18-12, Nishiochiai 2-chome,
Shinjuku-ku, Tokyo 161-8575, Japan
Phone: +81-3-5906-0211
FAX: +81-3-5906-0112

Natural Disaster Prevention

BACKGROUND & PURPOSE OF PROJECT

The flood danger level indicates the upper water level limit in which flooding can safely occur in current river conditions and is an important water level gauge used in river management which is referred to in relation to flood forecasts, flood prevention, and for evacuation direction decisions made by local municipalities.
Although there have been a number of disasters caused by heavy rain recently in Japan, it is hard to adopt drastic flood control measures to lessen damage because of prohibitive costs and time limits. It is becoming evermore important to minimize human suffering through the precise enactment of evacuation measures, and to achieve this it is necessary to set an accurate "flood danger level."
This project re-examined the danger water level in Yodo River, a class A river in Japan, on the basis of an evaluation into the height and quality of its levee.

CHARACTERISTICS, CONCEPT, STANCE & SKILLS OF PROJECT

Generally the flood danger level is set according to a "height evaluation", which is made by deducting the allowance height from the height of a levee. This project examined the setting of a flood danger level using a "qualitative evaluation", which is made based on the qualitative soundness of a levee in addition to its "height evaluation."
This "qualitative evaluation" aims to evaluate the level of safety of the levee in relation to the possibility of a breach in the case river water floods into the levee. In other words, this method aims to set a flood water level that ensures the safe flow of water considering the possibility of a levee breach by water infiltration caused even when the height of the levee is sufficient, such as when the levee width is insufficient, or where insufficient materials have been used in the construction of the levee.

EFFECT OF PROJECT

By implementing this project, evaluation from the perspective of "qualitative evaluation" has been introduced into the determination of flood danger levels in levees, which was previously only set on the basis of "height evaluation." As a result, we found it objectively reasonable to set a lower flood danger level than the level set previously considering the possibility of a levee breach at points where there is a problem with the levee width or the materials used to construct the levee, even where the height of the levee is sufficient.
However, the correct identification of levee materials remains a challenge because the construction history of a levee may be complex in some cases. It is necessary to further examine how to use bore data, data that only represents information of points bored, as qualitative information regarding levees, which are linear structures.