In tom-locatelli/fgr: R Version of the ForestGALES wind risk model

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Both the 'roughness'and the TMC methods require that some tree dimensions are known (i.e. provided by the user) or calculated within the model's workflow.

Top height, mean height, and equivalent mean height

Top stand height (e.g. the average height of the 100 trees per hectare with the largest dbh) is a commonly used measurement in forest management, especially in the UK. For the roughness method, ForestGALES^[Throughout the package documentation, we will use the name ForestGALES to refer to the model characteristics as described in e.g. Hale et al. (2015) or Gardiner et al. (2000, 2008). Conversely, when we refer to the functionalities implemented in this package, we use the name fgr] calculates the CWS and risk of damage for the mean tree in the stand. For this, top height is converted into mean height using linear regression coefficients derived from forest inventory data. Other mean tree characteristics are calculated from mean height. Therefore, knowledge of either top height or mean height is essential to calculate CWS with the roughness method.

For the TMC method, the equivalent mean height of the stand is used in the calculations of aerodynamic roughness ($z_0$) and zero-plane displacement ($d$). The equivalent mean height is derived from stand top height (defined as above) to define the height of wind moment absorption at stand level. Stand's top height is also used in the elevate function to convert the critical wind speeds to the wind speed 10 meters above zero plane displacement (to compare with for anemometers' data, see the Critical Wind Speeds vignette). Stand mean height is used in the TMC method to calculate stand mean canopy parameters used in the calculations of the $z_0$ and $d$, for use in both the elevate function and in the Turning Moment Ratios tm_ratio function.

Canopy width

In the roughness method, mean canopy width is calculated from species-specific coefficient of a linear regression against mean stand Dbh.

In the TMC method, the canopy width of individual trees is similarly calculated from species-specific coefficient of a linear regression against their Dbh. Stand mean canopy width is calculated with the same function, using the mean stand Dbh and the linear regression coefficients of the predominant species in the stand, and is used in the calculations of aerodynamic roughness ($z_0$) and zero-plane displacement ($d$).

Canopy depth

In the roughness method, mean canopy depth is calculated from species-specific coefficient of a linear regression against mean stand height.

In the TMC method, the canopy depth of individual trees is similarly calculated from species-specific coefficient of a linear regression against their height. Stand mean canopy depth is calculated with the same function, using the mean stand height and the linear regression coefficients of the predominant species in the stand, and is used in the calculations of aerodynamic roughness ($z_0$) and zero-plane displacement ($d$).

Stem diameter at tree base

The masses of the stem, of the overarching canopy, and of snow (when present), contribute to the additional turning moment captured by the Deflection Loading Factor. Some of these calculations are performed at tree base (e.g. the Second area moment of Inertia) which require that stem diameter at tree base is calculated from Dbh.

tom-locatelli/fgr documentation built on Oct. 2, 2020, 2:09 a.m.