Water improvement measures for fishing port basin and its application

摘要

As fishing ports are improved, water quality aggravation in the harbor becomes a serious problem. Usually, in the construction of a small fishing port, the width of a port entrance is made as narrow as possible. In addition the harbor is often divided into several sections by inner breakwaters or wharves to reduce wave height even more. These procedures transform the harbor into a semi-closed basin, and consequently the water becomes stagnant and the ebb and flow of the tide can no longer maintain a well-mixed water condition. In addition to harboring fishing vessels, fishing port harbors have been utilized for other purposes, such as places to pump water from for the washing of fresh fish, or cleaning transit yards, or to accommodate cages for the temporary storage of live fish. Several measures have been taken over the past 30 years to counter the deterioration of water quality. This paper will introduce some findings on the mechanism on water quality deterioration, based on a survey conducted at Ishinomaki fishing port. It suggests that the main causes of water deterioration are the sedimentation and decomposition of organic matters such as internally produced phytoplanktons, the poor fluidity of the harbor basin, and the anaerobic condition of the sludge. Secondly, the report discusses the countermeasures that have been taken to cope with these problems and the types of measures that introduce external seawater. Thirdly, two examples of water introduction using wave energy to improve or maintain the water quality of a harbor. Finally a method to estimate the degree of improvement we are now grappling with will be suggested.