Analysis of Infiltration Drainage and Evaluation of Tunnel Effects for a Doline in a Coral Limestone Region : A Case study of Ashichaga District of Itoman City, Okinawa Prefecture

Abstract

In coral limestone regions, river systems are often primitive and water is drained through underground infiltration from a doline in watershed areas. In these areas, if the infiltration capability is lowered, for example, by trash and dirt piled up at the mouth of a doline, heavy rain often triggers flood damages. In order to mitigate flood damages in a district of Itoman City, Okinawa Prefecture, a tunnel drainage was constructed. In this study, we conduct simulation analysis to examine the effect of the tunnel drainage on the degree and extent of flood damages. Our simulation model utilizes overland flow model and analysis of unsteady flow to assess rainfall outflow into the doline and applies Torricelli's theorem to model drainage capacity by explicitly considering both seepage holes and hydraulic pressure. Our simulation model can quantify the effect of tunnel drainage on flood damages and is deemed useful for a doline improvement plan intended to mitigate the damage from submergence in the areas where the infiltration capability has deteriorated.