10.2 Construction Diversion
10.2.1 The standard for construction diversion of a hydropower station shall be selected according to the following principles:
1 If the temporary diversion structure is Grade V，the flood standard shall be in accordance with the specification given in Table 10.2.1-1.
Table 10.2.1-1 Flood standards for temporary structures
2 If the dam has been filled high enough and does not need to be protected by a cofferdam，or the diversion and water release structures have been blocked，the temporary flood protection standard shall be determined as per Table 10.2.1-2.However，the standard may be raised or lowered，after specific demonstration，according to the potential impact of an accident on downstream areas.
Table 10.2.1-2 Temporary flood protection standards for dam body
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3 The water retaining and overflow standards for overflow cofferdams shall be defined according to different operation periods of the cofferdam with reference to Table 10.2.1-1.
4 For the standard for river closure，the monthly or 10-day mean discharge with a return period of 3 years-5 years during the river closure may be adopted.
5 The time of plugging diversion and water release structures shall be determined according to the general schedule of construction，under the prerequisite of meeting the requirement for flood retaining and water storage of reservoir.The design discharge for plugging and gate closure may be defined by using the monthly or 10-day mean discharge of a recurrence interval of 3 years-10 years at the time period of plugging.
6 The design standard for diversion during construction of a plugging project may be selected within a return period of 5 years-10 years for the plugging duration，in accordance with the importance of the project and the potential impact of an accident，etc.
10.2.2 The diversion method shall be selected after comprehensive analysis of cofferdam closure，temporary flood control of dam body，plugging，initial power generation and general construction progress according to the project layout，types of hydraulic structures and characteristics of river.After comparing the options，the following methods may be adopted:cofferdam diversion by stages，diversion by free flow channel coordinated with cut-off cofferdam，diversion by tunnels，diversion by culverts，dam bottom outlet diversion and gap diversion during construction，and diversion by the combination of various water release structures.
10.2.3 The types of diversion retaining and water release structures shall be finalized after technoeconomic comparisons of topographical，geological and hydrological conditions，project layout and construction conditions，etc.If the diversion structures will be incorporated into the permanent project，the incorporation method and detailed measures shall be developed.
10.2.4 Water storage during construction shall be associated with the plugging of diversion and water release structures.The plugging measures for diversion structures afterwards shall be proposed.During the plugging of diversion structures，measures shall be taken to deal with the downstream water demand.
