Field experiments on the functions and capabilities of a coastal drain system as a measurement for beach erosion control

在1998年至2000年期间，我们进行了三次沿海排水系统原型规模的现场试验。本实验的目的是调查海岸排水系统作为海滩侵蚀控制措施的有效性和能力，如果有效，它是如何工作的。实验地点是鹿儿岛县的柏原海滩和矶海滩。韦斯特比报告说，安装两周后，海岸线向前推进了几米。这促使我们计划进行实地实验。虽然我们的排水量比他的大，但我们看不到预期的大量沉积物的积累。这可能部分是由于地下水的排放方式。传统的方法是靠重力将地下水排入集水坑。在我们的实验中，泵连接到管道，并直接与连续抽吸排水。我们采用这种方法是为了使排水量变大，这是一个短期的实验优点。海滩表面的压实测量表明，近管道的一部分似乎被压实，并由于吸力变得紧密。这种压实效应是抵抗波浪冲刷作用的重要因素之一。这可能是一个新的观点，对海岸排水系统的功能。通过这些现场实验获得的数据将作为数值模型开发中的验证数据具有重要意义。同时，从实用的角度来看，这些原型实验的经验被认为是对系统安装的提示。通过这些实地实验，我们应该研究的与沿海排水系统功能相关的基本物理过程的问题变得更加清晰。

We conducted field experiments of a Coastal Drain System with prototype scale three times during the period between the fiscal year of 1998 and that of 2000. The objectives of the experiments were to investigate the effectiveness and capabilities of a Coastal Drain System as measures for beach erosion control, and, if it is effective, how it works.The sites of the experiments were Kashiwara beach and Iso beach in Kagoshima Prefecture. Vesterby reported several meters shoreline advance after two weeks of the installation. This motivated us to plan the field experiments. Although, our drainage were larger compared with the one of his, we could not see the accumulation of expected large amount of sediments. This may partly due to the way to drain groundwater. Conventional method drains groundwater into a sump by gravity. In our experiments, pumps were joined to pipes and drained directly with suction continuously. We adopted this method as it was expected to enable the drain discharge become larger, which is a merit for short-term experiment.The measurements of the compaction of beach surface showed that the part of near pipes seemed to be compacted and become tighten due to the suction. This compaction effect seemed to be one of important factors to resist an erosive wave action. This may be considered to be a new point of view on the function of a coastal drain. The data that was obtained through these field experiments will have importance as the verification data in the development of a numerical model. Also, from the practical point of view, the experiences of these proto type experiments are considered to become hints on the installation of the system. The issues we should examine about the fundamental physical processes that are related to the functions of a coastal drain system have become clearer through these field experiments.