Abstract
There was a destructive flood on Gidra river on June induced by 104-mm rainfall during 3 h on 7 June 2011. The total flood discharge was estimated to be 531,000 m3. The upper part of the Gidra river catchment is forested by more than 95%, but the forest floor has been disrupted to a large extent by intensive logging activities in the basin. Forest road density is up to 10 km/km2 in the catchment. The field inspections in the catchments revealed that approximately 25% of forest roads have been deepened down to the less permeable subsoil directly during their construction or by subsequent traffic and soil erosion. Forest roads affect runoff generation via two mechanisms: (1) generation of infiltration-excess runoff on road surfaces and (2) capturing of hillslope surface and subsurface water by road incisions. Infiltration-excess water runoff from all compacted surfaces was estimated to be about 54,000 m3 by simply multiplying the compacted area by the difference between the precipitation and infiltration. More challenging was to quantify the transformation of hillslope water to the road-surface runoff. We have suggested the methodological approach that combines the GIS analyses of the terrain with mathematical simulations of the subsurface water exfiltration from hillslopes to the road surfaces using HYDRUS 2D model. Simulations based on the variability of slope inclinations and slope lengths within catchment revealed that drainage of the upward hillslopes by forest roads and deeper logging lines increased the forest road runoff by another 6,000-15,000 m3 of water.