EM 1110-2-1100 (Part II)
30 Apr 02
explanation of these phenomena is not available, their impact is significant, with differences up to 30-40
percent from the significant wave approach.
e. Treatment of spectral wave mechanics in any detail requires use of a numerical model. However,
in using a significant wave approach, it can generally be assumed that:
(1) It may overestimate wave focussing effects.
(2) Careful estimates of the fraction of wave energy heading shoreward should be made for oblique
angles cases.
(3) Shoaling calculations may overestimate wave heights in high energy conditions.
f. Shifts in wave period may also occur. As a result, significant wave analysis tends to be conservative;
this may be why it has been an acceptable approach for design. However, for cost-sensitive projects, a more
complicated approach may be warranted.
g. The following precautions are suggested. In a significant wave analysis, if regions of highly focussed
wave energy occur with corresponding lobes of low energy, the regions of low energy should be carefully
considered. In the field, natural wave systems generally have significant directional spread, so calculated
values in the low energy lobes may significantly underestimate wave heights. In cases where irregular waves
are modeled spectrally, typically only the wave height Hs is estimated. In shallow water, larger waves do
occur (H1/10, etc.) and combinations of individual wave height, period, and bottom depth can result in
individual waves or groups of waves significantly larger than Hs (see Part II-2).
II-3-5. Advanced Propagation Methods
a. Introduction.
(1) As indicated in Part II-3-2, as waves propagate, they may continue to grow due to the continued
action of the wind or may lose energy due to breaking, bottom friction, or percolation. These effects cannot
be realistically incorporated through manual calculations. The preceding discussion indicates that
computations involving rays are tedious by hand and subject to many inaccuracies. Advances have been
made in computing wave transformation; they were briefly indicated in the preceding sections. Many of these
procedures may run efficiently on a personal computer or a work station and do not require a large mainframe
or supercomputer. Hence they can be applied readily by most engineers (ACES 1992).
(2) This section describes three computer programs that are available and in use by the Corps of
Engineers. Each program is briefly described and a reference indicates where the program can be obtained.
Each program is complicated and requires some effort to use properly. A short description is provided here
to indicate to the engineer the potentials of these codes. The three have been selected to provide a cross
section of the types of technology available. Other computer programs can be obtained and may be as
suitable for use as those described here.
(3) Examples of technology available to practicing engineers is provided. The Corps of Engineers does
not endorse the codes discussed or certify their accuracy. Indeed, the suitability and accuracy of any of
these codes depend upon the problem under study and the way in which the code is applied. With the
exception of very simple bathymetry, it is recommended that nearshore wave transformation studies
use a numerical code capable of handling the complexities required. However, the particular numerical
approach selected depends upon the problem.
Estimation of Nearshore Waves
II-3-19
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