Runoff and erosion from three unpaved road segments in northern Vietnam

Study region: Truong Son Commune, Luong Son District, Hoa Binh Province in north Vietnam. Study focus: Runoff and erosion from three road surface and three adjacent cutslope plots in a young plantation forest to evaluate how unpaved forest roads affect runoff and erosion. New hydrologic insights for the region: The 10-m long road surface plots generated similar amounts of runoff with runoff coefficients of 24-32 % over 24 rainstorms divided into an initial wetter period followed by a relatively drier period. Runoff coefficients for the two monitoring periods were not statistically different, and the relatively low runoff coefficients compared to previous studies were due to infiltration outside the narrow compacted tracks created primarily by motorbike traffic. Runoff from the base of the cutslopes varied by a factor of three, associated with the topographic convergence of the hillslopes. Road surface erosion over the two monitoring periods was consistent among the three plots and storm-based sediment production was strongly correlated with both storm precipitation and surface runoff. The amounts of runoff and sediment collected at the base of the cutslopes increased with increasing hillslope convergence, and the mass of collected sediment was 52-97 % of the corresponding amounts from the road surface plots. The results provide data on the extent of unpaved road runoff and erosion in this seasonally wet tropical environment, and the need to consider topographic convergence in road design and drainage.