12 December 2001
level and vary according to locale, and the National Geodetic Vertical Datum (NGVD),
which does not. It also discusses several processes that result in long-term changes in
relative mean sea level. An additional discussion of datums and relationship to coastal
geology is contained in Section IV-2-4 of the CEM. The selected datum and a rationale
for its choice should be stated specifically in the design documentation.
Storm Surge. High-wind systems and low barometric pressures over
shoaling water will create a temporary water-level rise along shorelines. Especially
susceptible are areas where large cyclonic storm systems (such as hurricanes and
typhoons) track across relatively shallow offshore water. A relatively short-duration
water-level rise (setup) will occur along coastlines during episodes of high-wave
attacks. The rise in water level is caused by breaking waves trapping a water mass
along the shoreline. This water rise can increase water heights in protected water
areas hydraulically linked to the coast, shoreward of the breaker line. This
phenomenon, and generated currents associated with it, can be significant in harbor
sites located behind reefs or large shoals. Section II-5-5 of the CEM discusses the
effect of tropical and extra-tropical storm activity on water surface elevation.
Seiche. Defined as a standing-wave oscillation of an enclosed body of
water that continues, pendulum fashion, after the cessation of the originating force,
seiche may be either seismic or atmospheric in origin. Seiche is a phenomenon
associated with ocean waves having periods in excess of those of normal sea swell.
Such waves, commonly known as "long waves," have periods ranging from 20 seconds
to several hours. Long waves exhibit relatively low heights, on the order of 0.03 to 0.12
meters (0.1 to 0.4 foot.) They are highly reflective, even off flat-slope beaches, and will
pass virtually unimpeded through porous breakwaters. Seiche occurs within a basin,
harbor, or bay during certain critical wave periods when the period of incident long-wave
energy matches the resonating period of the basin. The result is a standing wave
system comprising reinforced wave heights greater than those of the incident wave.
The water surface exhibits a series of nodes and antinodes with respect to the water
column. Antinodes are regions where the vertical motion is a maximum and the
horizontal velocities are minimum. Where wavelength is sufficiently greater than ship
length, a ship berthed at the antinode will experience a gentle rise and fall with the
standing-wave period. At the node, the ship will be subject to a periodic horizontal
surging action due to currents. A ship in combination with its mooring lines behaves as
a spring-mass system which, when excited, can resonate at certain critical frequency
ranges. During seiching action, the horizontal surging motion of a vessel located near a
node can interfere with loading operations and, in severe cases, cause the mooring
lines to part. Section II-5-6 of the CEM discusses further details of this phenomenon.
Tsunamis. In certain ocean regions, waves generated by seismic
disturbances or landslides occur. From event history, some shoreline locations are
more susceptible to damage from tsunamis than others. Probability approximations of
water-level height exist for some coastal locations. These are included in reports by the
U.S. Army Corps of Engineers (USACE) and licensing studies by Public Utility
Commissions. If warranted, a site-specific risk analysis can be performed which relies
heavily upon probability parameters for specifics of the underwater seismic movement.