Tunnel development over a 12 year period in a semi-arid catchment of the Loess Plateau, China

The tunnel systems in a semi-arid catchment of the Loess Plateau of China were repeatedly surveyed prior to the rainy seasons of 1989, 1999 and 2001. The surveys aimed to: (1) measure tunnel development over the 12 year period 1989-2001; (2) explore how the physiographical conditions affect the spatio-temporal variability of tunnel development; and (3) to identify the geomorphic processes associated with tunnel development. The ultimate goal was to quantify the geomorphic significance of tunnel systems in the catchment. Over the 12 year period, the number of tunnel inlets was more than doubled and most of the newly increased inlets were initiated in the few catastrophic storm events. However, tunnel enlargement can occur in storm or inter-storm periods, mainly through earth falls and slumps in inlets, and water erosion and roof cave-in collapses in tunnel paths. Tunnel development varied with material properties, land uses and topographic conditions. Net tunnel erosion may contribute at least 25-30% of the catchment sediment yield and was mainly produced by the initiation and enlargement of tunnel inlets rather than tunnel paths. To protect the areas against tunnel erosion, terracing of the upper slopes seems to be more effective than planting vegetation on the lower slopes.