三亚海岸演变与人工海滩设计研究*

三亚海岸位于海南岛南部,属弱潮海区,以来自开阔外海的偏南向风浪为海岸优势动力。三亚海岸经历了从基岩港湾海岸到岬角与港湾相间的海蚀-海积海岸的发育演化过程,沙坝(沙咀)发育始于中更新世初期,珊瑚礁发育始于8kaB.P. ,目前海岸总体处于相对平衡发展阶段。对三亚地区海岸地貌、动力及泥沙运动的调查研究,总结三亚湾及其周边不同类型海滩的特点,提取其海滩参数。依据处于不同发展时期的自然海滩形成演变的条件和规律,设计建构三亚白排人工海滩的关键参数。设计海滩总长度约400m,宽度{40~}50m,坡度4.5°~5.0°,相对高度约2m,填砂M_为0.5,以粒径1.0~}0.5mm的粗砂为主,总填砂量48000m³。从滩面物质和滩面坡度两个关键方面,利用代表性的数学模型,检验了设计海滩的稳定性。本项研究旨在服务白排人工海滩建设,研究方法对海滩侵蚀防护与同类海岸工程建设等具有参考价值。

EVOLUTION OF SANYA COAST AND ARTIFICIAL BEACH DESIGN

Beach is the most active geomorphologic part in the coastal zone, having its depositional feature controlled mainly by surf wave. Beach changes every minute, with its relatively stable sediment movement affected by the swash and backwash. Beach profile records surf wave characteristics, sediment resources, and grain size of the depositional material. A steep beach slope is associated with strong energy of surf wave, coarse sedimentary material, and strong erosional processes. Correspondingly, a gentle beach slope leads to fine material. If the slope is too flat, tidal dynamics would become the dominant dynamic factor in the nearshore area; fine material, such as silt and clay, would overlay the sand, and the beach would turn into a tidal flat. Thus, the relationship between local wave characteristics, sediment sources, and beach material are the key factor for the development process of a beach. By its bright sunlight, comfortable sandy beach, and delicious sea food, the so-called 3S, a beach has become the most attractive recreation place for world travelers. Among the 3S, a stable sandy beach is the most important feature. Due to sea level rising, beach erosion is becoming more and more serious in many places in the world in recent years. Based on various kinds of practice of coastal engineering to deal with the beach erosion problem during the past several decades, it has been proved that beach nourishment and artificial beach are the most effective and economic way at present and they have been applied in many places in America, Europe, and Asia with some useful good experiences. Sanya is a microtidal coast located in southern Haian Island. Wind waves from south are its dominant dynamic force. The coast of Sanya area has been developed from the original bedrock embayment coast to erosional-depositional coast with alternative headlands and bays. Sandy bars (spits) began to develop since early Middle Pleistocene, and coral reefs started to develop from 8kaB.P. At present, the coast of Sanya area is generally in a relatively stable stage. By local geologic and geomorphologic settings, dynamic processes, and sediment movement in the nearshore area of Sanya, sandy beaches have been investigated and classified into two main kinds, each of which has different models. Most of the sandy beaches are relatively stable, while some have been eroded slightly and some are still growing up with sediment accumulation. Based on this investigation, for a better natural development as well as economic development, an artificial beach has been designed in front of a breakwater for the artificial island in the southern part of the Baipai reef in Sanya Bay, with the hope that it would become a new attraction of the artificial island to improve its attractiveness and function for recreation. Due to the limited space and the nature situation of the platform of the Baipai reef, the designed arc-shaped artificial beach is about 400m long, 40~50m wide, including backshore, foreshore, and submarine parts. The backshore part is {20~}30m wide, the slope of foreshore part is 4.5°~5.0° with the width 9~10m. The submarine part becomes gradually flat extending to the coral reef platform. A 30cm-thick layer sand is planned to cover certain belt of the platform to protect the designed beach through reducing wave energy. The relative height of the artificial beach is about 2.0m. At least about 48000m³ sand is estimated to be necessary to build this artificial beach and more quantity of sand is expected to be borrowed to nourish a long lasting beach. Coarse sand with M_ as 0.5 would be the ideal borrow sand. Simulated with the U.S. and Japanese empirical formulae, the designed borrow sand and beach slope will lead to a stable artificial beach with relatively less quantity of lose sand every year.