Tide levels are used as reference elevations on maps, charts, and engineering drawings. Some key
reference elevations are important because of their wide use. These are defined in the Glossary and
illustrated on Figure 4. Not shown are reference levels for the Great Lakes where all water levels are
ultimately referenced to the International Great Lakes Datum (IGLD) (see Glossary). Each lake,
however, has a designated chart datum [Low Water Datum (LWD)] based on the IGLD. Depths and
water levels are commonly given as feet above or below the chart datum for that lake.
Storms tend to increase the Stillwater level because of atmospheric pressure differences, high
winds, and the effects of large breaking waves. Atmospheric pressure differences across a large water
body can commonly cause one- or two-foot rises in the water level in the lower pressure area. The stress
on the water's surface from high storm winds also tends to drive the water on shore to above normal
levels (storm setup) until balanced by the tendency for the water to f low back to a lower level. Finally,
these same high winds generate large storm waves. As these break, they tend to pile the water on shore
higher, raising the stillwater level further.
Enclosed water bodies (such as the Great Lakes) can also respond to storm forces by seiching.
This occurs when storm winds force the water surface higher at the downwind end of the lake. As the
storm passes, this pent-up water is released, causing it to move toward the opposite end of the lake,
resulting in oscillations as in a bathtub. This back and forth movement (seiching) will noticeably
continue for several cycles. Seiching effects are most noticeable on Lake Erie because its long axis lines
up with predominant storm tracks and wind directions, and its shallow depths lead to higher storm setup
levels.
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