Modeling forest road structure requirements according todifferent digital elevation model sources

One of the biggest obstacles to forest roads fulfilling their intended functions safely and economically within the planned timeframe is the negative impact of surface runoff and groundwater. Effective drainage systems are needed to remove this water without damaging the road platform, cut and fill slopes. Hydraulic structures, one of the drainage systems on forest roads, are structures that drain water from one side of the road platform to the other. The study aimed to identify differences that may arise in the locating and sizing of hydraulic structures based on different DEM sources, thereby providing decision-makers with a scientific basis for more accurate infrastructure planning. In this context, DEM data derived from 1/25,000 scale topographic maps were compared with SAM data obtained from airborne LiDAR data. The locations of hydraulic structures were determined by superposing the roads of the IUC Faculty of Forestry Education and Research Forest with stream branches obtained from both digital elevation models. The basin boundaries for each discharge point were then determined using the Hydrology tool in ArcGIS using the flow direction analysis, flow accumulation analysis, and threshold determination steps. Consequently, 19 hydraulic structures and their basins were created in the 1/25,000-based map area and 71 hydraulic structures and their basins in the LiDAR-based map. Rational method was used to calculate the maximum flow rates with 25-, 50- and 100-year recurrence periods that will pass through each hydraulic structure. The Talbot formula was then used as a first approximation to determine the cross-sections that could accommodate these flows. Dimensional analyses were conducted to verify whether the engineering structures could meet the calculated flow rates. Consequently, the type and final dimensions of each hydraulic structure were determined for three different recurrence periods. The results demonstrate that LiDAR data is not only an alternative for engineering planning on complex and sloping terrains like forest roads, but also a resource that offers more accurate solutions. On forest roads with high flood risk and extensive topographic variation, LiDAR-based models have enabled more reliable design of hydraulic structures in terms of both spatial and hydraulic calculations.