Study On Erosion Characteristics Of Surface Sediments In Muddy Tidal Flat

Land resources are important for economy development.Tidal flats,including salt marshes and bare flat,as reserved land resource with important social and economic values,have received ample research attention.Tidal flats also have important ecosystem services such as climate regulation,material production,environmental purification and biodiversity conservation.It is of great significance to study tidal flat erosion systematically under the background of sediment source reduction,sea level rise and extreme weather and the human activities.Such understandings can provide important scientific supports for rational,scientific planning and utilization of tidal flats,ecological environment protection and sustainable development.At the same time,it can also provide a scientific reference of coastal zone management.In this thesis,we collected columnar samples of bed sediments from the typically muddy tidal flats in the Yangtze Estuary and configured artificial sediment samples in laboratory.Using the U-GEMS(UMCES-Gust Erosion Microcosm System)micro-erosion system,we examined the effects of sediment particle composition,consolidation time and biological action(cave size and vegetation leaf)on the erosion characteristics of surface sediments.At the same time,we obtained the regional distribution of critical erosion shear stress of surface sediments in the tidal flats of the Yangtze Estuary.These results provide a scientific basis and reference for erosion and siltation study of tidal flat and related engineering management.The main findings and conlusions are summarized as follows:(1)For sediment samples with a clay content between 22.9%and 33.7%,and a median grain diameter between 7.9?m and 19.4?m,their critical erosion shear stress is>0.60 Pa.The erosion mass(20 mins)under each shear stress phase is uniform.The sediment erodibility is mainly controlled by clay content.When the clay content is between 8.6%and 17.8%,and the sediment median grain diameter is between 32.5?m and 98.6?m,the critical erosion shear stress is between 0.10 Pa and 0.45 Pa.Moreover,the erosion mass increased significantly when shear stress is greater than0.20 Pa.The sediment is much easier to be eroded because of larger sediment median grain diameter.(2)When the content of clay,silt and sand is 43.5%,48.8%and 7.7%respectively,the cumulated erosion mass decreased significantly,e.g.,by 64.8%after a consolidation time of 2 days.The critical erosion shear stress is 0.10 Pa within 2days of consolidation,and is 0.45~0.60 Pa after 2 days.When the content of clay,silt and sand is 8.2%,16.4%and 75.4%,respectively,the cumulated erosion mass decreased significantly by 37.9%after a consolidation time of 2 days.The critical erosion shear stress is 0.20 Pa within 2 days of consolidation,and is 0.20~0.45 Pa after 2 days.(3)Regarding the impacts cave sizes mimicking the presence of benthos,the relative erosion area(side wall area of caves)increases due to the decrease of cave size,thus the critical erosion shear stress decreases and the cumulated erosion mass increases.The erosion mass of shear stress>0.20 Pa accounted for 41.9%~86.7%of the total erosion mass.The critical erosion shear stress increases and the cumulated erosion mass decreases with increasing vegetation occupancy because vegetation can reduce the energy of water flow.There is no critical erosion occurred on the sediments surface with presence of vegetation.Moreover,the erosion mass of shear stress<0.20 Pa accounted for 52.1%~76.9%of the total erosion mass.(4)The critical erosion shear stress of surface sediments and the cumulated erosion mass in the Yangtze Estuary tidal flats gradually decreases and increases from north to south,respectively.The critical erosion shear stress in the north regions of the Chongming,Dongtan is>0.45 Pa because of its high clay content,and the erosion mass is<0.05 kg/m~2.The critical erosion shear stress in the vegetated and bare flat zones of the southern region of the Chongming,Dongtan is>0.45 Pa,and the erosion mass is<0.1 kg/m~2.The critical erosion shear stress in the tidal creek zone is 0.45 Pa with a low degree of consolidation,and the erosion mass is>0.2 kg/m~2.The median grain diameter of surface sediment in the Hengsha,Dongtan is between 15.8?m and21.6?m.The critical erosion shear stress is 0.20~0.30 Pa and erosion mass is 0.1kg/m~2.The critical erosion shear stress in Nanhui,Biantan is 0.05~0.20 Pa with a high sand content and the erosion mass is 0.02~0.38 kg/m~2.The critical erosion shear stress is 0.05~0.30 Pa in the presence of caves,which is smaller than the normal conditions without caves.