EM 1110-2-1100 (Part II)
30 Apr 02
problems. In such areas (e.g. the U.S. Pacific coast and Hawaii), seiche should be routinely considered in
design.
(3) Infragravity waves. Infragravity waves are discussed in Part II-4-5. They can be an important
component of surf zone processes, particularly during storms, and they are a forcing mechanism for harbor
oscillations and other seiching phenomena. Methods for estimating infragravity waves and incorporating
them into design are relatively immature at present. Infragravity waves can be considered in design by
conducting physical model tests with irregular waves if long waves can be sufficiently controlled, a
demanding task. They may also be estimated with some confidence from wind wave/swell conditions using
theory, numerical modeling, and/or empiricism. For example, Bowers (1992) considered long waves at three
coastal sites in intermediate depths typical of harbor entrances. He used theory to estimate a bound long wave
Hs and empiricism to estimate a free long wave Hs (including both edge waves and leaky waves described in
Part II-4-5). His general expression for free waves is
α
β
Hs Tp
Hs (free long waves) ' K
(II-9-6)
γ
d
For his three sites, K ranged from 0.0041 to 0.0066 and overall best-fit values for α, β, and γ were 1.11, 1.25,
and 0.25, respectively. Bowers observed that bound long waves increasingly dominate free long waves as
wind wave/swell Hs increases. For a 10-year return period in the 12-m to 13-m depth, Bowers estimated total
long wave Hs values of about 12 percent of the wind wave/swell Hs.
f.
Extreme water level. Extreme water levels are discussed in Part II-5.
(1) Design importance. Extreme high water levels cause flooding. They also facilitate wave damage by
raising the base level for runup and overtopping, by allowing increased depth-limited wave heights, and by
shifting the zone of wave attack further shoreward such that waves can damage dunes and coastal structures.
At some locations, extreme high-water levels can lead to pollution and health hazards when sewage treatment
ponds or other containment areas are breached.
(2) Estimation procedures. Extreme water levels are caused by some combination of astronomical tides,
storm surge (high wind stress, low atmospheric pressure, rainfall/runoff in enclosed or semi-enclosed areas),
and wave setup. Probabilities must be estimated as a joint probability of the various combinations that can
occur. Procedures for developing storm water level frequency-of-occurrence relationships are reviewed in
Part II-5-5.b, including the historical method, synthetic method, and empirical simulation technique (EST).
The EST is convenient for development of water level design criteria requiring the quantification of risk and
uncertainty associated with the frequency predictions. Traditionally, the distribution of extreme water levels
has been fitted to either a Pearson Type III (Engineer Manual (EM) 1110-2-1412) or log-Pearson Type III
(U.S. Water Resources Council 1976) distribution function. The EST approach does not require a theoretical
distribution function.
g. Water level climate. Water level climate is discussed in Part II-5.
(1) Design importance. The general water level climate at a site impacts navigation channel depths,
harbor depths, currents, harbor flushing, and physical and biological processes in the intertidal zone, including
marsh areas.
(2) Estimation procedures. The principal component of water level climate at most coastal sites is
astronomical tide (Part II-5-3). In areas with little or no tide, particularly very shallow areas, wind,
II-8-32
Hydrodynamic Analysis and Design Conditions
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