R/edge_gap_gust_factor.R

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

#' Combined effect of gap size, edge effect, and gustiness on maximum bending moment for the roughness method.
#' @title Edge Gap Gust Factor.
#' @param spacing Mean distance between trees (m).
#' @param mean_ht The mean tree height in the stand (m).
#' @param dist_edge Distance of tree from the upwind edge (m).
#' @param gap_size Length of the upwind gap (m).
#' @return \code{edge_gap_gust_factor}, the combined effect of edge, gap, and gustiness on the maximum bending moment exerted by the wind on a tree. Used in the roughness method.
edge_gap_gust_factor_fun <- function(spacing, mean_ht, dist_edge, gap_size, fgr_constants) {
  s_h <- spacing / mean_ht
  if (s_h < 0.075) {s_h <- 0.075}
  if (s_h > 0.45) {s_h <- 0.45}
  if (dist_edge > mean_ht*fgr_constants$tree_heights_inside_forest) {dist_edge <- mean_ht*fgr_constants$tree_heights_inside_forest}
  edge_gap_gust_factor <- ((2.7193 * s_h -0.061) + (-1.273 * s_h + 0.9701) * ((1.1127 * s_h + 0.0311)^(fgr_constants$tree_heights_inside_forest*mean_ht/mean_ht)) +
                             ((-1.273 * s_h + 0.9701) *(((1.1127 * s_h + 0.0311)^(dist_edge/mean_ht)) - ((1.1127 * s_h + 0.0311)^(fgr_constants$tree_heights_inside_forest*mean_ht/mean_ht)))) *
                             max_gap_factor_fun(gap_size, mean_ht)) /
    ((0.68 * s_h -0.0385) + (-0.68 * s_h + 0.4785) * ((1.7239 * s_h +0.0316)^(fgr_constants$tree_heights_inside_forest*mean_ht/mean_ht)))
  return(edge_gap_gust_factor)
}