Forest road decommissioning: modelling the effect on hydrological connectivity

Abstract

The decommissioning of forest roads is occurring in many forest environments, with the aim of reducing the negative impacts of road runoff on water quality and aquatic habitat. Prioritisation of decommissioning options is however problematic, as the relative merits of competing options are difficult to assess. This dissertation presents a method of quantifying the degree to which a road is hydrologically connected to the stream network and the uncertainty associated with this. The method permits comparisons between different roads network management options and is useful for assessing the likely result of decommissioning works. To demonstrate its utility, the model was applied to an actual road decommissioning and replacement project in south-eastern Australia. Road areas and drainage outlets were surveyed in the field and flow path lengths to streams derived from a 1 metre resolution LIDAR based digital elevation model (DEM). The results of the case study demonstrate that the road decommissioning was not effective in reducing runoff to the stream network and that the overall result of the works was counterproductive. The procedures developed in this dissertation are an extension of the ‘volume to breakthrough’ model presented by Hairsine et al. (2002) and allow the quantification of road/stream connectivity without the need for extensive parameterisation. Comparisons with empirical road-derived sediment deposition models developed in the US suggest that the methodology used here may readily translate beyond its original geographical context.