急傾斜な風化花崗岩森林斜面の流出2次ピーク発生についての数値シミュレーション Numerical Simulation of Secondary Discharge Peak Generation in a Steep Forested Hillslope of Weathered Granite

地形と物性分布が調べられている急傾斜な風化花崗岩森林試験斜面で,流出2次ピーク発生の数値シミュレーションをした.流出2次ピークは,斜面に浸透した雨水の流出遅れからなる.流出遅れの原因は2つあり,一つは,雨水が斜面の中に浸透し飽和帯を形成するのに時間がかかること,もう一つは,斜面中に不連続な飽和帯ができるとき,それが斜面方向に連続するのに時間がかかることである.明日香試験流域の実測では,流出2次ピークは,中程度の先行水分条件,中規模の降雨で起こっている.流出2次ピークの降雨ピークからの遅れ時間は,降雨量の増加に伴い短くなる傾向はあるが,大きくは変わらない.試験斜面のシミュレーションでは,これらの特徴の多くが再現された.また,流出2次ピーク現象の詳細が明らかになった.試験斜面のように,物性分布が不規則な斜面では,不連続な飽和帯ができるため,流出2次ピークが現れる.不規則な形状の斜面は,直線形状の斜面に比べ,雨水の流出に時間がかかる.実際の斜面の条件に近づけた試験斜面の数値シミュレーションで流出2次ピークが現われたことから,もし斜面に不連続な飽和帯が発生する条件があれば,急傾斜な上流斜面でも流出2次ピークが発生し得ると結論する.

Numerical simulations of secondary discharge peak generation were performed in an experimental steep forested hillslope of weathered granite for which the topographical form and distribution of physical properties are well known. A secondary discharge peak represents the delayed discharge of infiltrated rainwater in a slope. There are two possible causes of this delayed discharge. First, it takes time for rainwater to infiltrate from the upper to lower part of the slope and for the saturated water zone to develop. Second, when discontinuous saturated zones are generated in the slope, it takes time for the zones to become continuous along the slope. In the Asuka experimental watershed, the focus of this study, secondary discharge peaks developed under intermediate antecedent moisture conditions and following rainfall events of moderate magnitude. The lag time between the initial rainfall peak and the discharge peaks did not vary greatly, although it tended to become shorter with increasing amounts of rainfall. In simulations based on the experimental slope, these characteristics were reproduced closely, and the results of the simulation were used to examine the secondary discharge peak phenomenon in more detail. For a slope with an irregular distribution of physical properties, as in the experimental slope, a secondary discharge peak occurs because discontinuous saturated water zones are generated in the slope. Topography also affects the time it takes for rainwater to be discharged from a slope. It takes longer for rainwater to be discharged from a slope with irregular topography compared with one with a straight slope surface. Secondary discharge peaks also appeared in the numerical simulations of an experimental hillslope with slope conditions resembling the real slope. Thus, it was concluded that, if a slope has a tendency to generate a discontinuous saturated water zone, it is possible that a secondary discharge peak will occur not only in hollow slope but also in steep hillslope.