4.7　Analysis of Reservoir Sedimentation and Backwater Computation

4.7.1　If the ratio of storage capacity to annual sediment volume flowing into the reservoir（hereinafter referred to as“ratio of storage to sediment”）is less than 30，the sedimentation becomes a major issue for the project；therefore，a reservoir sediment regulating mode that aims to release sediment and reduce sedimentation shall be formulated according to the characteristics of flow and sediment，reservoir configuration，scale of releasing flow，and the impact of sedimentation on reservoir inundation and ecoenvironment，etc.If the ratio of storage to sediment is larger than 30，the sedimentation issue is less significant；then，if there are no important and sensitive inundated objects or other facilities in the reservoir，it is not obligatory to consider the impact of sedimentation，nor to study the reservoir sediment regulating mode.

4.7.2　For sediment-laden rivers with relatively large natural sediment concentration，i.e.the mean annual sediment concentration is larger than 1.0kg/m3，the analysis and computation of reservoir settling sediment should be carried out so as to propose the sediment prevention measures for the diversion of the project.

4.7.3　For high-water-head hydropower stations，the allowable concentration of sediment that passes through the turbines shall be proposed.

4.7.4　In the computation of sediment scouring and silting of the reservoir，various methods may be adopted based on the flow and sediment characteristics，the reservoir sediment regulating mode，and hydrological data availability，etc.If there is less data available，the analogous or empirical method may be applied.If more data are available，the mathematical modeling may be used for calculation，while the major parameters shall be calibrated by actually measured figures，to find out the locations and volumes of sedimentation，and its impact on regulated storage capacity，etc.

4.7.5　In the computation of reservoir backwater，the natural water surface profile before construction of the reservoir and the water surface profile of backwater in reservoir within the predicted duration of sedimentation upon completion of the reservoir shall be derived from the design discharge according to the river course conditions，characteristics and operation mode of reservoir.