Tunnel erosion and sediment production in the hilly loess region, North China

In this study, detailed monitoring of water and sediment delivery from major tunnel systems was conducted in a small semi-arid sub-basin, locally known as Yangdaogou, in the hilly loess region, North China. These deep-seated tunnels are located as much as 30 m below the slope surface, in deep Quaternary loess formations. In the Yangdaogou, these systems have large amphitheatre-like inlets, with mean diameter and depth of 4.8 and 4.99 m, respectively. These dimensions represent some of the largest tunnel inlets in the world. During 1989 and 1990, fifteen rainstorms were monitored at six tunnel outlets. Due to practical difficulties, only 35 sets of tunnel-storm data were obtained. Observed sediment concentration of tunnel flows ranges from 8.2 to 893.2 g/l. The peak sediment concentrations in tunnel flows are not distinctively higher than those in channel flows but considerably higher than those measured from untunneled sideslopes. No significant correlations between runoff and sediment yield can be found in most tunnels at both within-and between-storm levels. Such an erratic relationship is ascribed to the rapid shift of runoff and sediment source area, the occurrence of collapses within tunnels, and the initiation of new tunnel inlets. Based on the field monitored data, 57% of basin sediment production is delivered by the tunnel systems, suggesting that tunnel erosion is a major erosion process in the hilly loess region.