Study on the methodology of recent coastline delimitation

The coastline is defined as the mark line of the boundary between sea and land at high water level of mean spring tide,and its length is measured with the same accuracy as in a map of the 1∶50 000 scale.However this boundary between sea and land in the geographic environment might be mostly exposed on the land and only covered by seawater for 2~3 hours once a month.Why not to take a line that is exposed for half of the time and covered for the other half as the of the land and sea? This is because the terrestrial plants are hard to survive even if they have been inundated by seawater for a short time.Substantially the coastline is a boundary between the environments for halophilous organisms and the environments for fresh water living beings,which is the essential connotation of the term.Therefore it is certainly related to the territory of human beings who determine the boundary delimitation.The coastlines with various physiognomies are classified according to the principles as follows:(1) For the coasts with cliffs,the coastline is located at the connection between the cliff and the beach,i.e.the landward edge of the backshore,where the strong waves can reach at high tides.(2) For the coasts with beach ridges,in general,the coastline is located right above the mean spring water level at the seaward side of beach-ridge top,the utmost position that surf or upthrust flow can reach.If lagoon is developed near the beach,the lagoon margin should be considered for the coastline delimitation.(3) For the coast with tidal flat,the coastline is located in the transitional zone where the growing condition for halophyte has changed and Artemisia halodendron Turcz.,Tamarix Chinensis Lour.and Phragmites australis become dramatically sparse and short near the coastline,where there is "a waste-deposit line" with fragments of plants,shells or other waste."The waste-deposit line" is formed by the waste pushed by uprush into the tide flat and settled down during the spring high water period,and it is a typical mark line.In the same way,if the coast is developed with longshore bar(a sea-shell bar) on the tide flat,the lagoon margin should be considered for the coastline delimitation.(4) For the estuarine coast including adjacent water area,the coastline is defined on the center of the middle segment of the estuary.In the upstream of the middle segment,the fluviation is dominant,while in its downstream sea action does,and in between,the middle segment is a transitional zone.Since the middle segment of the estuary has a certain length,the coastline is defined at the boundary where salty water reaches in the dry season,and it is close to the central point of the middle segment,where the outlet gets wider,with a certain distance form the tidal boundary in the flood season.If the salinity in seawater is used for the delimitation,the location where the chlorinity of the estuarine water is ≥3 150 mg/L in dry season will be the coastline.It is the chlorinity with which the terrestrial plants can hardly survive.For rivers with more silt in water such as the Yellow River,the river course is narrow,water drop small,and the distance that the up-bound tide covers is short,and the up limit of the tidal water in flood season can be considered as the boundary between the sea and land.(5) Artificial coast.If there are permanent artificial structures on shore that encloses a large area of land mostly with towns or cropland,these structures are taken as the artificial coast.But narrow and long breakwaters,sand control dams and sightseeing dams are not included in this case.The delimitation of the coastline of salt field,where fresh water organisms can not survive and seawater needs to be abstracted either naturally or artificially for salt production,is a difficult issue.According to the connotation of the coastline delimitation,the natural passages(lagoon outlet or a bay) of the flood or ebb tidal water should be defined as sea.Therefore the landward margin of salt ponds will be considered as the coastline.But if underground brine is used for the salt production,it will be an exception.The marine aquacultural ponds on beach or tidal flat generally belong to sea area.