In Saroma Lagoon, the delay of freezing due to recent warming trends has led to a great deal of damage to aquaculture facilities and fishing boats in the lagoon, caused by inflow of ice floes. The ice boom method, by which incoming ice floesare trapped inside the lagoon inlet, was introduced as a preventive measure against inflow. The construction of 13 spans of ice booms began in 1994 and was expected to take several years. There are currently6 existing spans, 2 in the center installed in 1994, and 4 on the sides, 2 on either sid, installed in 1995. Various investi-gations were conducted by demonstration tests using full- scale experiments, and with even as few as 6 spans, installed in the center of the lagoon, effective ice floe control was confirmed.
This is the first attempt to introduce the ice boom method in an actual sea area anywhere in the world. The biggest design problem to be overcome is how to accurately estimate the load acting on the main wires when trapping ice floes. Factors which affect the tension of the main wires include fluid force of wind and flow, acting on the trapped ice floes, and the length of ice plate groups. The survey results showed that the constantly changing tidal current and the condition of inflow of ice floes influenced wire tension. However the measured tension was only 10% of the design value, as the whole facility has yet to be completed and conditions relating to oceanographic and meteorological phenomena did not reach their maximum values as estimated in the design.
This paper compares methods to calculate tension on the main wires and the results obtained in field surveys, and con- siders factors which affect tension, to compare and verify calculated and actual values.
1. Introduction
Saroma Lagoon is an inland sea- lake on the Sea of Okhotsk coast and has a circumference of 91km and an area of 152km 2. It is separated from the Sea of Okhotsk by a long, narrow sandbar and connects with the sea through 2 o. penings in the sandbar: the approximately 250- m wide first opening in the center of the sandbar and the approximately 50- m wide second opening. The area around Saroma Lagoon is designated as a quasi- national park and is famous for the many attractions along the shore, including sweeping natural landscapes and natural flower gardens.
The salinity in the lagoon is almost the same as that of the outside sea because the water is relatively deep and the wide inlets allow a large amount of seawater exchange with the Sea of Okhotsk. In addition, the sandbar maintains tranquillity in the lagoon, creating an optimum environment for scallop and oyster cultivation, making this area a popular location for natural sea farming. Although the lagoon freezes in winter, the first day of freezing period, which in the decade after 1965 used to arrive in the middle of November, has been increasingly delayed every year, due to winter warning trends. The freezing period now arrives in the middle of February. In 1989, the lagoon did not freeze at all.
Meanwhile, ice floes approach the coast of the Sea of Okhotsk. Although the approaching time varies year to year, it can usually be expected from late January through early February. After repeated landward and seaward movement a- long the shore, the ice floe period usually ends in March. When the lagoon freezes early, the inflow of ice floes can be blocked by the existing ice plates in the lagoon.However, when the lagoon cannot freeze sufficiently before the arrival of ice floes, as in recent years, inflow of ice floes through the lagoon inlets has damaged aquaculture facilities and fishing boats in the lagoon. Under these circumstances, the Hokkaido Development Bureau drew up plans for a breakwater (ice barrier) for the Saroma Lagoon fishing port to prevent the inflow of ice floes.
After examining various factors, including structure, design and location, the ice boom method, which uses floating structures to control ice floes, was chosen for use inside the lagoon's first inlet Of the 13 spans planned, 6 are now in operation, 2 spans installed in the center in 1994 and 4 on the sides, 2 each on either side, in 1995. At the same time, field observations and research have been conducted.
Although the ice boom method has been used for ice plate control in rivers in the United States and Canada, this was the first time it had been applied in an actual sea area. The biggest problem to be solved in design was how to accurately estimate the load acting on the main wires when trapping ice floes.
To this end, this paper focuses on the tension acting on the main wires of the ice boom, introduces its design method and discusses the present state of ice floe control and the results of the field observations. Based on these results, the characteristics and variable factors regarding tension will be analyzed to compare and verify design and actual values.
2. Outline of Ice Barriers
As shown in Figs.1 and 2, the Saroma Lagoon fishing port breakwater (ice barrier), which has a total length of 1,430m
