EM 1110-2-1100 (Part II)
30 Apr 02
(3) The added length is given by (Sorensen 1986, adopted from Miles (1948))
&W
πW
Rrc '
(II-7-19)
ln
π
g dc TH
where
W = channel width
dc = channel depth
II-7-6. Flushing/Circulation
a. Statement of importance.
(1) An important aspect of harbor design is the water circulation that occurs within the harbor and
between the harbor and the surrounding water body. Water exchange with the surrounding water body will
produce a flushing action in the harbor. Flushing is important to reduce the level of chemical, biological, and
floating solids pollution in the harbor. Typically, a criterion such as the exchange of X percent of the water
in the harbor within a certain time period or, alternatively, exchange of Y percent of the water in the harbor
each tidal cycle might be set as a design goal.
(2) Circulation patterns within a harbor should eliminate areas of stagnant water where pollution levels
will rise and fine sediment deposition may occur. This can occur, for example, in closed-end channels such
as pier slips and residential canals. Strong circulation is desired as long as adverse high-velocity currents are
not generated.
(3) Natural harbor circulation and flushing should be optimized. This can be accomplished by:
(a) Siting the harbor to make use of ambient currents that will pass the harbor entrance.
(b) Considering water circulation when determining the number, size, and placement of harbor entrances.
(c) Establishing the harbor planform and internal structure locations so that circulation is optimized and
potential pollution sources are located in areas of strong water circulation.
(d) Employing (where incident wave action allows) harbor protective structures such as floating
breakwaters and vertical barriers with bottom openings that permit flow into and out of the harbor.
(e) Installing culvert pipes through more massive harbor protective structures. This natural flushing can
be supplemented by installing pumping systems that bring exterior water into dead areas of the harbor or
remove polluted water from these areas.
b. Flushing/circulation processes.
(1) Tidal action.
(a) As the tide rises, ambient water will enter a harbor and mix with the water in the harbor. On the
subsequent falling tide, a portion of this ambient/harbor mixture will leave the harbor. The net result is the
exchange of some harbor water with water from outside the harbor. The efficiency of this exchange depends
II-7-48
Harbor Hydrodynamics
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