this source if the lagoon is less than 5 miles wide. Where the lagoon
is wider (especially 10 miles or greater) flooding from the lagoon side by
wind setup should be investigated before large dune construction projects
are undertaken.
4.622 Réservoir of Beach Sand. During storms, érosion of the beach'occurs
and the shoreline recedes. If the storm is severe, waves attack and erode
the foredunes and supply sand to the beach; in later érosion stages, sand
is supplied to the back of the island by overwash. (Godfrey, 1972.)
Volumes of sand eroded from beaches during storms hâve been estimated
in recent beach investigations. Everts (1973) reported on two storms tfuring February 1972 which affected Jones Beach, New York. The first storm
eroded an average of 27,000 cubic yards per mile above mean sea level for
the 9-mile study area; the second storm (2 weeks later) eroded an average
of 35,000 cubic yards of sand per mile above mean sea level at the same
site. Losses at individual profiles ranged up to 120,000 cubic yards per
mile. Davis (1972) reported a beach érosion rate on Mustang Island, Texas,
following Hurricane Fern (September 1971), of 12.3 cubic yards per linear
foot of beach for a 1,500-foot stretch of beach (about 65,000 cubic yards
per mile of beach). On Lake Michigan in July 1969, a storm eroded an average of 3.6 cubic yards per linear foot of beach (about 29,000 cubic yards
per mile) from an 800-foot beach near Stevensville, Michigan. (Fox, 1970.)
Because much of the eroded sand is usually returned to the beach by wave
action soon after the storm, these volumes are probably représentative of
temporary storm losses.
Volumes équivalent to those eroded during storms hâve been trapped
and stored in foredunes adjacent to the beach. Foredunes constructed
along Padre Island, Texas, and Core Banks, North Carolina, (Section 6.43
and 6.447) contain from 30,000 to 80,000 cubic yards of sand per mile of
beach. Assuming the présent rate of entrapment of sand continues for the
next 3 years at these sites, sand volumes ranging from 50,000 to 160,000
cubic yards per mile of beach will be available to nourish eroding beaches
during a major storm. Sand volumes trapped during a 30-year period by
European beachgrass at Clatsop Spit, Oregon, averaged about 800,000 cubic
yards per mile of beach. Thus, within a few years, foredunes can trap and
store a volume of sand équivalent to the volumes eroded from beaches during storms of moderate intensity.
4.623 Long-Term Effects. Dolan, (1972-73) advances *he 5on^
*
massive, unbroken foredune line restricts the lan war e ge
, .
zone during storms causing narrower beaches and t us increase
attriin the surf zone. The increased turbulence causes higher sand grain attr
tion and winnowing rates and leads to accelerated losses o
V1
’ .
erosive process that may be detrimental to the 1°n^“^an^ S ff ts of sedirier islands. However, as discussed in Section .
>
cnnrt nro
Wt size are usually of secondary importance in
PIn
cesses - processes which are important in barner is an
determining
addition, geographical location is probably more impor æj
j beaches
beach sand size than dune effects, since both fine and coarse sand beaches
4-115
is wider (especially 10 miles or greater) flooding from the lagoon side by
wind setup should be investigated before large dune construction projects
are undertaken.
4.622 Réservoir of Beach Sand. During storms, érosion of the beach'occurs
and the shoreline recedes. If the storm is severe, waves attack and erode
the foredunes and supply sand to the beach; in later érosion stages, sand
is supplied to the back of the island by overwash. (Godfrey, 1972.)
Volumes of sand eroded from beaches during storms hâve been estimated
in recent beach investigations. Everts (1973) reported on two storms tfuring February 1972 which affected Jones Beach, New York. The first storm
eroded an average of 27,000 cubic yards per mile above mean sea level for
the 9-mile study area; the second storm (2 weeks later) eroded an average
of 35,000 cubic yards of sand per mile above mean sea level at the same
site. Losses at individual profiles ranged up to 120,000 cubic yards per
mile. Davis (1972) reported a beach érosion rate on Mustang Island, Texas,
following Hurricane Fern (September 1971), of 12.3 cubic yards per linear
foot of beach for a 1,500-foot stretch of beach (about 65,000 cubic yards
per mile of beach). On Lake Michigan in July 1969, a storm eroded an average of 3.6 cubic yards per linear foot of beach (about 29,000 cubic yards
per mile) from an 800-foot beach near Stevensville, Michigan. (Fox, 1970.)
Because much of the eroded sand is usually returned to the beach by wave
action soon after the storm, these volumes are probably représentative of
temporary storm losses.
Volumes équivalent to those eroded during storms hâve been trapped
and stored in foredunes adjacent to the beach. Foredunes constructed
along Padre Island, Texas, and Core Banks, North Carolina, (Section 6.43
and 6.447) contain from 30,000 to 80,000 cubic yards of sand per mile of
beach. Assuming the présent rate of entrapment of sand continues for the
next 3 years at these sites, sand volumes ranging from 50,000 to 160,000
cubic yards per mile of beach will be available to nourish eroding beaches
during a major storm. Sand volumes trapped during a 30-year period by
European beachgrass at Clatsop Spit, Oregon, averaged about 800,000 cubic
yards per mile of beach. Thus, within a few years, foredunes can trap and
store a volume of sand équivalent to the volumes eroded from beaches during storms of moderate intensity.
4.623 Long-Term Effects. Dolan, (1972-73) advances *he 5on^
*
massive, unbroken foredune line restricts the lan war e ge
, .
zone during storms causing narrower beaches and t us increase
attriin the surf zone. The increased turbulence causes higher sand grain attr
tion and winnowing rates and leads to accelerated losses o
V1
’ .
erosive process that may be detrimental to the 1°n^“^an^ S ff ts of sedirier islands. However, as discussed in Section .
>
cnnrt nro
Wt size are usually of secondary importance in
PIn
cesses - processes which are important in barner is an
determining
addition, geographical location is probably more impor æj
j beaches
beach sand size than dune effects, since both fine and coarse sand beaches
4-115