[A-HW24] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW24-11] Conservation of Coastal Ecosystem and Coral Reef with the Management of Settling Basin: The Usefulness and its Economic Assessment

*Shigekazu Kawashima¹, Mitsuyo Saito², Takashi Hirose³, Shin-ichi Onodera⁴ (1.Miyagi University, 2.Okayama University, 3.University of the Ryukyus, 4.Hiroshima University)

Keywords:soil erosion, settling basin, economic analysis

Ishigaki Island, Okinawa is in a subtropical marine climate and surrounded by blue ocean where beautiful coral reef and lush greenery are serving as the habitat for a wide variety of wildlife. Due to the beautiful coastal ecosystem, Ishigai Island attracts more than 1 million tourists every year. Soil erosion, often referred to as “Red Soil Erosion”, has been considered as major environmental issues in Ishigaki Island, and it affects not only sugarcane/pineapple farming economy but also the degradation of coastal ecosystem and coral reef, eventually leading to economic loss in tourism and fishing industries. One of the countermeasures to mitigate such Off-site impacts of red soil erosion is to develop a settling basin. It treats sediment-laden run off and reduces the volume of sediment leaving a site, consequently protecting downstream ecosystem from excessive sedimentation and water quality degradation. As a fact, nearly 300 settling basins were already developed in Ishigaki Island. This study sheds light on the management strategy of a settling basin because many of settling basin remain unmanaged and filled with sediments, gradually degrading coastal ecosystem and coral reef. Economic model is developed to illustrate how settling basin is sustainably managed under alternative scenarios. This study suggests that more frequent dredging allow for mitigating the off-site impact of red soil erosion and reducing the overall environment cost.