急勾配林道における路面侵食の実態について

Abstract

急勾配林道を幹線道とした路網を実際に利用している例を対象に, 路面侵食, 特に侵食溝の実態を調査・検討したととろ, 次の様な傾向が得られた。1) 勾配が急になる程, 侵食溝の発生頻度は高くなるが, 侵食溝断面の大きさ及び形状は20％前後の勾配で最大値を取る凸型の分布をしていた。2) 調査路線がかなり低規格であるために路面上空が樹冠層により被われている箇所が多く見られた。このような箇所では侵食溝の発生が抑制され, その断面積・形状比とも小さくなる傾向があった。3) 横断勾配を外向きに付け, 雨水が集中する前に谷側に誘導する「その場排水」が簡便な排水方法として有効であると考えられる。さらに, 調査路線で取り入れられていた小丸太を路面の下に敷く, あるいは砕石を転圧する, といった路面補強工法が侵食溝の発達・拡大を抑制しているように思われた。

In this report, we referred to surface erosion, espacially to rill erosion generated on the steep grade forest road, taking up a case of steep primary road passable with 4WD car only. 13.9% was the mean road gradient of the investigated route. To analyze factors that had affected generation and cross-sectional scale of the erosion, we used the Hayashi's Quantification, 1, 2, and the Multiple Regression analysis. The results we've got are as follows: 1) As road gradient increased, the road surface tended to be subject to rill erosion. That is to say, generative frequency of rill erosion had positive correlation to road gradient. (See Fig. 4, Fig. 5) 2) As to less than about 20% gradient, it is true that the same relation as above was recognized between (cross-sectional) scale of rill erosion and road gradient, but this relation wasn't true of every case with more than the gradient, or rather, even negative correlation was recognized between them. (See Fig. 6, Fig. 7) The cause of this result is presumed that on the earth road surface with steeper gradient than about 20%, traffic cars should smooth shape of rill by wheelslip or skid. 3) Both generative frequency and scale of rill seemed to decrease when the road was covered by crowns of trees which lined it on side. (See Table 2-Table 4) Finally, we wish to make extra remark: Almost all along the investigated route, logs and crushed rocks were buried under or into the road surfaces to reinforce them. Besides the way of surface drainage that cross section of road surface is slightly inclined down to bank slope (of course it must be stabilized) so as not to concentrate runoff on rill, we consider that these manners are very effective to restrain surface erosion on the forest road.