Long term beach evolution mechanisms based on surface sediment analysis

日本的海岸侵蚀在过去30年中一直在加速。通过地理图的对比，估算出近15年来陆地面积的减少速度为24 km ~ 2。向沿海地区供应的沉积物减少被认为具有影响，特别是在日本，因为由于地质脆弱和河流坡度较陡，河流沉积物移动很大。因此，在由流域和近岸沉积单元组成的河流系统中建立泥沙控制是减轻海岸侵蚀的必要条件。在河流与海滩的泥沙平衡分析中，河流输沙量的估算是必不可少的。河流的泥沙供应受到各种人为影响，如水坝和堰的建设，河流路线的改变，以及河床的泥沙疏浚。港口、港湾和海岸防护设施的建设也改变了海岸沉积物的形态。 关于我们 动作为了制定有效的海岸侵蚀对策，首先必须定量地了解各种机制的贡献。然而，由于这些影响大多发生在过去40年的经济快速增长期间，并且在不同的时间和空间尺度上相互作用，因此很难将这些影响的影响定量地分开。基于地形测量资料的泥沙平衡分析，在多种过程复杂叠加的情况下，有时不能描述复杂的泥沙运动机制。在这种情况下，对沉积物质量的补充分析与对沉积物数量的补充分析相结合将是有益的。沉积物的质量是由物理和化学性质，如粒度，比重，形状，放射性和矿物constituent.In本研究中，区域泥沙运动和长期海滩变形的河流系统组成的流域和沉积物cell。通过一系列航空照片、河床、大坝水库和近岸区的水深测量数据，以及在包括海底和大坝上游河床在内的大面积区域取样的表层沉积物的物理和矿物性质，分析了沉积物的数量和质量。在综合分析的基础上，讨论了筑坝、挖沙、渔港、海岸防护工程等人为因素对海滩长期侵蚀的影响。少

Coastal erosion in Japan has been accelerating in the last three decades. The rate of decrease in land area, estimated by the comparison of geographical maps, is reported to be 24km2 in the recent 15 years. The reduction of sediment supply to the coastal area is considered influential especially in Japan because the fluvial sediment movement is significant owing to fragile geology and relatively steep slopes of rivers. The establishment of the sediment control in the fluvial system, composed of a watershed and a sediment cell in the nearshore zone, is therefore required to mitigate coastal erosion.In sediment budget analysis of a beach with a river, the estimation of sediment discharge from the river is essential. The sediment supply from the river is influenced by various anthropogenic impacts such as construction of dams and weirs, river route change, and sand dredging from the riverbed. The construction of port, harbors, and shore protection structures also alters coastal sediment m … More ovement. In order to develop an efficient countermeasure to coastal erosion, it is firstly essential to understand the contribution of each mechanism quantitatively. However, it is difficult to separate the effects of these impacts quantitatively because most of them occurred during the rapid economic growth in the last four decades and interacted each other with various scales in time and space. A sediment budget analysis based on topographical survey data sometimes fails to describe the complicated mechanisms of sediment movement when many processes are superimposed in a complex manner. Complementary analyses on sediment quality, combined with those on quantity, will be useful in such cases. The quality of sediment is described by physical and chemical properties such as grain size, specific gravity, shape, radioactivity, and mineral constituents.In the present study, regional sand movement and long-term beach deformation was investigated for a fluvial system composed of a watershed and a sediment cell. The quantity and the quality of sediments was analyzed by using a series of aerial photographs, bathymetry survey data of the riverbed, dam reservoirs and nearshore zone, and physical and mineral properties of surface sediments sampled in a wide area including the sea floor and the riverbeds upstream of dams. The anthropogenic impacts, such as dams, sand dredging, fishery harbors and shore protection structures on long-term beach erosion were discussed on the basis of the comprehensive analysis. Less