Most of the sédiment carried to the coast by rivers is deposited
in comparâtively small areas, often in estuaries where the sédiment is
trapped before it reaches the coast. (Strakhov, 1967.) The small fraction of sand in the total material brought to the coast and the local
estuarine and deltaic depositional sites of this sédiment suggest that
rivers are not the immédiate source of sédiment on beaches for much of
the world’s coastline. Many sources of evidence indicate that sandsized sédiment is not supplied to the coasts by rivers on most segments
of the U.S. Atlantic and Gulf coasts. Therefore, other sédiment sources
nust be important.
4.722 Erosion of Shores and Cliffs. Erosion of the nearshore bottom,
the beach, and the seaward edge of dunes, cliffs, and mainland (Fig. 4-44)
results in a sand loss. In many areas, érosion from cliffs of one area is
the principal source of sand for downdrift beaches. Kuenen (1950) estimâtes that beach and cliff érosion along ail coasts of the world totals
about 0.03 cubic mile or 160 million cubic yards per year. Although this
amount is only about 1 percent of the total solid material carried by
rivers, it is a major source in terms of sand delivered to the beaches,
since the sand fraction in the river sédiments is usually small, and is
usually trapped before it reaches the littoral zone. Shore érosion is an
especially significant source where older coastal deposits are being eroded, since these usually contain a large fraction of sand.
If an eroding shore maintains approximately the same profile above
the seaward limit of significant transport while it erodes, then the ero
sion volume per foot of beach front is the vertical distance from dune
base or berm crest to the depth of the seaward limit (h), multiplied by
the horizontal retreat of the profile, Ax. (See Figure 4-44.)
Figure 4-44 shows three équivalent volumes, ^ica^beacM’rofile.
sion of hAXo To the right in Figure 4-44 i*
profile.
The dashed line profile below it is the same
nTOfiies is Ax, the
The horizontal distance between solid an
as
p
érosion. The
horizontal retreat of the profile due to (assum
seaward limit is
unit volume loss, hAx, between dune base.and ^Xa-llelogram on
équivalent to the unit volume mdicated y
narallelogram, hAx,
the niddle of Figure 4-44. The unit volume
^ts parallelog^ ,
is équivalent to the shaded rectangle on t e
horiZOntal érosion,
vertical distance, h is 40 feet, and Ax -
, Der foot of beach
then the unit volume lost is 40/27, or 1.5 cubtc yards per
front.
*
«/^Ttain and potentially signifi
4.723 Transport from Offshore Slope.
contribution from the offshore
cant source in the sédiment budget is
distribution, and related evislope. However, hydrography, sedimen
contributions from the condence discussed in Section 4.523 in 1C
. negligible in many areas,
tinental shelf to the littoral zone are p
. ate in areas fairly close
Most shoreward moving sédiment appears
rt takes place within t e
to shore. Significant onshore-offshore transport^t^^ changes
t0
littoral zone due to seasonal and storm
4-121
in comparâtively small areas, often in estuaries where the sédiment is
trapped before it reaches the coast. (Strakhov, 1967.) The small fraction of sand in the total material brought to the coast and the local
estuarine and deltaic depositional sites of this sédiment suggest that
rivers are not the immédiate source of sédiment on beaches for much of
the world’s coastline. Many sources of evidence indicate that sandsized sédiment is not supplied to the coasts by rivers on most segments
of the U.S. Atlantic and Gulf coasts. Therefore, other sédiment sources
nust be important.
4.722 Erosion of Shores and Cliffs. Erosion of the nearshore bottom,
the beach, and the seaward edge of dunes, cliffs, and mainland (Fig. 4-44)
results in a sand loss. In many areas, érosion from cliffs of one area is
the principal source of sand for downdrift beaches. Kuenen (1950) estimâtes that beach and cliff érosion along ail coasts of the world totals
about 0.03 cubic mile or 160 million cubic yards per year. Although this
amount is only about 1 percent of the total solid material carried by
rivers, it is a major source in terms of sand delivered to the beaches,
since the sand fraction in the river sédiments is usually small, and is
usually trapped before it reaches the littoral zone. Shore érosion is an
especially significant source where older coastal deposits are being eroded, since these usually contain a large fraction of sand.
If an eroding shore maintains approximately the same profile above
the seaward limit of significant transport while it erodes, then the ero
sion volume per foot of beach front is the vertical distance from dune
base or berm crest to the depth of the seaward limit (h), multiplied by
the horizontal retreat of the profile, Ax. (See Figure 4-44.)
Figure 4-44 shows three équivalent volumes, ^ica^beacM’rofile.
sion of hAXo To the right in Figure 4-44 i*
profile.
The dashed line profile below it is the same
nTOfiies is Ax, the
The horizontal distance between solid an
as
p
érosion. The
horizontal retreat of the profile due to (assum
seaward limit is
unit volume loss, hAx, between dune base.and ^Xa-llelogram on
équivalent to the unit volume mdicated y
narallelogram, hAx,
the niddle of Figure 4-44. The unit volume
^ts parallelog^ ,
is équivalent to the shaded rectangle on t e
horiZOntal érosion,
vertical distance, h is 40 feet, and Ax -
, Der foot of beach
then the unit volume lost is 40/27, or 1.5 cubtc yards per
front.
*
«/^Ttain and potentially signifi
4.723 Transport from Offshore Slope.
contribution from the offshore
cant source in the sédiment budget is
distribution, and related evislope. However, hydrography, sedimen
contributions from the condence discussed in Section 4.523 in 1C
. negligible in many areas,
tinental shelf to the littoral zone are p
. ate in areas fairly close
Most shoreward moving sédiment appears
rt takes place within t e
to shore. Significant onshore-offshore transport^t^^ changes
t0
littoral zone due to seasonal and storm
4-121