Figure 1    The coastline distribution of northern China from 2000 to  2012

China, which has been a new growth pole of China’s economic development in recent years and the new driver following the Pearl River Delta in the south and Yangtze River Delta in the east。 There are three provinces and one municipality (i。e。, Liaoning Province, Hebei Province, Shandong Province, and Tianjin Municipality) along the coastline, this belt has gone through major changes in economy and infrastructures, and growth is occurring in various marine development activities, such as fisheries, ocean shipping, petroleum, natural gas, and sea-salt extraction operations, among other pursuits (Liu et al。, 2008)。 The creation of the Tianjin Binhai New Area, the Liaoning “five points and one line” Coastal Economic Zone, the Hebei Caofeidian Circular Economy Demonstrative Area, the Cangzhou Bohai New Area, and the Shandong Peninsula Blue Economic Zone advanced the scale and inten- sity of marine exploitation in the Bohai Rim (Wang et al。, 2011)。 Meanwhile, numerous ma- jor rivers in northern China flow into the Bohai Sea, including the Yellow River, Haihe River, Luanhe River, and Daliao River, among others。 Changes in natural conditions, including (but not limited to) estuarine siltation and erosion, and the influences of human actions, such as land reclamation, marine reclamation, port construction, and other activities, will inevitably cause constant changes in coastlines and thereby affect the coastal  environment。

2Materials and methods

2。1Acquisition and processing of the data sources

The remote sensing data sources used for the extraction of coastline information included Landsat thematic mapper (TM) data, China-Brazil Earth Resources Satellite (CBERS) charge-coupled device (CCD) data, and Huanjing (HJ)-1 CCD data。 The spatial resolution of the TM data and the HJ-1 CCD data was 30 m, and the spatial resolution of the CBERS CCD data was 19。5 m; both of these resolutions satisfy the requirements for 1:100,000 map- ping。 A total of 87 remote sensing images from all of the examined times were used in this study, and all images were compared with topographic maps for geometric corrections, and the error was never greater than 1 pixel。 Auxiliary data used in this study primarily include topographic maps, administrative maps, and local chronicles; a field survey of coastline in the Bohai Rim was also conducted to provide supporting information for interpretation of coastline positions on remote sensing images。

2。2The method of extracting remote sensing information regarding  coastlines

There have been many studies on coastlines changes in China, however, the coastline infor- mation extracted by most processing methods that are based on remote sensing imagery is the sea-land boundary when a satellite flies over an area (i。e。, the instantaneous “water edge”), rather than the geographic definition of the coastline(Ma et al。, 2007; Li et al。, 2009)。 In this study, in accordance with the “908 Special” definition, the coastline was regarded as the line that traces the boundary between land and sea during an average high spring tide (OSOA, 2005)。

Coastlines are pided into continental coastlines and island coastlines, and this study only examined the spatial and temporal variation of continental coastlines。 The following principles were used to determine the land-sea boundary for coastal estuaries: retain the

harbor-like characteristics of large estuaries; reflect the geomorphologic characteristics of sand spits, shoals, and lagoons of estuaries; manifest the trumpet-shaped morphology of an estuary; treat asymmetric canals based on the shapes of estuaries, assign the piding line between estuaries to locations where rivers become narrow or the maximum curvature of capes occurs。 The land of the tidal flats of the muddy coast (that is not submerged at high tide) was regarded as a collection of islands, and the coastlines of this land were not consid- ered in this study (Zhu et al。, 2001)。