12 December 2001
probably will not attempt to transit the channel during the storm. The effect of these
and other factors on design parameters are discussed in the CEM, Section II-7-7-b(4).
Additional analysis of this subject is contained in Principles of Naval Architecture, Vol. 3
SOURCES OF HYDRODYNAMIC INFORMATION. Because coastal
engineering design requires considerable knowledge of many physical sciences and
engineering disciplines, the CEM contains a summary of sources for available
information in Section II-8.
COMBINING DESIGN EVENTS. Consideration of specific event
extremes is the first step toward determination of design requirements. However, there
are situations where the probability of coincident events is significant enough to warrant
a further analysis of the combined effects of those events. As stated in the CEM,
Section II-5-5-a(3), the importance of the timing of a storm event with tide phase cannot
be over-emphasized because the resulting combined effect can be catastrophic.
Methods for determining the frequency of occurrence for a 25- or 50-year storm are
discussed in Section II-5-5-b of the CEM.
Earthquake and Low Tide. Coastal projects in areas with a high
probability of seismic activity need to consider potential impacts related to ground
deformation and severe liquefaction. The decision to allow for seismic loading in a
coastal project design may hinge on such factors as estimated repair costs versus
replacement costs, if loss of life and interruption of vital services are not considerations.
Flexibility of the structure is a consideration in areas where rubble mound structures
tend to be less affected by seismic activity than monolithic structures. Although not
discussed in available texts, there is the potential that the stability of bluffs and coastal
structures can be catastrophically affected if that activity occurs at a time of extreme low
Storm Surge and High Tide. Coastal areas are susceptible at certain
times of the year to the effects of storm surge associated with high surf, which, if
combined with an extreme high tide, can cause disastrous erosion of the shoreline.
The importance of considering this interaction in coastal design is discussed in Section
II-5-5-a(3) of the CEM.
The majority of coastal structures are designed to provide a level of
protection to the beach and the surrounding population and supporting structures. This
level of protection is generally based on a determination of the frequency-of-occurrence
of a storm surge of a specified maximum elevation selected through an assessment of
the risks of structural failure or consequences of overtopping versus project design
costs. Consequently, it is important to determine stage frequency or frequency-of-
occurrence relationships for the area in question. The designer must investigate and
determine such factors as how to define high tide (2 min per day or 2 hours per day)
and the probability of this happening with an extreme event such as a 25- or 50-year
hurricane. Essentially, the phasing of the storm and tide both impact the design of the
structure and the probability analysis of the design.