érosion of the nearshore bottom and beaches, but in the control volume
defined, this transport takes place within the control volume. Transpoi
from the offshore has been treated as a line source.
In some places, offshore islands or shoals may act as point sources
of material for the littoral zone. For example, the drumlin islands and
shoals in Boston Harbor and vicinity may be point sources for the nearby
mainland.
4.724 Windblown Sédiment Sources. To make a net contribution to the
littoral zone in the time frame being considered, windblown sand must
corne from a land source whose sand is not derived by intermediate s»teps
from the same littoral zone. On U.S. océan coasts, such windblown sand
is not a significant source of littoral materials. Where wind is important in the sédiment budget of the océan shore, wind acts to take away
sand rather than to add it, although local exceptions probably occur.
However, windblown sand can be an important source, if the control
volume being considered is a beach on the lagoon side of a barrier islan
Such shores may receive large amounts of windblown sand.
4.725 Carbonate Production. Dissolved calcium carbonate concentration
in the océan is near saturation, and it may be precipitated under favorable conditions.
In tropical areas, many beaches consist of calcium car
bonate sands; in temperate zones, calcium carbonate may be a significant
part of the littoral material. These calcium carbonate materials are ge
erally fragments of shell material whose rate of production appears to i
crease with high température and with excessive évaporation. (See Hayes
1967.) Oolitic sands are a nonbiogenic Chemical precipitate of calcium
carbonate on many low latitude beaches.
Quantitative estimâtes of the production of calcium carbonate sédiment are lacking, but maximum rates might be calculated from the density
and rate of growth of the principal carbonate-producing organisms m an
area. For example, following northeasters along the Atlantic coast of
the U.S., the foreshore is occasionally covered with living clams thrown
up by the storm from the nearshore zone. One estimate of the annual con
tribution to the littoral zone from such a source would assume an averag
shell thickness of about 0.04 foot completely covering a stnp of beach
100 feet wide ail along the coast. On an annual basis, this would be
about 0.15-cubic yard per year per foot of beach front. Such a quantity
is negligible under almost ail conditions. However, the dominance o
carbonate sands in tropical littoral zones suggests that the rate
p
duction can be much higher.
4.726 Beach Replenishment. Beach protection projects
•
Placing-sand on beaches. The quantity of sand placed on the ^ach i
such beach-fill operations may be a major element in
e
f
budget. Data on beach-fill quantifies may be available in Corps of Engl
neer District offices, in records of local 8overnm«nS,engl"“q r^
,
dredging company records. The exact computation o
le qu
4-123
defined, this transport takes place within the control volume. Transpoi
from the offshore has been treated as a line source.
In some places, offshore islands or shoals may act as point sources
of material for the littoral zone. For example, the drumlin islands and
shoals in Boston Harbor and vicinity may be point sources for the nearby
mainland.
4.724 Windblown Sédiment Sources. To make a net contribution to the
littoral zone in the time frame being considered, windblown sand must
corne from a land source whose sand is not derived by intermediate s»teps
from the same littoral zone. On U.S. océan coasts, such windblown sand
is not a significant source of littoral materials. Where wind is important in the sédiment budget of the océan shore, wind acts to take away
sand rather than to add it, although local exceptions probably occur.
However, windblown sand can be an important source, if the control
volume being considered is a beach on the lagoon side of a barrier islan
Such shores may receive large amounts of windblown sand.
4.725 Carbonate Production. Dissolved calcium carbonate concentration
in the océan is near saturation, and it may be precipitated under favorable conditions.
In tropical areas, many beaches consist of calcium car
bonate sands; in temperate zones, calcium carbonate may be a significant
part of the littoral material. These calcium carbonate materials are ge
erally fragments of shell material whose rate of production appears to i
crease with high température and with excessive évaporation. (See Hayes
1967.) Oolitic sands are a nonbiogenic Chemical precipitate of calcium
carbonate on many low latitude beaches.
Quantitative estimâtes of the production of calcium carbonate sédiment are lacking, but maximum rates might be calculated from the density
and rate of growth of the principal carbonate-producing organisms m an
area. For example, following northeasters along the Atlantic coast of
the U.S., the foreshore is occasionally covered with living clams thrown
up by the storm from the nearshore zone. One estimate of the annual con
tribution to the littoral zone from such a source would assume an averag
shell thickness of about 0.04 foot completely covering a stnp of beach
100 feet wide ail along the coast. On an annual basis, this would be
about 0.15-cubic yard per year per foot of beach front. Such a quantity
is negligible under almost ail conditions. However, the dominance o
carbonate sands in tropical littoral zones suggests that the rate
p
duction can be much higher.
4.726 Beach Replenishment. Beach protection projects
•
Placing-sand on beaches. The quantity of sand placed on the ^ach i
such beach-fill operations may be a major element in
e
f
budget. Data on beach-fill quantifies may be available in Corps of Engl
neer District offices, in records of local 8overnm«nS,engl"“q r^
,
dredging company records. The exact computation o
le qu
4-123