2.3　Flood

2.3.1　All or parts of the following design flood results at dam（plant）sites shall be provided as required for the design of hydropower stations:

1　Annual maximum peak discharges of design floods with different frequencies and their maximum volumes in a period of time.

2　Seasonal maximum flood peak discharge and flood volume in a period of time by each design flood frequency.

3　Annual and seasonal design flood hydrographs by each design flood frequency.

2.3.2　The computation method for a design flood shall be properly chosen based on the data available in the region where the hydropower station is located.

If more than 30 years of actually measured and interpolated flood data is available for the dam（plant）site or at the upstream and downstream nearby locations，the frequency analysis method shall be employed to calculate the design flood.

If there is no flood data for the hydropower station region，but precipitation data is available，the design flood may be derived from the design rainstorm by analyzing the rainstorm frequency.

2.3.3　Design flood may be derived based on comprehensive analysis of the region by using flood and rainstorm data actually measured or investigated in the vicinity of the hydropower station，in case that the actually measured flood and rainstorm data for the region of the hydropower station is limited.The design flood may also be derived from the design rainstorm based on the research results on rainstorm and runoffgeneration-and-concentration areas with the matching Rainstorm Runoff Calculation Charts nationwide or in provinces，autonomous regions and municipalities directly under the Central Government that have been approved by the competent authorities.

2.3.4　When deriving design flood from design rainstorm，design rainstorms of different durations may be calculated by using the design spot rainstorm and the spot-area relationship.The design spot rainstorm may be calculated from the approved isoline map of rainstorm statistic parameters.Temporal distribution of the design rainstorm may be obtained by amplifying design rainstorms in different durations with same frequency based on the region's comprehensive rainfall pattern or a typical rainfall pattern.

The duration of design rainstorm may be 24h or defined according to the basin area and duration of flow concentration.

Parameters of runoff-generation-and-concentration to derive design flood from design rainstorm may be found on the approved rainstorm runoff calculation chart.

2.3.5　Parameters and calculation results used in the computation of design flood shall be analyzed and examined in multiple ways to demonstrate rationality of the results.

2.3.6　For historical floods to be adopted in the computation of design flood，the historical flood investigation results that have been promulgated by the authorities of provinces（autonomous regions or municipalities directly under the Central Government）may be quoted directly.If actually measured data or investigated historical flood data is unavailable for the river where the hydropower station is located，investigations regarding historical floods shall be carried out at the dam（plant）site or at its upstream and downstream reaches.

2.3.7　In the computation of stage design flood，the stage shall be defined according to design requirements of the project，and the starting and ending dates shall comply with the pattern of changes in flood season.The stage should not be shorter than one month.The stage design flood may be used at cross-stage.

2.3.8　If there is a regulating storage reservoir upstream of the hydropower station，the regional composition of design flood shall be studied，and the design flood at the dam site under the impact of the water storage and regulation of the upstream reservoir be derived.