PROBLEM TO BE SOLVED: To easily eliminate a slow / fast parallel flow generated at the time of small and medium-sized floods and suppress a polarization phenomenon. SOLUTION: In a method for dealing with a polarization phenomenon in a river 1, the upper end of each of a plurality of protrusions 10 is arranged above the water surface W at the time of small and medium flooding, and the width of the river along the water surface W at the time of small and medium flooding. The protrusion length t2, which is the length with respect to the direction G2, is 1.25 m or more and 8 m or less, the distance between the plurality of protrusions 10 with respect to the direction along the riverbank is 25 m or more and 50 m or less, and the protrusion length t2 is spaced. A plurality of protrusions 10 are installed so that the value divided by 3 is 0.05 or more and 0.2 or less. [Selection diagram] Fig. 2
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