Countermeasure of Protection Against Local Scouring Downstream of Concrete Check Dams

Check dams are planned to control sediment and gravel runoff during floods. In accordance with the planning for the Sabo works, check dams have been constructed in each watershed, which may reduce serious sediment transport during floods. On the other hand, as the ratio of the installation of check dams to the planning is proceeded, sediment transport is limited, and degradation of the riverbed has been observed downstream of check dams. In general, localized scouring was observed downstream of the sub-dam even after protection blocks were installed downstream of the check dam. This may be caused by the lack of functionality to raise the main flow in the protection blocks. In concrete check dams, there are different types of ordinary closed check dams and check dams silted at a center part of dam. In terms of countermeasures against local scouring downstream of the sub-dam, there is no difference of design method for different types of check dams. Recently, the ramp with consecutively assembled boulders was proposed by Yasuda, the installation of the ramp below low drop structure is effective for both protection of river bed during flood stages and migration of multi-aquatic animals for normal stages. This paper presents that the installation of the ramp with consecutively assembled boulders downstream of a sub-dam is effective for different types of check dams. Experimental results show that the thee-dimensional main flow through the sub-dam can be controlled by the formation of seepage flow in the ramp composed by the assembled boulders, and that the main flow always rises toward the water surface at the downstream end of the ramp. The cross section of the ramp has a parabolic shape, and the velocity near the sidewall can be reduced in comparison with the center part of the ramp during flood stages. In addition, the migration for the multi-aquatic animals on the ramp is possible during normal stages as in the case of low drop structure.