136
R.H. Charlier and Chr. P. De Meyer
3.3
N-Line Model
This type of model has been developed to include the effect of tidal currents in
the longshore transport and to obtain a more horizontally two-dimensional
picture of coastal processes. This is contrary to the above mentioned one- and
two-line models, which consider only the longshore transport due to breaking
waves. However, the effect of tidal currents can certainly be important for the
development of the nearshore part of the coastal profiles, beyond the breaker
zone.
The set-up of the n-line model is as follows. The coastal area under study is
represented by a number of coastal profiles. Each coastal profile is schematized to
a number (n) of depth contours. Bars and troughs cannot be reproduced, the
consecutive lines increase in depth in seaward direction. Wave conditions are
determined from deep water conditions via refraction and shoaling. The
longshore current is driven by wave forces (shear stress due to radiation stresses)
and the tidal forces. The distribution of random waves inside the breaker zone is
based e.g. upon the early concept of Battjes (1974), according to which the local
root-mean-square wave height is obtained by deleting from the wave height
distribution the portion of waves which are higher than a certain maximum
value.
The sediment transport capacities are computed in sections between the
schematized profiles, both in longshore and cross-shore direction. The resulting
transport gradients are accounted for in depth changes in the schematized
profiles. Consequent discontinuities in the profiles are compensated for by a
smoothing procedure in each separate profile.
The Department of Public Works of The Netherlands (RWS) carried out a
number of computations with the N-line model to evaluate its possibilities. The
main conclusion was, that the principle of schematizing the water- and sedimenttransport in a horizontal plane is a sound approach for the development of a
morphological coastal model. However, a representation of the coastal profile in a
number of lines appears to be too strong a schematization for general cases of
curved coastlines, and is only suitable for practically uniform and straight coasts.
The ultimate advantage of the N-line model with respect to the KC/KL models is
the fact, that besides the waves, tidal or other currents can be taken into account
in the computation of the longshore transport. The longshore transport is locally
determined, which means that apart from the integrated value, the cross-shore
distribution of the longshore transport is determined. In cases with additional
structures, such as groins, part of the longshore transport can be blocked.
R.H. Charlier and Chr. P. De Meyer
3.3
N-Line Model
This type of model has been developed to include the effect of tidal currents in
the longshore transport and to obtain a more horizontally two-dimensional
picture of coastal processes. This is contrary to the above mentioned one- and
two-line models, which consider only the longshore transport due to breaking
waves. However, the effect of tidal currents can certainly be important for the
development of the nearshore part of the coastal profiles, beyond the breaker
zone.
The set-up of the n-line model is as follows. The coastal area under study is
represented by a number of coastal profiles. Each coastal profile is schematized to
a number (n) of depth contours. Bars and troughs cannot be reproduced, the
consecutive lines increase in depth in seaward direction. Wave conditions are
determined from deep water conditions via refraction and shoaling. The
longshore current is driven by wave forces (shear stress due to radiation stresses)
and the tidal forces. The distribution of random waves inside the breaker zone is
based e.g. upon the early concept of Battjes (1974), according to which the local
root-mean-square wave height is obtained by deleting from the wave height
distribution the portion of waves which are higher than a certain maximum
value.
The sediment transport capacities are computed in sections between the
schematized profiles, both in longshore and cross-shore direction. The resulting
transport gradients are accounted for in depth changes in the schematized
profiles. Consequent discontinuities in the profiles are compensated for by a
smoothing procedure in each separate profile.
The Department of Public Works of The Netherlands (RWS) carried out a
number of computations with the N-line model to evaluate its possibilities. The
main conclusion was, that the principle of schematizing the water- and sedimenttransport in a horizontal plane is a sound approach for the development of a
morphological coastal model. However, a representation of the coastal profile in a
number of lines appears to be too strong a schematization for general cases of
curved coastlines, and is only suitable for practically uniform and straight coasts.
The ultimate advantage of the N-line model with respect to the KC/KL models is
the fact, that besides the waves, tidal or other currents can be taken into account
in the computation of the longshore transport. The longshore transport is locally
determined, which means that apart from the integrated value, the cross-shore
distribution of the longshore transport is determined. In cases with additional
structures, such as groins, part of the longshore transport can be blocked.