1976年黄河改道以来三角洲近岸区变化遥感监测

MONITORING CHANGES OF NEARSHORE ZONES IN THE HUANGHE (YELLOW RIVER) DELTA SINCE 1976

通过对黄河三角洲沿岸 1 0个河口、潮汐汊道的 2 3景陆地卫星影像高潮线的解译与对比 ,探讨自 1 976年黄河改道以来 ,三角洲岸线的变化特征 ;同时 ,利用GIS软件处理 1 976年和 1 992年滨海区水深测量数据 ,并与黄河利津站同期入海沙量进行对照分析。结果表明 :1 )目前黄河三角洲总体处于侵蚀破坏阶段 ,侵蚀岸线长相当于淤积岸线的 2倍 ;2 )人类活动对三角洲岸线变化的影响不断增强 ,人工海岸长度大幅度增加 ,总岸线长度缩短 ,岸线趋于平直 ;3)5 2 6%的黄河入海泥沙堆积于入海口附近 ,其中 92 %的泥沙堆积在 1 5m水深线以内 ,向外迅速减少 ,2 0m水深线以外出现侵蚀 ;4) 87%黄河入海泥沙往东、东南方向扩散与沉积 ,往南、东北方向扩散与沉积的泥沙很少 ,河口的北、西北方向整体上处于侵蚀状态。

The Huanghe (Yellow River) is well known for its high sediment concentration and low water discharge. The deposition of a large amount of fluvial sediment in the nearshore shallow water zone has resulted in the rapid advance of the Huanghe (Yellow River) delta. Due to reduction of river fluxes resulted from human activities in its upper and middle drainage areas, the evolution of the delta has showed particular characteristics since the artificial migration of the Huanghe (Yellow River) course in 1976. Remote sensing is an effective method to obtain data on changes in subaerial delta. Multi-temporal satellite images can be deployed to study coastline change in the Huanghe (Yellow River) delta. In order to determine changes since the last river course shift in 1976, 23 cloud-free Landsat Multispectral Scanner (MSS) and Landsat Thematic Mapper (TM) images, including 1976-1996, 1998 and 2001 were used in this study. All images were registered to a 1∶50000 topographical map with 30 Ground Control Points (GCPs) per image and transformed to Gaussian orthogonal coordinates. During the geometric correction, MSS images were resampled to 60m×60m while TM images were resampled to 30m×30m, using the nearest neighbor method with a root mean square (rms) error less than 0.9 pixel for MSS or 0.5 pixels for TM. Based on coastline interpretation maps of the 10 estuaries and inlets around the Huanghe (Yellow River) delta, coastline changes in the delta during the period 1976-1998 were analyzed according to different coastline types. Results show that accretion only appeared in the Qingshuigou Estuary with a mean annual change rate of 0.98km/a. Coastline erosion was taking place in the Guoquzibei, Chaohe, Diaokou and Zimaigou estuaries at a rate of -0.51km/a, -0.29km/a, -0.26km/a and -0.24km/a respectively, while coastlines in the remaining estuaries and inlets were relatively stable. The progradation rate of coastline near the active river course depended on the amount of incoming sediment from the Huanghe (Yellow River). Due to the decrease of water and sediment discharge, the coastline in the vicinity of the Qingshuigou Estuary continuously retreated over the period 1993-1998. Up to 1998, the length of eroded coastal segments was twice as that of prograded ones, showing that the Huanghe (Yellow River) delta, despite being the fastest growing delta in the world, was actually in a destruction stage. Human activities played an increasingly important role in the evolution of the delta. Due to the rapid industrial and agricultural development in the delta, artificial coastline grew fast causing great changes in the nearshore hydrodynamic conditions and sedimentation environment, and shortening in the total coastline length. In 1996, artificial coastline accounted for one-third of the total coastline in the delta. Topographic changes in the subaqueous delta, sediment diffusion and transportation in the nearshore zone off the delta have profound impacts on the evolution of the whole delta. Water depth data of the subaqueous Huanghe (Yellow River) delta surveyed in 1976 and 1992 were analyzed and digital elevation models (DEMs) of the sea bed were generated using geographic information system (GIS) software. The amount of sediment deposited at different orientations and water depths were then calculated. Results showed that 52.6% of incoming sediment was deposited in the vicinity of the river estuary, of which 92% of deposited sediment was within the -15m isobath, while erosion of the sea floor occurred beyond -20m deep. About 87% of deposited sediments were found to the southeast and east of the river mouth while a small amount of incoming sediment was transported to the south and northeast of the river mouth. Eroded sea floors appeared to the north and northwest of the river mouth. Through the analysis, it is clear that human activities had great impacts directly and indirectly on delta evolution from a construction phase to a destruction one of the Huanghe (Yellow River) delta.