Departmental Bulletin Paper 放置年数の異なる森林作業道の路面凹凸とその発生要因
Unevenness and its occurrence factors on skid trails with different years of abandonment

増田, 治美  ,  篠原, 慶規  ,  久保田, 哲也

96pp.28 - 33 , 2015-03-30 , 九州大学農学部附属演習林
近年,森林管理における路網の重要性が見直されてきており,その整備が進みつつある。しかし,作業道において,長期的な路面侵食量やその要因を調べた研究はほとんどない。そこで本研究では,放置された年数や地形条件などが異なる福岡県内の作業道7路線を対象に,各路線で8~25横断面,計101横断面で,路面凹凸を計測し,平均侵食深(d_<ave>)と最大侵食深(d_<max>)を算出した。各路線のd_<ave>の平均値(D_<ave>)は,比較的放置年数が長い路線では,放置期間が長い方が,その値は大きくなる傾向があった。これは,放置期間の総降水量の影響を強く受けているためであると考えられる。また,路線内で各横断面のd_<ave>の違いを生み出す要因を調べたところ,比較的放置年数が長い路線では,各横断面の路面傾斜とd_<ave>の間に正の関係が見られた。これらの路線においては,路面傾斜と年平均d_<ave> (d_<ave>/放置年数)の関係に大きな違いが見られなかった。このことから,長期的に見ると,年平均の侵食量は,作設方法によらず路面傾斜によって一義的に決まる可能性がある。今後は,異なる地形・地質学的条件,気象条件の場所で同様のことが検証されることが望まれる。
Recently, forest managers have recognized the importance of road networks for forest practices, and constructed new skid trails aggressively. However, few studies investigated the soil erosion rate and its occurrence factors on skid trails. This study examined unevenness of the surface of skid trails at 101 cross sections on seven routes in Fukuoka prefecture, Japan. We measured roughness at 8-25 cross sections for each route, and calculated the average and maximum depths of roughness (d_<ave> and d_<max>, respectively) for each cross section. The averaged d_<ave> over all cross sections on a route (D_<ave>) was different among routes. On the routes with the relatively long abandoned period, d_<ave> with the long abandoned period tended to be larger than D_<ave> with the short abandoned period. This result suggests that the total precipitation strongly affect D_<ave>. On the routes with the relatively long abandoned period, there was a positive relationship between d_<ave> and the slope angle of each cross section. We found approximately the same relationship between the annual average d_<ave> (d_<ave>/the abandoned period) and the slope angle for these routes. This result suggests d_<ave> for each cross section could be determined by slope angle only, and not be strongly affected by differences of routes. Further studies are required to verify the above findings under various geological, topographical, and climate conditions.

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