Spatiotemporal Evolution of Forest Road Rutting and Flow Pathways Examined Using Unoccupied Aerial Vehicles ( <scp>UAVs</scp> )

ABSTRACT Flow pathways on unpaved forest roads are critical determinants of surface runoff and sediment transport. These flow pathways can be largely altered through road deformation caused by heavy traffic, with one of the most common types of deformation being ruts. Historically, rut development has been studied using cross‐sectional analyses. More recently, remote sensing techniques, such as structure‐from‐motion (SfM) or terrestrial LiDAR scanning (TLS), have demonstrated their utility in mapping ruts on forest roads. However, applications of these data are limited, especially with respect to flow pathways on the road surface. Here we used SfM, with validation from TLS, to examine the spatially comprehensive development of ruts and their effects on forest road flow pathways and relative sediment transport potential. We carried out a small‐scale experiment at two field sites in western Washington using unoccupied aerial vehicles (UAVs) to obtain digital elevation models (DEMs) of mainline logging road surfaces over 3 seasons. These UAV‐derived DEMs were used in an elevation change analysis and a simple flow routing model to examine the evolution of ruts and the impacts thereof. We found that: (1) the relationship between measures of rut incision and time since grading was nonlinear at both sites for all seasons with sufficient data; (2) as ruts developed, the flow pathways on the road surface were altered; (3) the relative transport potential of the road surfaces increased overall as ruts developed; and (4) drainage system metrics reveal a threshold rut incision depth for increased transport potential and flow network change. Our results demonstrate that a great deal of useful information can be extracted by using SfM DEMs for the analysis of rut evolution. Additionally, our results allow us to examine how rutting may affect the utilisation of erosion control treatments in roadside ditch lines and the sediment yield of the road surface.