Effect of Astronomical Tides 



30. The maximum still-water elevation produced during tsunami activity 

 is the result of a superposition of tsunami and astronomical tide. Therefore, 

 the statistical effect of astronomical tides on total tsunami runup must be 

 included in the predictive scheme presented in this report. Since the wave 

 forms calculated by the model did not have a simple form (e.g., sinusoidal), 

 the statistical effect of the astronomical tide on tsunami runup had to be 

 determined through a numerical approach. 



31. A computer program was developed to predict time-histories of the 

 astronomical tides throughout the study area. The program was based upon the 

 harmonic analysis methods used in the past by the National Ocean Survey (NOS) 

 for mechanical tide-predicting machines (Schureman 1971). Tidal constants 

 available from NOS were used as input to the computer program. A year of 

 tidal elevations was then predicted for grid locations in the study area. The 

 year 1964 was selected because all the major tidal components for tides in 

 Alaska had a node factor of approximately 1.00 during this year, thus making 

 it an average year. The node factor is associated with the revolution of the 

 moon's node and has an 18.6-year cycle. Since a tsunami can arrive at any 

 time during this 18.6-year period (arrival at a low of the node factor being 

 equally as likely as an arrival at a high), the statistical effect of the 

 temporally varying node factor on the predicted runup elevations is shown by 

 Houston (1980) to be very small. 



32. The tidal time-histories calculated at each numerical gage location 

 were subdivided into 30-min segments. Each of the twelve 24-hr wave forms was 

 allowed to arrive at the beginning of each of these 30-min segments and then 

 superimposed upon the astronomical tide for the 24-hr period. The maximum 

 combined tsunami and astronomical tide elevation over the 24-hr period was 

 determined for tsunami wave forms arriving during each of these 30-min start- 

 ing times. Each of these maximum elevations had an associated probability 

 equal to the probability that a certain intensity tsunami would be generated 

 during a particular 30-min period of the year. 



33. The many maximum elevations with associated probabilities were used 

 to determine exceedance frequency distributions of combined tsunami and astro- 

 nomical tide elevations. The maximum elevations were ordered and frequencies 

 summed, starting with the largest elevations, until a desired frequency was 



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