shelter for a floating dredge to pump the impounded material across the
navigation opening back onto the downdrift beach. This method is used
at Channel Island Harbor near Port Hueneme, California.
1.56 GROINS
The groin is a barrier-type structure that extends from the backshore
into the littoral zone. The basic purposes of a groin are to interrupt
longshore sand movement, to accumulate sand on the shore, or to retard
sand losses. Trapping of sand by a groin is done at the expense of the
adjacent downdrift shore unless the groin or groin System is artificially
filled with sand to its entrapment capacity from other sources. To reduce
the potential for damage to property downdrift of a groin, some limitation
must be imposed on the amount of sand permitted to be impounded on the
updrift side. Since more and more shores are being protected, and less
and less sand is available as natural supply, it is now désirable, and
frequently necessary, to place sand artificially to fill the area between
the groins, thereby ensuring a more or less uninterrupted passage of the
sand to the downdrift shores.
Groins hâve been constructed in various configurations using timber,
Steel, concrète or rock. Groins can be classified as high or low, long
or short, permeable or imperméable, and fixed or adjustable.
A high groin, extending through the breaking zone for ordinary or
moderate storm waves, initially entraps nearly ail of the longshore
moving sand within that intercepted area until the areal pattern or surface profile of the accumulated sand mass allows sand to pass around the
seaward end of the groin to downdrift shores. Low groins (top profile no
higher than that of desired beach dimensions) fonction like high groins,
except that sand also passes over the top of the structure. Permeable
groins permit some of the wave energy and moving sand to pass through the
structure.
1.57 JETTIES
Jetties are generally employed at inlets in connection with navigation improvements. When sand being transported along the coast by waves
and currents arrives at an inlet, it flows inward on the flood tide to
form an inner bar, and outward on the ebb tide to form an outer bar.
Both formations are harmful to navigation through the inlet, and must be
controlled to maintain an adéquate navigation channel. The jetty is similar to the groin in that it traps sand moving along the beach. Jetties
are usually constructed of Steel, concrète, or rock. The jetty type
dépends on foundation conditions, wave climate, and économie considérations. Jetties are much larger than groins, since jetties sometimes extend from the shoreline seaward to a depth équivalent to the channel
depth desired for navigation purposes. To be efficient in maintaining
the channel, the jetty must be high enough to completely obstruct sand
movement.
1-17
navigation opening back onto the downdrift beach. This method is used
at Channel Island Harbor near Port Hueneme, California.
1.56 GROINS
The groin is a barrier-type structure that extends from the backshore
into the littoral zone. The basic purposes of a groin are to interrupt
longshore sand movement, to accumulate sand on the shore, or to retard
sand losses. Trapping of sand by a groin is done at the expense of the
adjacent downdrift shore unless the groin or groin System is artificially
filled with sand to its entrapment capacity from other sources. To reduce
the potential for damage to property downdrift of a groin, some limitation
must be imposed on the amount of sand permitted to be impounded on the
updrift side. Since more and more shores are being protected, and less
and less sand is available as natural supply, it is now désirable, and
frequently necessary, to place sand artificially to fill the area between
the groins, thereby ensuring a more or less uninterrupted passage of the
sand to the downdrift shores.
Groins hâve been constructed in various configurations using timber,
Steel, concrète or rock. Groins can be classified as high or low, long
or short, permeable or imperméable, and fixed or adjustable.
A high groin, extending through the breaking zone for ordinary or
moderate storm waves, initially entraps nearly ail of the longshore
moving sand within that intercepted area until the areal pattern or surface profile of the accumulated sand mass allows sand to pass around the
seaward end of the groin to downdrift shores. Low groins (top profile no
higher than that of desired beach dimensions) fonction like high groins,
except that sand also passes over the top of the structure. Permeable
groins permit some of the wave energy and moving sand to pass through the
structure.
1.57 JETTIES
Jetties are generally employed at inlets in connection with navigation improvements. When sand being transported along the coast by waves
and currents arrives at an inlet, it flows inward on the flood tide to
form an inner bar, and outward on the ebb tide to form an outer bar.
Both formations are harmful to navigation through the inlet, and must be
controlled to maintain an adéquate navigation channel. The jetty is similar to the groin in that it traps sand moving along the beach. Jetties
are usually constructed of Steel, concrète, or rock. The jetty type
dépends on foundation conditions, wave climate, and économie considérations. Jetties are much larger than groins, since jetties sometimes extend from the shoreline seaward to a depth équivalent to the channel
depth desired for navigation purposes. To be efficient in maintaining
the channel, the jetty must be high enough to completely obstruct sand
movement.
1-17