For Chiuliao first levee, the slope sliding failure is the most likely mode to occur under all flood water level heights. However, the levee may undergo foundation sliding failure with a water level difference coefficient of 0.25 to 0.3 over a long flood return period. This is because of the high water levels and large local scouring depths. Therefore, the updated levee design cross section with rows of piles and thickened backfill material was designed to overcome possible failure modes.