EM 1110-2-1100 (Part II)
30 Apr 02
(8) HIGHER HIGH WATER (HHW). The higher of the two high waters of any tidal day. The single
high water occurring daily during periods when the tide is diurnal is considered to be a higher high water.
(9) HIGHER LOW WATER (HLW). The higher of two low waters of any tidal day.
(10) HIGH TIDE, HIGH WATER (HW). The maximum elevation reached by each rising tide.
(11) LOW TIDE, LOW WATER (LW). The minimum elevation reached by each falling tide.
(12) LOWER HIGH WATER (LHW). The lower of the two high waters of any tidal day.
(13) LOWER LOW WATER (LLW). The lower of the two low waters of any tidal day. The single low
water occurring daily during periods when the tide is diurnal is considered to be a lower low water.
(14) MIXED TIDE. A tide in which the presence of a diurnal wave is conspicuous due to a large
inequality in either the high or low water heights, with two high waters and two low waters usually occurring
each tidal day. In strictness, all tides are mixed, but the name is usually applied without definite limits to the
tide intermediate to those predominantly semidiurnal and those diurnal.
(15) SEMIDIURNAL TIDE. A tide with two high and two low waters in a tidal day with comparatively
little diurnal inequality.
(16) TIDAL DAY. The time of the rotation of the earth with respect to the moon, or the interval between
two successive upper transits of the moon over the meridian of a place, approximately 24.84 solar hours
(24 hr and 50 min) or 1.035 times per solar day. Also called a lunar day.
(17) TIDAL PERIOD. The interval of time between two consecutive, like phases of the tide.
(18) TIDAL RANGE. The difference in height between consecutive high and low (or higher high and
lower low) waters.
(19) TIDAL RISE. The height of the tide as referenced to the datum of a chart.
II-5-4. Water Surface Elevation Datums
a. Introduction. Water level and its change with respect to time have to be measured relative to some
specified elevation or datum in order to have a physical significance. In the fields of coastal engineering and
oceanography this datum represents a critical design parameter because reported water levels provide an
indication of minimum navigational depths or maximum surface elevations at which protective levees or
berms are overtopped. It is therefore necessary that coastal datums represent some reference point which is
universally understood and meaningful, both onshore and offshore. Ideally, two criteria should be expected
of a datum: 1) that it provides local depth of water information, and 2) that it is fixed regardless of location
such that elevations at different locations can be compared. These two criteria are not necessarily compatible.
The following list of datums represents those commonly in use in the United States.
(1) Tidal-observation-based datums are computed from measured time series of tidal elevations. As
such, they vary with geographical location and exposure. Geographic variability of tide records can be seen
in Figures II-5-8 and II-5-9. Although datums based on tidal series do indicate site-specific conditions, they
Water Levels and Long Waves