ON SAFETY AGAINST SEEPAGE FAILURE OF GENTLY SLOPING LEVEE BASED ON LEVEE VULNERABILITY INDEX IN THE TOKACHI RIVER DURING 2016 FLOOD AND FUTURE LEVEE DESIGN FOR LEVEE FAILURE RISK REDUCTION

Abstract

 In the Tokachi River large flood in 2016, there was no significant damage to the levee in the downstream area where the gently sloping levee has embanked. It means that the gently sloping levee has high stability not only for the soft ground measures but also for the seepage flow during a flood event. In this study, the downstream of the Tokachi River is targeted, the history of earthquake disasters and levee maintenance are examined, and the soil quality and structure of levee are grapsed. Then, using the water level hydrograph, the cross section shape of the levee and the average pearmeability coefficient, the longitudinal distribution of the levee vulnerability index is calculated. We clarify the reason why the gently sloping levee has not caused the seepage failure due to the 2016 large flood based on levee vulnerability index t^*. It is concluded that the wide and gentle slope levee is very effective for seepage failure reduction. Also, the importance of embanking the levee with a structure whose t^* is less than 0.01 is discussed.