浙江瓯江口南岸潮滩相带的时空变化

SPATIAL AND TEMPORAL VARLATIONS OF TIDAL FLAT IN THE OUJIANG ESTUARY IN CHINA

瓯江口南岸潮滩可分为三个沉积地貌相带：高潮滩物质由砂、泥混合沉积组成，沿岸沙堤增长；中潮滩物质以砂质为主，坑、洼地貌遍布，滩脊发育，是潮滩中最活跃，对水动力条件反映最敏感的相带；低潮滩物质由泥质组成，滩面平缓，是稳定的淤积带。潮滩相带的沉积物、地貌特征具有明显的时空变化，瓯江入海的水沙对瓯江口南岸潮滩演变产生明显的影响。

The tidal flat on the southern bank of the Oujing estuary can be divided into 3 morphological facies zones according to the bedforms and sediment distributions: (ⅰ) mixed sand-mud high tidal flat;(ⅱ) sandy middle tidal flat: (ⅲ)muddy low tidal flat. Due to variable behaviours in response to tidal processes ai different sites within the same facies zone, 8 subfacies belts can been distinguished.On the sandy middle tidal flat,surficial sediments are scoured and frequently reworked by rectilinear tidal currents and waves. It is the most active and sensitive facies zone in all of the 3 facies zones. The surface sediment activity is 2. 9-5. 5 cm thick on the daily basis under normal conditions.The bedforms on the surface are also more variable than the other two facies zones. Bumps and puddles, ridges and runnels as well as creeks are present over this facies zone. The disturbed(mixed) layer reaches more than 50cm measured by Pb210. However,the muddy low tidal flat is a steady accumulation zone with an advncing rate of 10m/a toward the sea. The deposition rate is 0. 31-2. 97cm/a which is the largest in all of the 3 facies zones.The morphological and sedimentary features in the facies zones change obviously with space and time. Freshwater and sediment discharges from the Oujiang River play important roles in the morphological evolution of the tidal flats at the southern bank of the Oujiang River. The river influence on tidal flat decreases with the increasing distance from the Oujiang River mouth ,and disappears in the area to the south of transect Ⅲ. The mixed sand-mud tidal flat changes into the muddy tidal flat going from the north part to the south part in the study area. By comparing the data sets taken during two investigations (Dec. 1990 and May 1991 ), the facies zones are shown to move landward and the surficial sediments in the middle tidal flat became coarser from December 1 990 to may, 1991. It means that the tidal current was enhanced on the tidal flat in winter. In addition, storm events and human impacts contribute significantly to the tidal flat development.