The once-in-20-year flood means that according to the plan, the control facilities such as maximum rainfall, reservoir flood storage capacity and coastal flood discharge capacity are relatively stable within 20 years. If the flood level with a 50-year return period is designed, the factors such as the maximum precipitation with a 50-year return period and the decrease of reservoir flood regulation capacity caused by sediment deposition need to be considered. Because there are too many things that may happen in 50 years, such as vegetation destruction and waterway blockage.