transport direction can vary from season to season, day to day or hour
to hour. These reversais of transport direction are quite common for
most United States shores. Direction may vary at random, but in most
areas the net effect is seasonal.
The rate of longshore transport is dépendent on both angle of wave
approach, and wave energy. Thus, high storm waves will generally move
more material per unit time than low waves. However, if low waves
exist for a much longer time than do high waves, the low waves may be
more significant in moving sand than the high waves.
Because reversais in transport direction occur, and because different
types of waves transport material at different rates, two components of
the longshore transport rate become important. The first is the net rate,
the net amount of material passing a particular point in the prédominant
direction during an average year. The second component is the gross rate,
the total of ail material moving past a given point in a year regardless
of direction. Most shores consistently hâve a net annual longshore transport in one direction. Determining the direction and average net and
gross annual amount of longshore transport is important in developing
shore protection plans.
In landlocked water of limited extent, such as the Great Lakes, a
longshore transport rate in one direction can normally be expected to be
no more than about 150,000 cubic yards per year. For open océan coasts,
the net rate of transport may vary from 100,000 to more than 2 million
cubic yards per year. The rate dépends on the local shore conditions
and shore alignment as well as the energy and direction of wave action.
1.46 EFFECT OF INLETS ON BARRIER BEACHES
Inlets may hâve significant effects on adjacent shores by interrupting the longshore transport and trapping onshore-offshore moving sand.
On ebb current, sand moved to the inlet by waves is carried a short distance out to sea and deposited on an outer bar. When this bar becomes
large enough, the waves begin to break on it, and sand again begins to
move over the bar back toward the beach. On the flood tide, when water
flows through the inlet into the lagoon, sand in the inlet is carried a
short distance into the lagoon and deposited. This process créâtes shoals
in the landward end of the inlet known as nrtddteground shoots or 'tnnev
bars, Later, ebb flows may bring some of the material in these shoals
back to the océan, but some is always lost from the stream of littoral
drift and thus from the downdrift beaches. In this way, tidal inlets
may store sand and reduce the supply of sand to adjacent shorelines.
1.47 IMPACT OF STORMS
Hurricanes or severe storms moving over the océan near the shore may
greatly change beaches. Strong winds of a storm often create a storm surge
This surge raises the water level and exposes to wave attack higher parts
of the beach not ordinarily vulnérable to waves. Such storms also generate
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to hour. These reversais of transport direction are quite common for
most United States shores. Direction may vary at random, but in most
areas the net effect is seasonal.
The rate of longshore transport is dépendent on both angle of wave
approach, and wave energy. Thus, high storm waves will generally move
more material per unit time than low waves. However, if low waves
exist for a much longer time than do high waves, the low waves may be
more significant in moving sand than the high waves.
Because reversais in transport direction occur, and because different
types of waves transport material at different rates, two components of
the longshore transport rate become important. The first is the net rate,
the net amount of material passing a particular point in the prédominant
direction during an average year. The second component is the gross rate,
the total of ail material moving past a given point in a year regardless
of direction. Most shores consistently hâve a net annual longshore transport in one direction. Determining the direction and average net and
gross annual amount of longshore transport is important in developing
shore protection plans.
In landlocked water of limited extent, such as the Great Lakes, a
longshore transport rate in one direction can normally be expected to be
no more than about 150,000 cubic yards per year. For open océan coasts,
the net rate of transport may vary from 100,000 to more than 2 million
cubic yards per year. The rate dépends on the local shore conditions
and shore alignment as well as the energy and direction of wave action.
1.46 EFFECT OF INLETS ON BARRIER BEACHES
Inlets may hâve significant effects on adjacent shores by interrupting the longshore transport and trapping onshore-offshore moving sand.
On ebb current, sand moved to the inlet by waves is carried a short distance out to sea and deposited on an outer bar. When this bar becomes
large enough, the waves begin to break on it, and sand again begins to
move over the bar back toward the beach. On the flood tide, when water
flows through the inlet into the lagoon, sand in the inlet is carried a
short distance into the lagoon and deposited. This process créâtes shoals
in the landward end of the inlet known as nrtddteground shoots or 'tnnev
bars, Later, ebb flows may bring some of the material in these shoals
back to the océan, but some is always lost from the stream of littoral
drift and thus from the downdrift beaches. In this way, tidal inlets
may store sand and reduce the supply of sand to adjacent shorelines.
1.47 IMPACT OF STORMS
Hurricanes or severe storms moving over the océan near the shore may
greatly change beaches. Strong winds of a storm often create a storm surge
This surge raises the water level and exposes to wave attack higher parts
of the beach not ordinarily vulnérable to waves. Such storms also generate
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