Long term beach evolution mechanisms based on surface sediment analysis

点击翻译按钮获取中文摘要

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