In Okinawa region, red soil have discharged into rivers and reefs around islands and caused a serious environmental problems. Authors have estimated the effects of check dams on the red soil discharge control, and clarified the followings.
1) Check dams with surface drainage system decreases 40-80% in turbidity or 50-80% of red soil in weight.
2) At rain event, turbid water of more than 1, 000 mg/l in SS flows into the bottom layer of check dam reservoir and the turbidity of surface water kept less than 300 mg/l in SS. This vertical distribution of turbidity could be a cause of high effects of check dam on red soil discharge control.
3) Above described effects are of the check dams with vast volume of reservoir against its catchment area (more than 10, 000 m
33/1 km
2) and it is impossible to install check dams with such volume to all basins. To control red soil discharge, measures in farmland where are the main source of red soil outflow should be taken.
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