Surge runup on a dry bed will have a much higher velocity than the 

 values given by Houston and Garcia (1974) for a tsunami which acts like 

 a rapidly rising tide. Keulegan (1950) gives 



u = 2(gh) 1/2 (318) 



where h is the surge height at any point and u the water velocity at 

 the same point. Fukui, et al . (1963) give a lower value of velocity as 



u = 1.83(gh) 1/2 (319) 



The higher value would be conservative. 



2. Interaction with Shore Protection Structures . 



Breakwaters and seawalls may provide coastal areas protection from 

 tsunamis. When a tsunami occurs, breakwaters may decrease the volume of 

 water flowing into a harbor and onto the coastline. Proper placement of 

 breakwaters may also decrease wave heights by changing the natural period 

 of an inlet discussed in Section VI, 7. However, breakwaters may also 

 affect the resonant period of a harbor so that wave heights are increased, 

 and seawalls may reflect waves within a harbor. A high seawall along a 

 coastline may prevent flooding of the backshore areas. 



A tsunami may damage shore protection structures; therefore, care 

 must be exercised in the design of the structures. Numerous instances 

 of tsunamis damaging or destroying protective structures have been 

 recorded. The 1946 tsunami in Hawaii overtopped and breached the break- 

 water at Hilo, removing 7.25-metric ton (8 tons) stones to a depth 0.9 

 meter (3 feet) below the water surface along nine sections of the break- 

 water crest with a total length of over 1,800 meters (6,000 feet) (U.S. 

 Army Engineer District, Honolulu, 1960). Matuo (1934) refers to the case 

 of an earthern embankment at Yosihama on the northeast coast of Honshu, 

 Japan, which had been constructed to protect a section of coastline. The 

 1933 Sanriku tsunami overtopped the embankment, and it was swept away, 

 flush with the original ground level. 



Iwasaki and Horikawa (1960) investigated areas along the northeast 

 coast of Honshu after the 1960 tsunami. They indicated that a sea dike 

 at Kesennuma Bay failed during the 1960 tsunami because the water from 

 the incident waves, which had overtopped the dike, caused extensive 

 erosion receding at a gap in the dike. The receding water gradually 

 widened the gap. They also noted that a quay wall at Ofunato failed 

 because of scouring of the backfilling, and that a quay wall constructed 

 of reinforced concrete sheet piles at Hachinohe collapsed due to a lack 

 of interlocking strength after backfilling was washed away. 



Iwasaki and Horikawa also indicated that receding water may seriously 

 scour the seaward base of a revetment or seawall. The combination of this 



158 



