Island crest élévation is determined by the nature of the sand forming the beach, and by the waves and water levels of the océan. The beach
and waves internet to détermine the élévation of the limit of wave runup -
the primary factor in determining island crest élévation. Normally the
island crest élévation is almost constant over long sections of beach.
However, duneless barrier island crest élévations vary with geographical
area. For example, the crest élévation typical of Core Banks, North
Carolina, is about +6 feet MSL; +4 feet MSL is typical for Padre Island,
Texas; +11 feet MSL is typical for Nauset Beach, Massachusetts.
Landward of the upper limit of wave uprush or berm crest are the back
shore and the déflation plain. This area is shaped by the wind, and infrequent ly by the flow of water down the plain when the island crest is
overtopped by waves.
(e.g., Godfrey, 1972.) Obstructions which trap
wind-transported sand cause the formation of dunes in this area. (See
discussion in Section 6.4 Sand Dunes.) Beachgrasses which trap windtransported sand from the beach and the déflation plain are the major
agent in creating and maintaining foredunes.
4.62 ROLE OF FOREDUNES
Foredunes, the line of dunes just behind a beach, hâve two primary
functions in shore processes. First, they prevent overtopping of the
island during some abnormal sea conditions. Second, they serve as a
réservoir for beach sand.
4.621 Prévention of Overtopping. By preventing water from overtopping,
foredunes prevent wave and water damage to installations landward of the
dune. They also block the water transport of sand from the beach area to
the back of the island and the flow (overwash) of overtopping sea water.
Large réductions in water overtopping are effected by small increases
in foredune crest élévations. For example, the hypothetical 4-foot dune
shown in Figure 4-41 raises the maximum island élévation about 3 feet to
an élévation of 6 feet. On this beach of Padre Island, Texas, the water
levels and wave runup maintain an island crest élévation of +4 feet MSL
(about 2 feet above MHW). This would imply that the limit of wave runup
in this area is 2 feet (the island crest élévation of 4 feet minus the
MHW of 2 feet). Assuming the wave runup to be the same for ail water
levels, the 4-foot dune would prevent significant overtopping at water
levels up to 4 ft MSL (the 6-foot effective island height at the dune
crest minus 2 feet for wave runup). This water level occurs on the average once each 5 years along this section of coast. (See Figure 4Thus, even a low dune, one which can be built with végétation and sand
fences in this area in 1 year (Woodard et al., 1971) provides considerable protection against wave overtopping. (See Section .an
Foredunes or other continuons obstructions on barrier islands may
cause ^inacceptable ponding from the land side of t e is an
needed
goon between the island and mainland is large enoug
o supp
from
wind setup.
(See Section 3.8.) There is little danger of flooding from
4-113
and waves internet to détermine the élévation of the limit of wave runup -
the primary factor in determining island crest élévation. Normally the
island crest élévation is almost constant over long sections of beach.
However, duneless barrier island crest élévations vary with geographical
area. For example, the crest élévation typical of Core Banks, North
Carolina, is about +6 feet MSL; +4 feet MSL is typical for Padre Island,
Texas; +11 feet MSL is typical for Nauset Beach, Massachusetts.
Landward of the upper limit of wave uprush or berm crest are the back
shore and the déflation plain. This area is shaped by the wind, and infrequent ly by the flow of water down the plain when the island crest is
overtopped by waves.
(e.g., Godfrey, 1972.) Obstructions which trap
wind-transported sand cause the formation of dunes in this area. (See
discussion in Section 6.4 Sand Dunes.) Beachgrasses which trap windtransported sand from the beach and the déflation plain are the major
agent in creating and maintaining foredunes.
4.62 ROLE OF FOREDUNES
Foredunes, the line of dunes just behind a beach, hâve two primary
functions in shore processes. First, they prevent overtopping of the
island during some abnormal sea conditions. Second, they serve as a
réservoir for beach sand.
4.621 Prévention of Overtopping. By preventing water from overtopping,
foredunes prevent wave and water damage to installations landward of the
dune. They also block the water transport of sand from the beach area to
the back of the island and the flow (overwash) of overtopping sea water.
Large réductions in water overtopping are effected by small increases
in foredune crest élévations. For example, the hypothetical 4-foot dune
shown in Figure 4-41 raises the maximum island élévation about 3 feet to
an élévation of 6 feet. On this beach of Padre Island, Texas, the water
levels and wave runup maintain an island crest élévation of +4 feet MSL
(about 2 feet above MHW). This would imply that the limit of wave runup
in this area is 2 feet (the island crest élévation of 4 feet minus the
MHW of 2 feet). Assuming the wave runup to be the same for ail water
levels, the 4-foot dune would prevent significant overtopping at water
levels up to 4 ft MSL (the 6-foot effective island height at the dune
crest minus 2 feet for wave runup). This water level occurs on the average once each 5 years along this section of coast. (See Figure 4Thus, even a low dune, one which can be built with végétation and sand
fences in this area in 1 year (Woodard et al., 1971) provides considerable protection against wave overtopping. (See Section .an
Foredunes or other continuons obstructions on barrier islands may
cause ^inacceptable ponding from the land side of t e is an
needed
goon between the island and mainland is large enoug
o supp
from
wind setup.
(See Section 3.8.) There is little danger of flooding from
4-113