On the Inspection of River Levee Safety in Japan by MLIT

Abstract

The ministry of land, infrastructures, transportation and tourism (MLIT) of the Japanese government maintains governmental managed river levee for the safety against flood whose total length is more than 10,000 km. The design standard of river levee had been traditionally done based on so called shape based specification, where levee is judged to be safe as long as a section satisfies the specified dimensions. In 2002, this policy had been changed and a new inspection guideline which is based on the modern soil mechanics principles had been introduced. The method mainly consists of the non-stationary seepage analysis followed by the circular slip line stability analysis. The exit gradient and uplift pressure are also checked based on specified safety factors. The way of the safety assessment is that, first a river is divided into so called a continuous strip (CS) which is judged to have similar configurations, geotechnical and hydraulic conditions. The typical length of a CS is half to several km long. Then a representative cross section (RCS) that is considered to represent all the CS section is selected. At RCS, detailed soil investigations are carried out, and the inspection is done to assess the safety of the section for stability and piping. A CS is judged to be safe if all the verification items satisfy the specified safety factor, but judged to be NG (no good) if any of the items could not fulfill the threshold value. The first round assessment at year 2011 indicated, only 60% of the all levee is judged to be safe for all verification items. 25% of the levee could not satisfy the stability requirement, 30% piping requirement, and 14% the both requirements. In the paper, the details of the verification methods are described. Furthermore, some recent flood events are introduced. Finally, further challenges for the second screening of the safety of levee are discussed.