ForestGALES - Wind Risk Decision Support Tool

How ForestGALES works

Calculating wind risk

ForestGALES operates within a standard risk framework whereby risk is defined as the interplay between the vulnerability of a system and the hazard it is exposed to. Risk is therefore calculated in two stages. Firstly, the vulnerability of trees to wind damage is computed as the critical wind speed at which trees will be damaged, either by uprooting or stem breakage. Secondly, the localised wind climate is derived to represent the wind hazard. Multiple ways exist to characterise wind climates. Traditionally and most commonly, this is done with a Weibull distribution of mean wind speeds in a specific location. In British forestry, the DAMS windiness score was developed to provide an accessible interpretation of the Weibull distribution of wind speeds. Risk is then defined as the probability of exceeding the calculated critical wind speeds given the localised wind climate. The return periods of damaging storms based on stand location can then be calculated from this probability.

ForestGALES calculates the vulnerability to wind damage using information from the trees:

• species
• height
• stem diameter
• age (to estimate height and diameter when not known directly)

and then the site:

• soil type
• rooting depth
• spacing between trees
• thinning
• type of forest edge (acclimated or ‘green’ vs unacclimated or ‘brown’).

Detailed Aspect Method of Scoring (DAMS)

The probability of damaging winds occurring at the site is then calculated using information on the wind climate. Throughout Britain, this is classified in the DAMS scoring system, a modelled windiness score calculated from tatter flag observations, elevation, aspect, topographical exposure, valley shape and direction.
DAMS values can be:

• calculated for a specific location directly; or
• looked up from the DAMS scores for the whole of Britain.

ForestGALES Desktop 2.5 and ForestGALES Online 2.5 already feature the DAMS dataset.  ForestGALES 3.0/fgr requires downloading DAMS scores in raster format (as GeoTIFF). These are freely available in two coordinate reference systems (CRS): one compatible with Ordnance Survey data (DAMS in EPSG: 27700), and also in a CRS compatible with Google Maps, OpenStreetMap, etc. (DAMS in EPSG: 3857).

Outside of Britain, the wind hazard can be modelled with the scale and shape parameters of the Weibull distribution of mean hourly wind speeds. These can be calculated from wind data obtained from local meteorological stations, or from accessible sources such as the Global Wind Atlas. Weibull parameters can be supplied directly to ForestGALES 3.0/fgr, and to ForestGALES Desktop 2.5 when used in Research Mode.

Risk = Vulnerability x Hazard

The critical wind speeds of damage (for stem breakage and uprooting) calculated for the stand or for individual trees within the stand are calculated in ForestGALES to model the vulnerability of the system to wind damage. The risk of wind damage is then calculated as the probability of exceeding these critical wind speeds given the mean wind climate at the stand’s location. However, it is well known that wind damage is caused by wind gusts that represent the extreme winds rather than the mean. The relationship between the force exerted on trees by the mean wind and that delivered by wind gusts, and the multiple factors influencing this relationship, have been extensively studied in the research fields of tree stability and of wind impacts across different landscapes. The state-of-the-art scientific understanding of these phenomena is incorporated in ForestGALES to convert the effects of mean winds on trees to those of damaging gusts.

While ForestGALES Online 2.5 only allows simple, static wind risk calculations at one point in time, in ForestGALES Desktop 2.5 and in ForestGALES 3.0/fgr wind risk can be calculated over time and for different silvicultural scenarios. This information can be used to assist stand management decisions, for example felling age, silvicultural practice and cultivation.