3.3　Engineering Hydrological Investigation and Surveying

3.3.1　The task of engineering hydrological investigation and surveying is to provide reasonable and reliable hydrological investigation results for the design，construction and operation of overhead transmission lines under design conditions.

3.3.2　For 330kV-750kV overhead transmission lines，the design standard for flood control shall not be lower than 100-year reoccurrence interval.

3.3.3　Where the overhead transmission lines cross over a navigable river，canal，reservoir or lake，the reoccurrence interval of flood of the design maximum stage of waterway shall comply with GB 50139 Navigation Standard of Inland Waterway.In the case of a gulf，the national standards concerning navigation standards of sea areas shall be followed.

3.3.4　Where the overhead transmission lines pass through important water-conservancy facilities，traffic areas or marine areas，special study shall be carried out in accordance with relevant laws and regulations.

3.3.5　New technologies and methods should be actively and cautiously promoted in the hydrological investigation and surveying of lines.

3.3.6　The basic data referenced in the hydrological analysis and calculation shall be analyzed in terms of reliability，consistency and representativeness，and the calculated results shall be justified.

3.3.7　The elevation system of design water level shall be identical with that of plan and section drawings of lines.

3.3.8　In case of emergencies such as catastrophic flood，storm surge or huge evolvement of bank，field investigation shall be carried out in time，the hydrological investigation results provided during the design stage shall be rechecked，and counter measures for such emergency events shall be proposed.