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FAQ/ How to calculate overflow width

If a river overflows, how do you determine the width of the overflow?


The overflow section is the section centered on this cross-section with the midpoint as the midpoint between adjacent upstream cross-sections and the midpoint with the adjacent downstream cross-section as the end point. In addition, the overflow section is further subdivided, and the part where the ground height in the embankment is higher than the water level of the river channel cross-section is excluded from the overflow section. The length of this overflow section is the overflow width.

This is the specification according to the Flood Expected Area Diagram Creation Manual (4th Edition) July 27.

Manual for Drawing Flood Expected Areas (4th Edition) July 27, p. 18

(1) Overflow width ・Water overflow width
The overflow width and overflow width at the expected flood point of overflow and overtopping are set equally upstream and downstream of the point using the distance between the cross-sections of each river channel (distance between geodesies) as the basic unit. When overflowing and overflowing at the same time in adjacent cross-sections, the continuous overflow width and overflow width shall be used. However, if there is a cross-section of a cross-sectional workpiece between each river channel cross-section, or if the ground height in the embankment is higher than the embankment height of the river channel cross-section due to mountains, etc., it shall be appropriately subtracted from the overflow width and overflow width.
In addition, in overflow flooding / overflow flooding, there is no change in the laying height of the flood part.

Last update: 2023-03-17