ABSTRACT
In the Meiji Era (late 19 th century), a series of large-scale groins were constructed in the downstream basin of the Kiso River flowing through the Chubu region of Japan in order to conserve the river banks and ensure draft depth for river transport. Groins of 200 m in length, 10 m in width and 1.5 m in height are constructed in a zone influenced by a maximum 2 m tidal variation. Currently, embayments and creeks have been formed with the expansion of forest around these large-scale groins, resulting in the conservation of a favorable natural environment. In particular, scouring found downstream of the groins is considered to play an important role in the formation of embayments. This downstream scouring behavior cannot be seen in the studies concerning groin constructions in other rivers, via model-testing, or through hydraulic analysis. Thus, clarification of the generation mechanism of deep scour behind large-scale groins can contribute to groin site planning that takes the environment into consideration. In this study, we utilized electromagnetic velocimeter and Acoustic Doppler Current Profiler (ADCP) to measure flows generated between groins and in their downstream section during normal periods and during flood events. As a result, it was verified that the scours in the downstream basin was the result from a longitudinal vortex including the flow in a transverse direction when overflow occurs on the groin. Moreover, 2-dimensional and 3-dimensional numerical analyses were conducted to reproduce the results of field observations and the mechanism for secondary flow formation downstream of the large-scale groins was considered.
