3 October 2005
The magnitude and direction
in harbors and
nearshore areas are in most cases a function of location and time. Astronomical tides,
river discharges, wind-driven currents, and other factors can influence currents. For
example, wind-driven currents are surface currents that result from the stress exerted
by the wind on the sea surface. Wind-driven currents generally attain a mean velocity
of approximately 3 to 5 percent of the mean wind speed at 10 meters (33 feet) above
the sea surface. The magnitude of this current strongly decreases with depth.
Currents can be very site specific, so it is recommended that currents be
measured at the design site and combined with other information available to define the
design current conditions.
Water Levels. At most sites some standard datum, such as mean low
water (MLW) or mean lower low water (MLLW), is established by formal methods.
Water levels are then referenced to this datum. The water level in most harbors is then
a function of time. Factors influencing water levels include astronomical tides, storm
surges, river discharges, winds, seiches, and other factors.
The design range in water levels at the site must be considered in the
Waves. Most DOD moorings are wisely located in harbors to help
minimize wave effects. However, waves can be important to mooring designs in some
cases. The two primary wave categories of interest are:
a) Wind waves. Wind waves can be locally generated or can be wind
waves or swell entering the harbor entrance(s). Small vessels are especially
susceptible to wind waves.
b) Long waves. These can be due to surf beat, harbor seiching, or other