[A-HW28] Hydrology and Water Environment

[AHW28-09] Rainfall-runoff characteristics in high-relief meso-scale mountain catchments underlain by accretionary sedimentary rocks

★Invited Papers

*Yosuke Yamakawa¹, Koji Koshimizu², Takafumi Hajika³, Miyu Nakamura¹, Suneth Neranjan¹, Taro Uchida¹ (1.University of Tsukuba, 2.Hiroshima Prefecture, 3.Osaka Metro Co., Ltd.)

Keywords:Rainfall-runoff processes, High-relief mountains, Meso-scale catchments, Accretionary sedimentary rocks, Electrical conductivity

In recent years, the increasing frequency of extreme rainfall events—likely driven by climate change—has heightened the incidence of sediment-related disasters in mountainous regions, particularly those associated with extensive damage, such as “sediment flooding damage” and “large-scale landslides”. Sediment flooding damage often occurs at the outlets of meso-scale watersheds, particularly those with catchment areas of approximately 1 to 100 km², and the rainwater runoff process at this scale largely defines the phenomenon. However, in such meso-scale catchments, the substantial discharge of water and sediment poses challenges for hydrological observations. Furthermore, in areas with high-relief, accessibility issues further restrict the accumulation of hydrological observation data. On the other hand, large-scale landslide induced by rainfall predominantly occur in accretionary sedimentary rocks. It has been noted that the spatially heterogeneous behavior of groundwater, governed by the unique geological structure of these formations, plays a crucial role in determining the occurrence of such landslides. However, many aspects of the underlying phenomena remain inadequately understood. This study investigates the hydrological processes associated with those two mass movement phenomena, observed respectively, both in the upper reaches of the Ooi River, Shizuoka Prefecture, Japan. This region, composed of accretionary sedimentary rocks, is highly susceptible to large-scale landslides. At the first study site, time-series data on river water levels and electrical conductivity during flood events were collected from seven adjacent watersheds within a tributary of the Ooi River. The catchment areas of these watersheds range from approximately 0.2 to 9.0 km². The peak delay time—the time difference between peak rainfall and peak runoff—was analyzed to characterize stormwater runoff. The results indicated that peak delay time generally increased with watershed area. Building on trends identified in previous studies of low-relief mountainous areas, the results suggest that peak delay time is predominantly influenced by watershed size, regardless of topographic undulations. This implies that in mountainous regions with long slopes, flood propagation occurs at a similar speed to that in areas with shorter slopes. Additionally, based on the analysis of runoff sources at the site using the electrical conductivity of river water as a tracer, it was suggested that, for most events, during flood events, the precipitation component preceding the event (i.e., old water) accounts for more than 50% of the runoff. At the second site, a total of 16 adjacent watersheds, with catchment areas ranging from approximately 0.6 to 4.7 km², were studied. These watersheds flow from the east and west into the Ooi River, which generally flows southward. Around the site, the bedding planes generally dip to the northwest. On a broader scale, the tributaries on the left bank of the Ooi River exhibit a dip slope structure, whereas those on the right bank display an anti-dip slope structure. Many of the slopes on the left bank are characterized by gravitational slope deformation. In contrast, such topography is rarely observed on the right bank, where the topographic gradient is relatively steep. River flow measurements were conducted during multiple periods without rainfall, across these tributary basins. As a result of the analysis conducted across multiple basins, a positive correlation was observed between the ratio of dip slopes and the coefficient of variation of specific discharge (over multiple periods). This suggests that tributary basins dominated by dip slopes tend to exhibit a more sensitive response to antecedent rainfall, with flow rates increasing and decreasing accordingly. In contrast, it was suggested that tributary basins dominated by anti-dip slopes tend to exhibit a flow rate with less fluctuation over time.