Numerical models have to be calibrated against field data which
should be of sufficient accuracy. Calibration is used to fine tune the model
by adjusting some of the parameters such as the eddy viscosity, roughness
coefficient etc. The calibrated model is then verified against another
independent set of field data which have not been used in the calibration.
In coastal engineering, continuity of sediment volume is normally
used to model erosion or accretion of a beach. The so-called one-line
model assumes a typical beach profile like equation (15.6) assuming to be
unchanging up to the closure depth. But the beach itself is assumed to
move on a horizontal plane onshore or offshore depending on erosion or
accretion. A formula like equation (15.3) is used for the quantity of sedi-
ment motion alongshore. The model should also incorporate computation
of wave transformation along the beach. The one-line model predicts the
movement of the beach cross-shore. On the other hand, N-line coastal
models introduce refinements by taking account of the cross-shore sedi-
ment transport. Kamphuis (2000) lists some available computer programs
based on one-line or N-line models.
Roelvink and Broeker (1993) compared the performance of five
commonly used profile change models. Cross-shore sediment transport is
important in these models. Comparison of the computed results with the
results of laboratory results showed that these models can only predict
short-term changes. They also find wide differences between the predictive
capabilities of the various models used.
Numerical models are also used to investigate the impact of structures
such as offshore or shore-connected breakwaters on the sediment motion
and hence the coastal behaviour. Although significant progress has been
made so far in the development of these models, research is actively
pursued to improve their predictions. Physical models are also used to
investigate coastal behaviour under controlled conditions. However, it may
not be possible to simulate in the physical model all the processes present
in the field. Currently design of coastal structures makes use of results
obtained from numerical and physical models, and field measurements.
For a general discussion of models in hydraulic engineering see
Chapter 16.
15.2.3 Groynes
One of the methods of saving a beach or a shoreline from erosion is to
construct a series of groynes, usually built perpendicular to the shoreline.
They trap sand transported by the longshore currents, but downdrift
erosion may take place due to starvation of beach material.
Groynes are constructed from timber, concrete, stone or steel; riprap
or concrete blocks are also used, the choice of material depending on
634 COASTAL ENGINEERING
