Not as much attention has been given to wave rundown; however, the 

 drawdown of the water level may result in the seaward collapse of sea- 

 walls, result in damage to ships in a harbor, or expose seawater-intake 

 pipelines. It should also be noted that a gradual increase in water 

 level, with very low velocity currents, may be followed by a sudden 

 withdrawal of water producing very strong currents. 



During the 1946 tsunami in Hawaii, waves at Hanamaulu Bay rose 2.7 

 meters at a breakwater and wharf, but the water receded to a level 5.6 

 meters below normal sea level between waves (Shepard, MacDonald, and Cox, 

 1950) . Most of the damage was caused by the violent withdrawal of the 

 water. 



The rundown elevations will depend on the wave train generated at 

 the tsunami source. For the 1946 tsunami, the tide gage record at 

 Honolulu, Hawaii, indicated some very narrow, deep wave troughs with 

 the initial troughs having greater amplitudes than the initial crests. 



Consideration must also be given to the current velocities of the 

 runup. Ishimoto and Hagiwara (1934) investigated the large 1933 tsunami 

 at Kamaisi, Japan, and estimated current velocities with a maximum value 

 of 1 meter per second. Houston and Garcia (1974) estimated that small 

 tsunamis in southern California acting as rapidly rising tides would 

 have maximum current velocities of about 0.5 meter per second. The 

 current velocity for the 1933 tsunami, which was about double the veloc- 

 ity estimated by Houston and Garcia for small tsunamis, destroyed some 

 buildings when the water depth reached a height of 2 meters (6.15 feet). 



Water overflowing a coastal barrier will have a current velocity 

 determined by the difference in height between the top of the barrier 

 and the ground level behind the barrier, as well as the quantity of 

 water overtopping the barrier, rather than acting like a rapidly rising 

 tide. The barrier will also limit the height of the runup; however, 

 large drain openings must be provided to prevent water levels from 

 building up behind the barrier if it is overtopped by successive waves . 

 Magoon (1965) cites one example south of Crescent City, California, 

 during the 1964 tsunami where water flowed over narrow coastal dunes. 

 The quantity of water overflowing the dunes was insufficient in some 

 instances to fill the low areas to landward, reducing the resulting 

 runup height . 



Where the slope is very long compared to the wavelength, and friction 

 effects must be considered, it can be seen that for low velocities the 

 retarding effect of the slope roughness (deceleration) may prevent the 

 water from rising to a runup height equal to the wave height at the shore- 

 line (i.e., drawdown will start at the shoreline, reversing the direction 

 of flow). As previously noted, the currents associated with the rundown 

 might have much higher velocities than the currents associated with the 

 runup. No estimates are available for the rundown currents. 



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