hydrostatic pressure created by flooding behind a structure combined with 

 the loss of equalizing forces at the front of a structure due to extreme 

 drawdown of the water level when the waves recede, and by erosion at the 

 base of the structure. Major damage may also be caused by debris carried 

 forward by the tsunami in the nearshore area. 



To determine the potential damage to structures located along a shore- 

 line, the probable increase in water level caused by the tsunami, i.e., 

 the runup height, must be estimated. Estimates of tsunami runup are also 

 needed for flood zone planning along the shoreline, and for operation of 

 the tsunami warning system to evacuate people from endangered areas. 



1 . Tsunami Runup on a Shoreline . 



The height of a tsunami will vary from point to point along a 

 coastline. The numerical models for prediction of tsunami height at 

 the shoreline, i.e., the elevation of water at the shoreline due to the 

 tsunami, must be applied to a sufficient number of points along the 

 shoreline to determine this variation. When the variation is large 

 between adjacent points, calculations for tsunami heights should be 

 carried out at additional shoreline points between those points. After 

 the height of the tsunami at a point along the shoreline has been deter- 

 mined, the vertical runup height at that point can be estimated. 



When the tsunami height along a section of coastline is relatively 

 constant, and the variations in onshore topography are relatively minor, 

 the runup height may be assumed to be constant along that section of 

 coastline as a first approximation. Variations in tsunami height and 

 shoreline topography will actually cause some variation in runup charac- 

 teristics along any section of coastline. An example of how extreme 

 this variation can be is given by Shepard, MacDonald, and Cox (1950) for 

 Haena, on the Island of Kauai, Hawaii, where there was a gentle rise of 

 water level on the western side of the bay, but less than 1 mile to the 

 east, waves rushed onshore, flattening groves of trees and destroying 

 houses. An example of the variation in runup height is given by Wilson 

 and T0rum (1968) for Kodiak City, Alaska (Fig. 49). The mean runup 

 height at Kodiak City was a little more than 6 meters (20 feet) above 

 mean lower low water (MLLW) , with variations from about 5 to 8 meters 

 (17 to 27 feet). Because these variations are difficult to predict, the 

 predicted runup heights may contain substantial errors. Where tsunamis 

 of a known height have produced variations in runup at a particular sec- 

 tion of coastline, the higher heights should normally be used for conserv- 

 ative design. 



It should be noted that the characteristics of the waves may vary from 

 one wave to another at the same coastal point. Shepard, MacDonald, and 

 Cox (1950) cite a case in Hawaii where the first waves came in so gently 

 that a man was able to wade through chest -high water ahead of the rising 

 water. Later waves were so violent that they destroyed houses and left 

 a line of debris against trees 150 meters (500 feet) inland. 



147 



