Impact Ice Forces of Drift Ice Acting on Gates of Sea Embankments
Masakuni Hanada?, Takahiko Sasajima?, Tetsuya Hayakawa?, Kunihiro Kawai? and Hiroshi Saeki? (?Hokkaido University,?Hokkaido Development Bureau,?Nishimura-gumi Co., Ltd.)
ABSTRACT
The Okhotsk Sea coast of Hokkaido is covered by drift ice during winter. However, when drift ice has reached or has receded from the shore and when the amount of drift ice is small, the amount of ice in the water in front of a structure is also small. Thus waves can cause ice floes to collide with the breakwaters near the harbor entrances and with the members of special-type breakwaters, such as slit caisson breakwaters. This study aimed to clarify the impact forces of drift ice acting on upright structures, such as the gates of sea embankments.
The Okhotsk Sea coast of Hokkaido is covered by drift ice during winter. When the Okhotsk Sea has a great deal of drift ice, the ice floats through the Nemuro Strait and may reach as far as the Pacific coast of Hokkaido.
On March 4, 1952, around 10:23 A.M., a major earthquake (M = 8.0) occurred in the southeastern part of Hokkaido, causing great damage on the east coast, and killing 30 people and seriously injuring 90 others. This earthquake is called the Tokachi-oki Earthquake (longitude 143° 52' E and latitude 42°09' N). During this earthquake, the entire coast from Cape Erimo and the east coast in the Hidaka district to Nemuro was struck by a tsunami of wave height 1 - 2 m (over 3 m locally), which was not considered a very large tsunami However, serious loss of life and injury, as well as property damage, were caused because Kiritappu is a low-lying land-tied island, and because the drift ice along the whole coast ran up onto the shore due to the tsunami and collided with houses and buildings near the shore.
Today, there are sea embankments in this region with gates for boats to pass through during their daily activities. When tsunami waves threaten to pass through the embankment, the gate is closed by lowering a steel plate. However, this gate part is believed to have a deficiency in its strength. Safety of the sea embankment can be increased if we can determine the ice force when ice masses carried by tsunami waves collide with the gate. When drift ice has reached or has receded from the shore and when the amount of drift ice is small, the amount of ice in the water in front of a structure is also small. Thus waves can cause ice floes to collide with the breakwaters near the harbor entrances and with the members of special-type breakwaters, such as slit caisson breakwaters.
This study aimed to clarify the impact forces of drift ice acting on upright structures, such as the gates of sea embankments.
2. Experimental Methods
An experimental device was a steel skeleton construction with a steel plate 180 cm wide, 65 cm deep and 16 mm thick at the lower part of the device supported by a hinge at one end and by a load cell at the other end (Figure 1). Rectangular ice plates (width 150 cm; variable thickness up to a maximum of 30 cm) were dropped from a specified height through the frame of the experimental device. The ice plates were forced to collide with the central part of the steel plate. Five webs were provided laterally on the back of the steel plate to minimize the deformation at the time of the collision. The impact load was measured by the load cell, and sampled and recorded at 10μ sec.
The impact velocity of the ice floe was set at 4 m/s, 6 m/s and 8 m/s in our experiment, based on the results from 1) the studies by Saeki et al.1) and by Takahashi et al.2) on the velocity of ice floes when ice floes run up onto the shore due to tsunamis and collide with structures such as sea embankments, 2) the study by Kunimatsu et al.3) on the size of drift ice at the Okhotsk coast, and 3) the study by Sakai et al.4)on the horizontal velocity of ice floes overtopping breakwaters.
