXS, maximum wind speed during strong windevent; ELV100, elevation; HEIGHT, stand height; MWSPEED, BCHydro mean annualwind speed; BRGCN, direction from cutblock boundary to cutblock centroid.Spatial autocorrelation was determined <strong>using</strong> semivariance, which expresses thedegree of relationship between points on a surface (Carr 1995). Semivariance is halfthe variance of the differences between all possible points spaced a constant distanceapart. Values increase quickly with distance for damage, stand, and managementvariables that vary within small distances, and more slowly for variables that vary overlarger distances such as elevation and wind speed. To reduce the spatialautocorrelation among observations, the minimum neighbourhood over which samplepoints were selected to estimate a grid node was held at 125m (e.g. five 25m*25msegments. Hence in a 125m*125m panel only one segment was selected for analysisor testing.6
Determination of independent variablesCalculation of Topex2000:Topex is a measure of topographic exposure at a given location. There are two basicforms of topex, ‘classical topex’, and ‘topex-to-distance’. Classical topex is the sum ofthe angle to skyline in degrees for the 8 cardinal directions. The minimum value is 0,meaning that the lowest topex score is 0 and this would occur on a mountain top withhigh exposure. Higher topex scores indicate increasing topographic shelter.In topex-to-distance, classical topex is modified in the following ways in order tofacilitate map-based calculations:1. the maximum distance in each direction is limited - in Topex1000 to 1000m, inTopex2000 to 2000m, etc.;2. the greatest angle to ground within this distance is used;3. negative values are permitted.A low score still indicates higher exposure – a mountaintop would have a verynegative score.Derivation of management variablesBlocks contiguous to each other were dissolved together into single blocks prior toboundary segmenting to better represent the wind (fetch) effects within openings. Aset of variables describing segment edge orientation, segment position relative toopening centroid, number of directions of boundary exposure and fetch werecalculated <strong>using</strong> coordinates of segment centroids.Determination of dependent variablesBecause windthrow is a rare event and measures of damage such as ‘% of segmentarea loss’ and ‘% crown loss’ are non-normally distributed, normal regressionapproaches are not appropriate for windthrow prediction. Logistic regression allows usto predict discrete outcomes (e.g. segment is damaged or undamaged) and is moresuitable to the problem of rare events. In this study, logistic regression was used toestimate model coefficients and generate the probability of windthrow occurrence foreach segment unit. In order to address the issue of damage severity, a set ofdependent variables were created to represent the presence of damage for differentseverity thresholds. Table 3 explains how these dependent variables were obtained.7
Page 3 and 4: models have been successfully devel
Page 5: angle between peak wind direction a
Page 9 and 10: anking of risk groups. For practica
Page 11 and 12: Contingency tablesTrends in the pro
Page 13 and 14: k)l)0.2060001612000Proportion0.150.
Page 15 and 16: Observed0.450.40.350.30.250.20.150.
Page 17 and 18: was not possible to evaluate this e
Page 19 and 20: ReferencesAnderson, C. J., Coutts,
Page 21 and 22: Appendix 1. Procedures used to buil
Page 23 and 24: Table A1-3. Map formulas used to ca
Page 25: *a pdf version of this map is attac

Windthrow Hazard Mapping using GIS, Canadian Forest Products ...

Create successful ePaper yourself

Delete template?

Save as template?