Seven-year deterioration of a hiking trail and measures to mitigate soil erosion, Mount Kurodake, Daisetsuzan National Park, Hokkaido, Northern Japan

Abstract

To stabilize the hiking trail near Mount Kurodake (43° 41' 40″ N, 142° 55' 30″ E), water bars and steps were installed on the trail between the 7th lift station and the mountaintop in 1989. Water bars, or cut-off drains, are formed by making a small trench, primarily reinforced with a pair of wooden boards (90%) or logs (5%). Most steps are made of either boards (95%) or logs (4%). The materials of the water bars and steps must have been merged into the profile of the trail when they were installed; however, many of them are now exposed due to severe soil erosion and form barriers to walking. This study first examines the function of the water bars and steps seven years after their installations, estimates soil erosion rates for the period from 1989 to 1996, discusses the sequence of soil erosion and trail widening, discusses major problems with the stabilization methods, and recommends some measures for furture trail management. Although none of the 40 water bars have broken, 67% of board water bars had ceased to function by 1996. About two-thirds of 300 board steps no longer function because they have slipped down and/or bent. The trail was widened an average of 72.5 cm during the seven yrs. Net soil loss for five sites ranged from 380 to 3900 cm2, which gives a soil erosion rate of from 54 to 557 cm2/yr. The water bars and steps installed in this area have the following problem: (1) their length is too short; (2) too few water bars and too many steps were installed; and (3) their angle against the trail is too shallow. A water bar should extend beyond trails, so that runoff bypassing bars can be avoided. However, trail widening was so rapid in the area that surface water bypassed the bars and steps, resulting in extensive scouring and gullying. The number of installed water bypassed the bars and steps, resulting in extensive scouring and gullying. The number of installed was bars is only 40 for a 1800 m-long trail, whereas steps number 317. Surface water had to flow on the trail for too great a distance from one water bar to the next, causing scouring and gully erosion at many sites. Water can be effectively drained when water bars are installed at an angle of 30-45° to a cross section of the trail. The actual angle in this area ranges from 4° to 22° (average: 12.3°), which led to siltation in the trench. The trench of the water bars was eventually buried by soil and litter, allowing surface water to flow over the water bars and to cause severe scouring. Future trail management should include: (1) installation of additional water bars at a proper angle; (2) use of longer planks both for water bars and for steps; and (3) frequent, regular maintenance of the trail.