TY - JOUR
T1 - Assessment of the effectiveness of general breakwaters in reducing Tsunami inundation in Ishinomaki
AU - Takagi, Hiroshi
AU - Bricker, Jeremy D.
N1 - Funding Information:
The authors thank Koki Matsumoto, Syohei Nagase and Yuto Takahashi for their assistance during the field survey in Ishinomaki. Funding for this research was partially provided by The Obayashi Foundation.
Publisher Copyright:
© 2014 World Scientific Publishing Company and Japan Society of Civil Engineers.
PY - 2014/12/16
Y1 - 2014/12/16
N2 - The 2011 Great East Japan Earthquake and Tsunami caused devastation all along the coast of eastern Japan. Ishinomaki City was one of the most severely damaged municipalities, though the height of the tsunami in this city was smaller than that in Iwate and northern Miyagi. A large difference in the extent of building damage was found comparing two areas of Ishinomaki: one not protected by breakwaters, and the other behind the breakwaters of a large Fishery port. In this paper, the authors perform numerical simulations to determine whether these general breakwaters, which were designed not to block tsunamis but wind waves, reduced the tsunami's energy and contributed to a reduction in inundation of the areas behind the port. Before assessing the effectiveness of breakwaters against tsunamis, simulated inundation heights in each of these two areas were compared with heights measured by The 2011 Tohoku Earthquake Tsunami Joint Survey Group. It was found that a simulation with larger Manning's n value (n = 0.30) can evaluate inundation more precisely than when this value is small (n = 0.06), as often used by Japanese coastal engineers. Comparing the region protected by breakwaters with the unsheltered area, the results of a 2D shallow water equation model do not show a significant difference in inundation mainly because the tsunami intrudes through the openings in the breakwater and fills up the port basin with seawater in a very short time. Therefore, the effectiveness of general breakwaters in reducing tsunami impact should not be overestimated. However, a hypothetical study shows that water levels could be greatly reduced if the port were fully enclosed by breakwaters. This implies that a port could substantially reduce tsunami inundation if the breakwater openings were equipped to be closed before tsunami arrival. The authors also demonstrate that the difference in the extent of building damage in the two areas of Ishinomaki considered can be explained by the difference in drag forces due to the topography as well as the difference in land use, rather than by the presence of breakwaters.
AB - The 2011 Great East Japan Earthquake and Tsunami caused devastation all along the coast of eastern Japan. Ishinomaki City was one of the most severely damaged municipalities, though the height of the tsunami in this city was smaller than that in Iwate and northern Miyagi. A large difference in the extent of building damage was found comparing two areas of Ishinomaki: one not protected by breakwaters, and the other behind the breakwaters of a large Fishery port. In this paper, the authors perform numerical simulations to determine whether these general breakwaters, which were designed not to block tsunamis but wind waves, reduced the tsunami's energy and contributed to a reduction in inundation of the areas behind the port. Before assessing the effectiveness of breakwaters against tsunamis, simulated inundation heights in each of these two areas were compared with heights measured by The 2011 Tohoku Earthquake Tsunami Joint Survey Group. It was found that a simulation with larger Manning's n value (n = 0.30) can evaluate inundation more precisely than when this value is small (n = 0.06), as often used by Japanese coastal engineers. Comparing the region protected by breakwaters with the unsheltered area, the results of a 2D shallow water equation model do not show a significant difference in inundation mainly because the tsunami intrudes through the openings in the breakwater and fills up the port basin with seawater in a very short time. Therefore, the effectiveness of general breakwaters in reducing tsunami impact should not be overestimated. However, a hypothetical study shows that water levels could be greatly reduced if the port were fully enclosed by breakwaters. This implies that a port could substantially reduce tsunami inundation if the breakwater openings were equipped to be closed before tsunami arrival. The authors also demonstrate that the difference in the extent of building damage in the two areas of Ishinomaki considered can be explained by the difference in drag forces due to the topography as well as the difference in land use, rather than by the presence of breakwaters.
KW - 2011 Great East Japan Earthquake and Tsunami
KW - Tsunami
KW - breakwater
KW - building damage
KW - drag force
KW - numerical simulation
KW - port
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U2 - 10.1142/S0578563414500181
DO - 10.1142/S0578563414500181
M3 - Article
AN - SCOPUS:84929380132
VL - 56
JO - Coastal Engineering in Japan
JF - Coastal Engineering in Japan
SN - 0578-5634
IS - 4
M1 - 1450018
ER -