Attention should be drawn to the use of Mean Low Water (MLW)
as datum in the previous
discussion. This has been the datum used by the National Ocean Survey
in the past for east coast
navigation charts. In the future, however, the NOS will begin to adopt
Mean Lower Low Water
(MLLW) as datum for all nautical charts in the United States. This change
will occur gradually as
charts are periodically revised and reissued.
On the Great Lakes, the Monthly Bulletin of Lake Levels for the Great Lakes (see Water Levels in
the Other Help Section) summaries of water levels for the previous year and the current year to date, as
well as projected lake levels for the next six months. For each lake, a curve is also 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 Michigan.)
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 or seiche values should be added to obtain a final water level. Figure 14 contains
suggested values from Help Yourself [U. S. Army Corps of Engineers (1978d)] superimposed on a map
of the Great Lakes - The design lake level, therefore, will be the sum of the lake level found in the
previous step and the storm setup value.
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 design wave will be the lesser of (a) the maximum
height generated by wind acting along the critical fetch or (b) the maximum breaker height that can reach
the site during design water level conditions. In other words, if the wind can produce a larger wave than
can be supported at the site, the available depth will control, not the wind.
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