A model for optimizing forest road drain spacing

Abstract: Unsealed roads are necessary infrastructure required to manage forested catchments. They form linear features in the landscape that intersect, concentrate and redirect flow paths which can alter catchment hydrology and in-stream water quality. However, water quality is only affected if runoff and sediment generated on the roads are delivered to the streams. Road runoff and eroded sediment can enter the stream at 1) stream crossings, 2) via gullies at drain outlets that concentrate the flow all or part of the way to the stream, or 3) via diffuse overland flow. Hairsine et al. (2002) developed a simple probabilistic model for diffuse overland flow. It uses the concept of the ‘volume to breakthrough’, which is the volume of runoff required to enter an area before discharge is observed at the downslope boundary of that area. The model only requires variables of distance of drain outlet from the stream, road contributing area (or road length), road infiltration rate and a designer rainfall event to calculate runoff volume. The model has been applied to a number of forest catchments to assess the adequacy of road drainage. Where road drainage is determined to be inadequate, the model can be used to determine the location of “new drains ” by maximizing contributing road area as determined by the distance the drain outlet is from the stream. However, this method results in many new drains being placed close to existing drains along road segments. The aim of this paper is to describe a new model that optimizes the placement of new drains along unseale