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

在1998财政年度至2000财政年度期间，我们进行了三次沿海排水系统的原型试验。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，以及如果它是有效的，它是如何工作的。实验地点是鹿儿岛县的柏原海滩和伊索海滩。韦斯特比报告说，安装两周后，海岸线向前推进了几米。这激发了我们计划实地实验的动机。虽然，我们的排水量比他的大，但我们没有看到预期的大量泥沙的积累。这可能在一定程度上是由于排干地下水的方式。传统的方法是通过重力将地下水排入蓄水池。在我们的实验中，泵被连接到管道上，并通过连续抽吸直接排水。我们采用这种方法是为了使排水流量变大，这是短期试验的优点。对滩面压实的测量表明，由于吸力的作用，靠近管道的部分似乎被压实并变紧。这种压实效应似乎是抵抗侵蚀波作用的重要因素之一。这可以被认为是对沿海排水沟功能的一种新的观点。通过这些现场试验获得的数据将在开发数值模式时作为验证数据具有重要意义。此外，从实用的角度来看，这些原型实验的经验也被认为是对系统安装的提示。通过这些实地实验，我们应该研究与沿海排水系统功能有关的基本物理过程的问题变得更加清楚。

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.