EM 1110-2-1100 (Part II)
30 Apr 02
Figure II-5-28. Sediment transport magnitude-frequency relationship - December 1992
northeaster
II-5-6. Seiches
a. Seiches are standing waves or oscillations of the free surface of a body of water in a closed or
semiclosed basin. These oscillations are of relatively long period, extending from minutes in harbors and
bays to over 10 hr in the Great Lakes. Any external perturbation to the lake or embayment can force an
oscillation. In harbors, the forcing can be the result of short waves and wave groups at the harbor entrance.
Examples include 30- to 400-sec wave-forced oscillations in the Los Angeles-Long Beach harbor (Seabergh
1985). Dominant long-period seiche conditions on the Great Lakes have resonant modes with periods
varying from 2 to 12 hr. These oscillations result primarily from changes in atmospheric pressure and the
resultant wind conditions and occur over the entire basin. The frequency of oscillation is a function of the
forcing, together with geometry and bathymetry, of the system.
b. In areas of simple geometry, modes of oscillation can be predicted from the shape of the basin. For
example, Figure II-5-30 shows surface profiles for various simple geometric configurations.
c. For the one-dimensional case of a closed rectangular basin with vertical walls and uniform depth
(Figure II-5-30b), the natural free oscillating period Tn can be written as
2 lB
Tn '
(II-5-26)
n gh
Water Levels and Long Waves
II-5-51
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