Abstract:Flood risk is a very important subject in hydrology. Firstly, the exact meaning of flood risk has been discussed, and two concepts of flood risk in the current situation, time risk and annual risk, have been pointed out. In the former, Both risks resulting from annual maximum flood and submajor flood are considered simultaneously in the design. In the latter, only risk resulting from annual maximum flood is considered in the design. Secondly, frequency or probability is defined as flood risk degree in this paper. While time frequency corresponds to time flood risk degree, annual frequency corresponds to annual flood risk degree. Lastly, the analysis and calculation show that the standards for flood control of annual flood risk degree and time flood risk degree are different from each other. The difference between annual flood risk degree based on annual maximum flood value and that based on annual maximum and submajor flood value has been discussed. The former accords with current standard for flood control of water-related engineering, whereas the latter is not consistent fully with the current standard for flood control.