turbulence for additional movement. The downdrift movement of material is thus caused by zig-zag
motion up and down the beach, and the turbulence and action of the wave-generated longshore current.
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 Change
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. If the most severe winds striking a site are
generally along the longest fetch length, structures should be built 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, the water level with no waves present, changes because of astronomical tides,
storms, and periodic lake level variations. Tides are caused 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. 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
the two successive highs or two 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 tidal 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 moon are out of phase, somewhat canceling their individual effects and
causing correspondingly smaller tidal ranges. Differences in tidal range are also caused by the varying
distance to the moon as it orbits the earth, the declination of the moon's orbit, and the declination of the
earth's orbit about the sun.
Tide levels are used as reference elevations on maps, charts, and engineering drawings. Key
reference elevations or datums (Figure 4), which are important because of their wide use, are defined in
the Glossary. Not shown are reference levels for the Great Lakes where all water levels are ultimately
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