EM 1110-2-1100 (Change 1)
31 Jul 03
(1) Pattern of geographic distribution of individuals within a species. (2) Distortion of the shape of a seismic wave
train or ocean wave train because of variations of velocity with frequency.
DISTRIBUTARIES
Multiple branches of a river.
DITCH
A channel to convey water for irrigation or drainage.
DIURNAL
Having a period or cycle of approximately one TIDAL DAY.
DIURNAL CURRENT
The type of tidal current having only one FLOOD and one EBB period in the tidal day. A ROTARY CURRENT is
diurnal if it changes its direction through all points of the compass once each tidal day.
DIURNAL INEQUALITY
The difference in height of the two high waters or of the two low waters of each day. Also, the difference in velocity
between the two daily flood or EBB CURRENTS of each day.
DIURNAL TIDE
A tide with one high water and one low water in a tidal day. (See Figure II-5-16)
DIVERGENCE
(1) In refraction phenomena, the increasing of distance between orthogonals in the direction of wave travel.
Denotes an area of decreasing wave height and energy concentration. (2) In wind-setup phenomena, the decrease in setup
observed under that which would occur in an equivalent rectangular basin of uniform depth, caused by changes in planform or
depth. Also the increase in basin width or depth causing such decrease in setup.
DIVERGING WAVE
Waves that move obliquely out from a vessel's sailing line. (See Figure II-7-40)
DIVERSION CHANNEL
A waterway used to divert water from its natural course. The term is generally applied to a temporary arrangement
e.g. to by-pass water around a dam site during construction.
DOCK
The slip or waterway between two piers, or cut into the land, for the reception of ships.
DOLPHIN
A cluster of piles.
DOWNDRIFT
The direction of predominant movement of littoral materials.
DOWNSTREAM
Along coasts with obliquely approaching waves there is a longshore (wave-driven) current. For this current, one
can define an upstream and a DOWNSTREAM direction. For example, on a beach with an orientation west-east, the sea is to
the north. Suppose the waves come from NW, then the current flows from West to East. Here, UPSTREAM is west of the
observer, and east is downstream of the observer.
A-24
Appendix A Glossary of Coastal Terminology
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