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大藪川流域において，水系網，流域面積，流域勾配に関する地形的特徴を明らかにした．地形解析の対象は，大藪川源流に位置する3流域，シキノタニ，ボウズダニ，コウチノタニである．水系網の解析は，Horton－Strahler方式による流路次数に基づいて，Hortonの第一法則（分岐比）と第二法則（流長比）とを用いておこなった．流域面積と流域勾配とは，3流域それぞれの主流路に流入する支流路ごとに計測し，主流...路に沿うこれらの位置的変化について解析をおこなった．その結束，分岐比はR_b＝4．0～6．0で，一般にとりうる値より大きく，流量比R_1＝1．7～2．0で，一般にとりうる値より小さいことが分かった．また，大薮川では東から西に向かってR_bが増加し，北から南に向かってR_1が増加することも分かった．流域面積と流域勾配については，3流域ともそれぞれの中流部から上流部の間で流域面積の大きな支流路が流入し，1kmあたり3～5本の割合で土石流発生限界勾配（tanα＝0.25）より急な支流絡が流入する事が分かった．このことから，洪水時の流量は合流点で著しく増加し，同時に支流絡から主流絡への土砂生産がきわめて発生しやすいものと推測された．
Geomorphological characteristics of drainage network, drainage area and channel gradient of river basin were analyzed at the Oyabu river located Kyushu mountain range. Geomorphology of three river basins, Shikino-tani, Bouzu-dani and Kouchino-tani, included in the Oyabu river basin were investigated. Analysis of the drainage network, based on the Horton-Strahler method, employed bifurcation ratio and length ratio confirmed by the Horton's law. Drainage area and channel gradient of the river basin, measured about each branch stream along the main flume, were analyzed. Results were as follows. Average bifurcation ratio showed larger values (R_b=4,0-6.0) than the general measuring values. Average length ratio showed smaller values (R_1=1.7-2.0) than the general measuring values. Over the whole the Oyabu river basin, the bifurcation ratio increased from east to west, and the length ratio increased from nouth to south. In each of three river basins, drainage areas of branch flumes showed large values from middle to upper reach of main flumes and steep branch flumes formed junctions with main flumes at intervals of 3-5/km. These results lead that the value of flood discharge increase at the junctions.show more
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