琉球石灰岩域における降雨流出とドリーネの浸透排水に関する研究

摘要

琉球石灰岩域では，河川が未発達な地域が多く,流域の多くは，すり鉢状の地形をなしているため，降雨はドリーネの浸透孔から，地下に排水される事例が多い.このような地域では，台風等の豪雨の度に，ドリーネの浸透能力の機能低下が原因で，周辺農地は深刻な湛水被害に見舞われ，農業経営上，大きな問題を引き起こしている.%River systems are often primitive in "Ryukyu" coral limestone soil regions. Therefore, water is drained through underground infiltration from a doline. A lower infiltration capability coupled with typhoon and heavy rain often triggers flood damages, which jeopardizes agricultural crop production in these areas. Therefore, it is urgent to evaluate the effect of various measures on flood damage mitigation these areas and implement the effective measure. In order to evaluate the effectiveness of these measures, we need to clarify the relationship between rainfall-discharge and infiltration drainage through a doline in these regions. However, previous studies used a simple model assuming only penetration apertures at the bottom of infiltration ponds and were not able to sufficiently capture the roles of side apertures and limestone pipes for drainage which are characteristics to be seen in these areas. The objective of this study is to clarify and improve our understanding of the relationship between rainfall-discharge and infiltration drainage through a doline in these regions considering the complex drainage system in these areas. We considered rainfall-discharge and infiltration drainage as a series of phenomena within a model. First, we estimated the inflow to a doline based on the amount of discharge. Then, we evaluated the residues by computing the differences between the inflow to a doline and infiltration drainage through a doline. Finally, we developed a mathematical simulation model and simulates an input-output relationship of these drainage systems. In order to reflect this complex systems in the model, we varied the size of side apertures depending on the water level of a seepage basin, modeled the relationship between the cross sectional area of side apertures and the water level, and incorporated this mechanism into our simulation model. We applied this simulation model to two areas, 1) "Ashi-chaga" area where a tunnel drainage system has been introduced for mitigating flood damages, 2)"Makabe" area where several measures have been discussed for mitigating flood damages. Our simulation model was able to closely approximate the observed water level changes by explicitly modeling the effect of side apertures in "Ashi-chaga" area. Our simulation results indicate that the peak discharge for the December's rainfall event (the maximum rainfall per hour: 54.0mm, the total rainfall: 187.0mm) was 8.51m~3/sec. We applied parameters estimated for this December's rainfall event to the heavy rain event of September, 1999 (the maximum rainfall per 24 hours: 477.0mm, the total rainfall: 555.0mm) and simulated changes in water levels. This simulation closely projected the peak water level of the heavy rain event (simulated water level was 6.8m, while the observed water level was 6.7m). Furthermore, this simulation results indicate that the peak discharge of this event was 25.91m~3/sec. In this region, the tunnel had been constructed as a drainage measure. However, we have not experienced the heavy rain event like September, 1999 and the effectiveness of this tunnel has not been tested yet. So, we used our simulation model to examine the effect of the tunnel drainage on the degree and extent of flood damages. Our simulation model suggests that the tunnel contribute to the reduction of the maximum flood area by 65% and 82% of the total discharge. In "Makabe" area, we simulated infiltration of our study site by modeling two adjacent infiltration ponds with different infiltration capabilities, overflow among these two adjacent ponds through a subterranean drain system, and pipe flow drainage system. Our simulation model was able to closely approximate the observed water level changes by modeling this complex multiple drainage ponds system. We used the rain event data of August, 2007(rainfall per day: 457mm, the maximum rainfall per hour: 83.5mm), which caused the flood damage over 20 ha, and simulated the changes in water level. Our simulation results show that the peak water level was at the evaluation of 26.3m, and flooding occurred on the section of approximately 110m of prefectural road. Although, the rainoff-discharge relationship at each pond has not been quantified, our simulation results indicate that the peak discharge and the cumulative discharge at the 1st pond were 28.4m~3/sec and 445,029m~3 respectively, while the peak discharge and the cumulative discharge at the 8th pond were 6.2m~3/sec and 113,706m~3 respectively. Furthermore, we identified the characteristics of the limestone-cave which has been observed at the downstream direction of the 1st pond thorough our simulation analysis. Our simulation model closely projected changes in water levels when the base height of this pipe is set at the elevation of 18.5m (the base height of this pond is EL=16.5m)， the diameter of this pipe is set at 80cm and the roughness coefficient (n) of 0.1is used. Our simulation results indicate that this modeling approach is useful for evaluating proposed measures for mitigating flood damages in these regions. These results will help us improve our understanding of the complex relationship between rainfall-discharge and infiltration drainage through a doline on and will provide scientific knowledge for evaluating flood damage mitigation measures in these areas.