Method for preventing repeated (tsunami, storm surge, and river) disasters by forces of nature

A method for preventing repeated river disasters, comprising:

• constructing a levee body surrounding a part of an area with a water level variable due to tide levels to form a surrounded water area;
• constructing a water channel connected to the surrounded water area for putting river water and water from a sea in and out of the surrounded water;
• area, wherein one end portion of the levee body is connected to one of a riverside of a river or a coast proximate to an estuary and another end portion of the levee body is in the river and forms a water inlet and outlet of the water channel such that water from the river and seawater flow into the surrounded water area from the water inlet and outlet at a time of a high tide level and the river water and the seawater inside the surrounded water area flow out of the water inlet and outlet to the river at the time of a low tide level, thereby scouring sediments at a bed of the river on a downstream side from the water inlet and outlet and enlarging a flood discharge section of the river;
• wherein a position of the water inlet and outlet of the channel is changed in a direction of flow of the river by varying a length of the other end portion of the levee body;
• constructing with a plurality of separation levees extending in a direction of flow of the river a plurality of sectioned water channels;
• further enlarging the flood discharge section of the river, comprising:
• placing movable dams on both end portions of each of these sectioned water channels in the river flowing direction, such that an amount of flowing water in each sectioned water channel is able to be changed through an operation of opening or closing each of the movable dams one or more times to scour riverbeds of one more of the sectioned water channels and thus deepen those one or more sectioned water channels, wherein the water is retained on sides of the sectioned water channels;
• and
• lowering one or more times tops of the separation levees on sides that are upstream, which allows overflowing of the separation levees, scouring of the separation levees together and causing the sediments to flow out.