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

[AHW24-12] Sediment yield and sedimentation rate at several settling basins in the northern part of Okinawa Island

*Takashi Hirose¹, Kenta Nakasone², Shigekazu Kawashima³, Shin-ichi Onodera⁴, Mitsuyo Saito⁵ (1.University of the Ryukyus, 2.The Daido Fire and Marine Insurance Co., Ltd., 3.Miyagi University, 4.Graduate School of Advanced Science and Engineering, , Hiroshima University, 5.Graduate School of Environmental and Life Science, Okayama University)

Keywords:red soil erosion, settling basin, Okinawa Island, sediment yield, bare land

Red soil erosion is one of the environmental problems of the Ryukyu Islands. Coral reefs have been received damage and fishery has been affected by soil flowing out into the sea. Red soil erosion in late years happens mainly by farmland. The settling basin is one of the prevention measures for red soil erosion. The settling basin traps sediment before going out to outside the area and reduces sediment outflow to outside, and constant capacity is necessary for covering area. It is necessary to restore capacity by dredging for function maintenance that settling basin capacity decreases because the red soil which began to flow deposits and makes functional decline. It is necessary to know sediment yield and sedimentation rate in the settling basin to do effective operation.
In this study, thickness of sediment is measured in 14 settling basins with different setting times in the northern part of Okinawa Island, and sedimentation rate and denudation rate are estimated.
The results of these investigations are as follows:1) It takes 8-25 years to deposit about 80% of capacity of the settling basin. 2) The denudation rate is 0.03-0.17cm/year (300-1,700mm/1,000 years). 3) The bare land area in the covering area of settling basin have very good relationships with Sediment yield and sedimentation rate.