Regular survey of suspended solid concentrations in streamwater during line thinning with construction of spur road

Abstract

We investigated suspended solid (SS) concentrations, except flooding, before and during line thinning in a Japanese cedar and broad-leaved forested watershed in Ibaraki Prefecture, Japan. Line thinning was conducted at a thinning rate of 35% during 2012 and 2013. Spur roads were constructed along all tributaries (ephemeral streams). The SS concentrations in the regular survey during the thinning conducted in 2012 did not increase significantly compared to the values before conducting thinning. Whereas, SS concentrations were significantly higher during and after the thinning that was conducted in 2013 than the values before conducting thinning. Furthermore, even though 11 hours had passed since the end of the rainfall, high SS concentration (175.4 mg L ⁻¹ ) was observed. This reason may be that the spring water after rainfall eroded the surface of the spur roads and the disturbance by forestry machinery for spur roads construction or timber collection. Therefore, it was concluded that the SS concentrations in streamwater except flooding during thinning might be higher than the values before thinning.