On the Great Lakes, the Monthly Bulletin of Great Lakes Levels (see a7iso, OTHER HELP Section)
summarizes past and projected water levels. For each lake, a curve is given for the long-term average
lake level (1900 to the present) (see Figure 13). A suggested design water level is the greater of (a) the
water level midway between the long-term average and the recorded maximum average monthly water
level, or (b) the highest monthly water level that has occurred during the preceding year. For instance, on
Lake Michigan, the highest average water levels occur in July when they are about +2.0 feet above chart
datum (576.8 feet) [Low Water Datum (LWD) is +576.8 feet IGLD for Lake Mighigan]. The maximum
observed monthly water level for July on Lake Michigan was observed in 1974 at +4.2 feet. A water
level midway between them is +3.1 feet. The maximum observed monthly water level during the
previous year was +3.0 feet, so the chosen water level should be the greater of the two, or +3.1 feet
(579.9 feet). Storm setup values should be added to obtain a final water level. Figure 14 contains
suggested values superimposed on a map of the Great Lakes. The design water level, therefore, will be
the sum of the water level found in the previous step and the storm setup value from Figure 14.
Wave Heights
Waves at a site are generated either by wind action or moving vessels. At most locations,
however, wind action is more critical for design. The first step in evaluating the wave height is to
measure (on a calm day) the water depth about 50 feet from the low water line or lakeshore and then
determine the lake or tide level at the time of measurement (from Tide Tables or Monthly Bulletin of
Lake Levels for the Great Lakes). Next, add to the measured depth the difference between the design
water level and the level at the time of observation and multiply by 0.8 to find the maximum design
breaking wave height. For instance, at a site on Lake Erie, if the measured depth is 3.5 feet when the
lake level is 571.0 feet and the design lake level is 572.5 feet, the design depth is 3.5 + (572.5 - 571.0) or
5.0 feet. The maximum design breaking wave at the site is then 5.0 x 0.8 or 4 feet.
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