长兴、横沙潮滩是典型的河口潮滩。其沉积物粒度在总体上较四周河道细，且具有从潮下带向潮上带沉积物质逐渐变细的趋势，但由于风浪和植被等因素的影响，局部高程的变化往往较为复杂；在沿岸方向上，以面向口外的开敞岸段最粗，顶江迎流的洲头滩次之，顺江两岸物质相对较细。在此基础上，提出了分汊河口冲积岛潮滩的基本沉积模式和河口潮滩与正常海岸潮滩的区别。

Based on the data of field observation and sediment samples. the paper deals with the grain parameters. the sedimentary structures and the formation processes. Meanwhile a basic model for tidal flat sedimentation on the alluvial islands in bifurcate river mouthes was established. Each profile has a general trend that sediments become finer landward though the details are different. Averagely,M_Z(φ) increases from 3. 36 in the subtidal zone to 7. 03 in the supratidal zone. This trend is either due to the weakening of water energy of currents and waves or the appearance of the vegetation at the high Fart of the fla. Along the middle tidal line, the average M_Z(φ) are respectively 3. 29, 4. 69,5. 79 and 6. 45 in the open east Hengsa coast, the bifurcate coast of west Changxin. the north and south branch coasts. δ_Ⅰ= 0. 463.M_Z-0. 785, r= 0. 94 (n=40), which is similar to the Chongming Island and the south coast of the Yangtze Estuary. 90% of the SK_Ⅰ is positive that is different from beach sediment.there are apparent diversities among sedimentary structures on the east Hengsa coast. west Changxin coast and the north and south coasts although each profile shows the reduction of water energy from the low to the high places. The bare flat of the eastern Hengsa is dominated by thick sand bands in which flaser bedding and cross bedding sometime occur and the marsh is mainly alternate thick beds of sands and silts. In the west Changxin coast. flaser beddings and wavy beddings usually appear on the bare flat, alternate thick beds of fine sands and fine silts which are composed of alternate thin beds or wavy beddings occur in the Scirpus zone and horizofital laminers occur in the reed zone. In contrast. structures on the bare flats on the north and south coasts consisting of wavy beddings and alternate thin beds of silts and mudy silts and horizontal laminers of muds occur in the intertidal marshes and no beddings can be found in the supratidal marshes. This difference is also due to the variation of hydrodynamics.Abundant roots of plants in the marsh sediments often deform or destroy the layers. Sediments are slightly disturbed and shells are scarce because of the low biomass of benthic fauna and the unsteadyness of the flats.