12 December 2001
Open Coastlines. When harbors are situated on open coastlines, a high
degree of artificial work is required to provide shelter. Consequently, as the coastline
itself becomes more winding and offshore islands appear, the degree of natural shelter
provided in turn reduces the harbor's exposure to wind and waves. Thus a
corresponding decrease in the amount of artificial protection is required.
Bays, Estuaries, and Navigable Rivers. When the harbor is situated
entirely within an enclosed bay or estuary having a narrow opening into the sea, an
environment of total natural protection results. Depending on the orientation of the
naturally occurring protective features, these harbor sites need little or no additional
protection. Nonetheless, a degree of entrance improvement is usually required to
ensure safety during storms and as the entrance widens, the degree of protection
required increases as well.
Hydraulic Impoundments. Hydraulic impoundments are defined as
harbor basins in which the vessel mooring depth is constantly maintained behind locks,
versus the case in harbors where free-flowing water linkages to the sea or other large
bodies of water exposed to storms exist. Since harbors are not located in bays,
estuaries, and rivers, the hydraulic-impoundment harbors are not influenced by tidal
fluctuations. Either admitting or releasing water through the locks maintains water
levels. Though constructed worldwide economically efficient for commercial port
facilities, the constricted access of hydraulic impoundments makes then generally
undesirable for military purposes.
Roadsteads. In cases when protection is provided only as a moored-ship
refuge, the protected harbor area is referred to as a "roadstead." Roadsteads need a
bottom in the protected area in the protected area that is suitable for sufficient anchor
holding power. Examples of roadstead anchorages in a natural bay and a protected
harbor are shown in Figure 2-2.
HYDRODYNAMIC INVESTIGATIONS. Effective coastal engineering
studies require appropriate understanding of the hydrodynamics of the project area and
its impact on design parameters. A summary of the considerations and procedures for
hydrodynamic investigations is contained in Section II-8 of the CEM.
SHIP DYNAMICS IN CHANNELS. Ships moving in shallow or restricted
waterways experience dynamic behavior different from that exhibited in open water that
can significantly affect control of the ship. The width and depth of the channel, if
insufficient, can interfere with the normal passage of water around the hull of the ship
and inhibit steering control. The sinkage and trim of a vessel in a navigation channel
depend primarily on vessel speed, the ratio of the channel cross section area to the
vessel wetted cross section area, and the ratio of the water depth to the vessel draft.
The maximum vessel draft is determined through consideration of the effects on vessel
draft by such factors as squat, variation in salinity, effects of wave motion, and loading.
In addition, some judgment needs to be exercised in considering the conditions that
could realistically be expected to occur. For example, it wouldn't be practical to design
a ship channel for extremes of vessel draft during a hurricane, because the ships