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(EM 1 1 10-2-1414). Methods to estimate the probabilities of exceedence of 

 such critical conditions along with detailed guidance on the determination and 

 selection of water levels and wave heights for coastal engineering design are 

 presented in EM 1110-2-1414, Water Levels and Wave Heights for Coastal 

 Engineering Design. Such critical conditions may be tolerable on a more 

 frequent basis for shore stabilization structures compared to navigation 

 structures, since such structures are primarily used to prevent erosion, not to 

 protect people's lives. A decrease in design level may also offer a substantial 

 cost savings over the traditional SPM design approach. These factors need to 

 be considered when selecting design water levels and wave heights for 

 detached breakwaters used as shoreline stabilization structures. 



Water levels 



The entire range of possible water levels is needed for the structural design 

 of beach stabilization structures. High-water levels are used to estimate 

 maximum depth-limited breaking wave heights and to determine crest 

 elevations. Low-water levels are generally used for toe protection design. 

 Water levels can be affected by astronomical tides, storm surges, seiches, 

 river discharges, natural lake fluctuations, and reservoir storage limits. 



Design water levels are typically described statistically in terms of 

 frequency, or probability that a given water level will be equalled or 

 exceeded, or its return period in years. Thus, for example, the water level 

 that is exceeded once in 50 years (a 2-percent probability of being exceeded in 

 any 1 year) might be specified as a design water level. Significant deviations 

 from predicted astronomical tide levels will occur during storms because of 

 meteorological tides (storm surges) caused by strong onshore winds and low 

 atmospheric pressure. Consequently, design water levels for a structure may 

 include a storm surge with a specified return period. Detailed information on 

 the prediction of astronomical tides and storm surge are available in EM 1 1 10- 

 2-1414 and EM 1110-2-1412. 



Waves 



Wave data required for structural design differ from data needed for 

 functional design. Structural design generally focuses on larger waves in the 

 wave climate since these waves represent critical conditions which may 

 endanger the structure's stability. Structural stability criteria are most often 

 stated in terms of extreme conditions which a coastal structure must survive 

 without sustaining significant damage. The conditions usually include wave 

 conditions of some infrequent recurrence interval, say 25 or 50 years. 



Wave height statistics to determine design conditions will normally be 

 based on hindcast wave data since a relatively long record is needed for 

 extrapolation and wave gauge records rarely cover a sufficient duration. WIS 

 has developed hindcast data spanning 20 years for all three ocean coasts and 



Chapter 4 Structural Design Guidance 



