3.3　Land Flood Survey

3.3.1　In land flood survey，the following information shall be collected:the hydrographic map of drainage areas，topographic map of drainage areas and river reaches to be surveyed；physical geography conditions of drainage areas，locations and hydrological characteristics of water bodies in the area，characteristics，causes and historical records of rainstorms and floods，flood survey reports and data from hydrological stations；current status and planning of drainage areas and watercourses；survey and engineering data，construction quality and safety and operation data of water-related projects；ice-formation period，ice float period，way of ice break-up，distribution scope and duration of ice dams and ice jams.

3.3.2　The flood survey shall be conducted upstream and downstream the site.This survey should include main and branch waterway and even larger area if necessary.

3.3.3　The river reach to be surveyed shall be with straight watercourse，stable riverbed and under good control，without major tributary，backwater，distributary or banked-up water and with similar composition of riverbed material and bank vegetation.

3.3.4　For survey of floods，the focus shall be laid on the occurrence time of each extraordinary flood and the corresponding recurrence interval，flood marks，flood courses，changes in scour and silting，and riverbed roughness.The information on rainfall，water and disaster when the flood occurs shall be investigated.The source，cause，main flow direction and driftage of the floods shall be identified，and the existence of overland flow，distributary，crevasse，stagnant water as well as significant changes in the natural environment of drainage areas and watercourses shall be determined.The runoff floods shall be distinguished from dam-break floods caused by other causes.

3.3.5　The flood survey should be conducted in old residential areas and river reaches with more flood marks.At least three reliable or relatively reliable and representative flood marks shall be identified along one river bank for one flood survey，and the accuracy of the flood marks shall be validated.In desolate areas，flood marks may be identified by river deposits，flood scour marks，and flood biochemical marks on both banks.

3.3.6　The flood survey in plain areas shall focus on river network characteristics，historical water logging disasters，and current status and planning of local flood and water logging control works.