Low Water Datum (LWD) - The elevation of each of the Great Lakes to which are referenced the depths
shown on navigation charts and the authorized depths of navigation projects.
Marsh - Area of soft, wet, or periodically inundated land, generally treeless, and usually characterized by
grasses and other low growth.
Mean Higher High Water (MHHW) - Average height of the daily higher high waters over a 19-year
period. Only the higher high water of each pair of high waters of a tidal day is included in the mean.
Mean High Water (MHW) - Average height of the daily high waters over a 19-year period. For
semidiurnal or mixed tides, the two high waters of each tidal day are included in the mean. For diurnal
tides, the single daily high water is used to compute the mean.
Mean Lower Low Water (MLLW) - Average height of the daily lower-low waters of a 19-year period.
Only the lower low water of each pair of low waters of a tidal day is included in the mean. Long used as
the datum for Pacific coast navigation charts, it is now gradually being adopted for use across the United
Mean Low Water (MLW) - Average height of the low waters over a 19-year period. For semidiurnal and
mixed tides, the two low waters of each tidal day are included in the mean. For a diurnal tide, the one
low water of each tidal day is used in the mean. Mean Low Water has been used as datum for many
navigation charts published by the National Ocean Survey, but it is being phased out in favor of Mean
Lower Low Water for all areas of the United States.
Mean Sea Level - Average height of the sea surface over a 19-year period. Not necessarily equal to
MEAN TIDE LEVEL.
Mean Tide Level - Plane midway between MEAN HIGH WATER and MEAN LOW WATER. Not
necessarily equal to MEAN SEA LEVEL. Also called half-tide level.
Mixed Tide - A tide in which there is a distinct difference in height between successive high and
successive low waters. For mixed tides there are generally two high and two low waters each tidal day.
Mixed tides may be described as intermediate between semidiurnal and diurnal tides.
Module - A structural component, a number of which are joined to make a whole.
Neap Tides - Tides with decreased ranges that occur when the moon is at first or last-quarter- ;4nl in
opposition to each other. The neap range is smaller than the mean range for semidiurnal and mixed
Nearshore - In beach terminology, an indefinite zone extending seaward from the shoreline well beyond
the breaker zone
Nourishment - Process of replenishing a beach either naturally by longshore transport or artificially by
delivery of materials dredged or excavated elsewhere.