111. Sediment Transport
.......
133
3.1.4
Computation of the coastal constants
The essential role played by the coastal constants So~ and s~ could be noticed in
the mechanisms described above. Hence, an accurate computation of those values
is important and they have to be constant in time but not basically constant in
space (along the x-axis).
The longshore transport So~ along a straight coastline can be computed as a
function of the wave climate by adding up the transports due to the waves from
various directions, taking into account the frequency distribution. The resulting
transport then is the coastal constant Sox.
The longshore transport may be computed for each wave direction by means of
the CERC formula. It is, however, advisable to check the resulting transport So~
by means of other methods, such as volumes of erosion or accretion based on
measurenlents.
Finally, the longshore transport is a function of the wave height and direction in
the breaker zone. These values can be deduced from deep water data by means of
refraction and diffraction computations.
The second coastal constant sl can be computed by applying a rotation Aa of the
coastline and calculating the difference in resulting longshore transport
ASlcaused by this rotation. The coastal constant sl then is equal to "
AS s
s 1 - -
(77)
Aa
3.2
Two-Line Theory
The limitations of the one-line theory are numerous. Often it schematizes reality
too strongly. This can apply to initial and boundary conditions, as well as the
physical wave and beach characteristics. Bakker (1968) was concerned in
particular about a coast upon which the longshore sand transport is only partially
blocked by groins which were shorter than the width of the breaker zone. Bakker
proposed a so-called two-line theory for the solution of this problem. Instead of
schematizing a coastline with a single curve, two curves are used ~igure 3 I).
.......
133
3.1.4
Computation of the coastal constants
The essential role played by the coastal constants So~ and s~ could be noticed in
the mechanisms described above. Hence, an accurate computation of those values
is important and they have to be constant in time but not basically constant in
space (along the x-axis).
The longshore transport So~ along a straight coastline can be computed as a
function of the wave climate by adding up the transports due to the waves from
various directions, taking into account the frequency distribution. The resulting
transport then is the coastal constant Sox.
The longshore transport may be computed for each wave direction by means of
the CERC formula. It is, however, advisable to check the resulting transport So~
by means of other methods, such as volumes of erosion or accretion based on
measurenlents.
Finally, the longshore transport is a function of the wave height and direction in
the breaker zone. These values can be deduced from deep water data by means of
refraction and diffraction computations.
The second coastal constant sl can be computed by applying a rotation Aa of the
coastline and calculating the difference in resulting longshore transport
ASlcaused by this rotation. The coastal constant sl then is equal to "
AS s
s 1 - -
(77)
Aa
3.2
Two-Line Theory
The limitations of the one-line theory are numerous. Often it schematizes reality
too strongly. This can apply to initial and boundary conditions, as well as the
physical wave and beach characteristics. Bakker (1968) was concerned in
particular about a coast upon which the longshore sand transport is only partially
blocked by groins which were shorter than the width of the breaker zone. Bakker
proposed a so-called two-line theory for the solution of this problem. Instead of
schematizing a coastline with a single curve, two curves are used ~igure 3 I).