l/I. Sediment Transport
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4
Beaches Backed by Dunes
139
As is known, "storms" erode beaches, which means by- combinations of storm
tidesError! Reference source not found, and stormwaves. To the contrary, they
build up with normal or low tides and by swells. The hydrodynamic behavior for
the two conditions differs greatly. During storms material concentrations are
high, and the material is fed into the wave uprush by eroding dunes or beach
berms. Swells on the other hand generate much less concentration of suspension
loads. They "shear the material" off the bottom and wash it up on the beach
thereby building it up.
Vellinga (I 983) established a predictive model for beach and dune erosion during
storm surges. In many ways his findings have coiffirmed certain known facts.
(a) as seen in the field :
(1) A steep slope causes heavier erosion than a more gentle one.
(2) Coarser material on a steep beach washes out faster but it also settles
faster.
(3) Sand material when eroded from a dune and from the upper beach
during a storm, deposits in the lowest part of the beach and in the
nearshore shallow water, thereby establishing a mass balance twodimensionally and three-dimensionally, assuming uniform material
characteristics and a "long" straight shore.
Co) as seen in the laboratory :
(4) Reproduction on a "scale model" using native beach sand only gives
qualitative similarity. Using finer sands a better similarity is
obtainable.
(5) The present available longshore transport formulas are unreliable,
particularly for storm conditions which ignore the differences in
erosion and transport caused by differences in tides as well as
differences imposed by wave characteristics.
Because of the great predictive-importance of the Vellinga-model :for beach and
dune erosion, this computational model will be briefly outlined.
In 1972 the Technical Advisory Conunittee for the Sea Defense (TAW) published
a provisional guideline for the computation of dune erosion during storm surges.
On the basis of a limited number of beach profile measurements after storm
surges, the following erosion profile was defined ;
h =
0.415 (y + 4.5) °s - 0.88
(89)
......................
4
Beaches Backed by Dunes
139
As is known, "storms" erode beaches, which means by- combinations of storm
tidesError! Reference source not found, and stormwaves. To the contrary, they
build up with normal or low tides and by swells. The hydrodynamic behavior for
the two conditions differs greatly. During storms material concentrations are
high, and the material is fed into the wave uprush by eroding dunes or beach
berms. Swells on the other hand generate much less concentration of suspension
loads. They "shear the material" off the bottom and wash it up on the beach
thereby building it up.
Vellinga (I 983) established a predictive model for beach and dune erosion during
storm surges. In many ways his findings have coiffirmed certain known facts.
(a) as seen in the field :
(1) A steep slope causes heavier erosion than a more gentle one.
(2) Coarser material on a steep beach washes out faster but it also settles
faster.
(3) Sand material when eroded from a dune and from the upper beach
during a storm, deposits in the lowest part of the beach and in the
nearshore shallow water, thereby establishing a mass balance twodimensionally and three-dimensionally, assuming uniform material
characteristics and a "long" straight shore.
Co) as seen in the laboratory :
(4) Reproduction on a "scale model" using native beach sand only gives
qualitative similarity. Using finer sands a better similarity is
obtainable.
(5) The present available longshore transport formulas are unreliable,
particularly for storm conditions which ignore the differences in
erosion and transport caused by differences in tides as well as
differences imposed by wave characteristics.
Because of the great predictive-importance of the Vellinga-model :for beach and
dune erosion, this computational model will be briefly outlined.
In 1972 the Technical Advisory Conunittee for the Sea Defense (TAW) published
a provisional guideline for the computation of dune erosion during storm surges.
On the basis of a limited number of beach profile measurements after storm
surges, the following erosion profile was defined ;
h =
0.415 (y + 4.5) °s - 0.88
(89)