近期长江口南港河槽沉积地貌变异及其可能原因

南港是长江口两个主要入海通道之一,也是长江深水航道的必经之路。基于2012和1984年的地形资料,利用Arc GIS技术对南港进行冲淤分析;并用同一种方法(筛析法-移液管)对相应年份所采表层沉积物样品进行粒径测试对比。结果表明:近28年来南港地形发生了重大变化,复式河槽间的沙脊因刷深而趋于消亡,主航槽则淤浅数米;沉积物中值粒径均值从81μm下降为24μm,特别是原沙脊位置上的沉积物从砂变细为砂质粉砂或粘土质粉砂。分析认为,采砂是上述变化的主要原因:沙脊采砂使复式河槽"中脊"消失,水动力减弱,河槽中央过水断面增大、分流增多,两侧航槽流速减小导致淤浅;沉积物变细是较粗颗粒被采走、动力减弱和淤积发生的综合结果。

VARIATIONS AND CAUSES OF SEDIMENTATION CHARACTERISTIC IN SOUTH CHANNEL OF YANGTZE ESTUARY

The South channel is one of the two major entrances of Yangtze River into the sea, and the necessary passage of the North deep-water navigation channel. In this paper, we collected the topographic maps and 12 sediment samples from 1984 and 2012, and compared the grain size of riverbed surficial sediment using the same method (sieve and pipette analysis) of the two years. The erosion and/or deposition of the two year was analyzed by using the ArcGIS software. The results showed that there was a major geomophological change in the South Channel during the past 28 years. Sand ridges in compound channels were dying out due to sediment erosion, whereas the main channel tended to be shallow to several meters due to siltation at the same time. Moreover, the median grain size of surficial sediment decreased from 81 μm to 24 μm, especially the sediment type on the original ridge changed from sand to sandy silt, even clayey silt. We determined that sand extraction mainly caused these changes in South channel because this activity led to the extinction of mid-ridges in the compound channels, weakened hydrodynamic environment, and amplified the area of the crossing-section. Siltation on both sides of the channel arose due to lessening in flow velocity. The sediment size became finer depending on combination of coarse sediment dredging, weakening current and slitting in the channel.