Currents
The water at the shore is constantly in motion due to currents as well as waves. Tides produce
currents in sheltered bays connected to the open sea. As the tide begins to rise in the ocean (flood tide),
the bay's water surface elevation lags behind, generating a current into the bay. As the tide falls (ebbs),
the ocean surface drops more quickly so that the bay surface becomes higher and current flows out of the
bay. Tidal currents are generally not strong enough to cause erosion problems except -in the throat area
of tidal inlets connecting bays to the ocean.
Seasonal Changes
The most notable seasonal change at sheltered sites is the frequency, direction, and severity of high
winds. Summer storms generate strong winds that often approach from entirely different directions than
winter squalls. The manner in which storm winds align with fetch lengths at the site figures prominently
in evaluating the potential for wave damage. I f the most severe winds striking your site are generally
along the longest fetch length, you should build more strongly than if severe winds rarely approach from
that direction.
The formation of thick ice sheets is a notable seasonal change on the Great Lakes which produces
tremendous horizontal and vertical forces on shore structures and must be anticipated in design. Ice is
also important to shoreline processes because waves cannot form or reach shore to move sediment when
large amounts of surface ice are present.
Water Level Variations
The stillwater level, or the water level with no waves present, changes because of astronomical
tides, storms, and periodic lake level variations.
Tides are generated in ocean basins by the gravitational attraction between the earth, moon, and
sun, and are classified as diurnal, semidiurnal, or mixed. Diurnal tides have only one high and low each
day, while semidiurnal tides have two approximately equal highs and two approximately equal lows
daily. Mixed tides, on the other hand, exhibit a distinct difference in the elevation of either successive
highs or successive lows. In addition, at locations with mixed tides, the characteristics of the tide may
change to diurnal or semidiurnal at certain times during the lunar month.
In addition, the tide range, or difference between the high and low, tends to fluctuate throughout
the lunar month. Spring tides have larger than average ranges with higher high and lower low tides.
Neap tides are exactly opposite with smaller ranges, lower highs, and higher lows. Spring tides occur
with full and new moons because the gravitational attraction of both the sun and moon act along the same
line, tending to exaggerate the difference between the high and low tides. At neap tides (during quarter
moons), the pull of the sun and the moon are out of phase, largely canceling their individual effects and
causing correspondingly smaller tide ranges. Differences in tidal range are also caused by the varying
distances to the moon as it orbits the earth, the declination of the moon, and the declination of the sun.
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