渤海湾海岸带近现代地质环境变化

淤泥质海岸带地区10～10²a级别的相对快速的“缓变型地质环境变化”,主要包括水体、湿地和盐沼-湖退化或消失、贝壳堤和牡蛎礁被破坏、村镇和城市面积剧增、海岸线(平均大潮高潮线)被侵蚀后退、潮间带和近海河道被淤积、潮间带变宽变缓等。这些变化,已对我国东部沿海地区的经济和社会发展造成不利影响,并将对该类地区的可持续发展构成潜在威胁。在该类地区正在进行的1∶50000区域地质调查,充分发挥其高精度和规范化的优势,并辅以对近百年来不同时期的地形图、航空照片和遥感影像的数字化处理和对比研究,试图对这些变化做出形象、直观和定量的二维(2D)表达。本文介绍的是正在渤海湾海岸带进行的工作,充分显示出地质调查服务社会的公益性、基础性和战略性特点。对这一研究方向所显示的巨大潜力的进一步开发,将可能形成供政府决策部门和各方面用户广泛利用的、可二次开发的平台,从而使该类地区的可持续发展建立在对近现代地质环境变化详细了解的扎实基础之上。

RECENT GEO-ENVIRONMENTAL CHANGES IN THE BOHAI BAY COAST

Geological environmental changes during the recent time in the Bohai Bay muddy coast have been very striking.The main characteristics are: decrease of wetlands and marshes, destruction of cheniers and oyster reefs, landward recession of the MHWST shoreline and seaward progradation of the MLWST shoreline (i.e. widening of the intertidal zone), vertical accumulation of mud and fine sand on the intertidal flat and in estuaries, and extension of cities and towns. These changes, usually on the scale of 10 to 10 2 a, are different from either the fast changes like earthquake, slide and mud flow or long life geological processes, such as development of cheniers and oyster reefs which need at least several hundred years. We, thus, called these ten to hundred year changes as the 'relatively fast changes', being easily perceived by one or two generations. These are indeed even more dangerous than those instantaneous disasters and long time processes for the muddy coastal zones where have been densely populated and most economically developed both in China and on the world. On the basis of shallow strata geological mapping (1∶50,000), including high resolution Eijkelkamp hand auger investigation for the mid and late Holocene coastal muddy sediments, systematic digital processing was carried out by using MAPGIS and PCI softwares for the maps of the period 1915～1989 with various scales from 1∶100,000, 1∶50,000 to 1∶10,000, and the TM/MSS images of remote sensing of the period 1976～1997 with the scale 1∶100,000. This systematic work was refined by a set of aerophotos of 1950's and 1970's, respectively. Consequently, two dimensional (2D) vectorial pictures were obtained. Through comparisons, these pictures show remarkable geo environmental changes along the Bohai Bay coast in the past hundred years, including 1) the area of the natural wetland and marshes/lagoons decreased by 50%; 2) as two famous components, the cheniers and oyster reefs were seriously damaged; 3) the shoreline of the mean high water spring tide (MHWST) has been retreated for several hundred meters while the shoreline of the mean low water spring tide (MLWST) has been extended farther seaward. The intertidal zone was widened from 3 to 5 km; and 4) cities and towns dramaticallly extended. As a part of the existing coastal geological mapping investigations along the Bohai Bay coast, the digital processing of various maps and topographic images is useful for the coastal sustainable development and infrastructure management