Checking the coupled model

Interactions of waves and currents

Waves can cause setup, i.e. an increase or decrease of the average water level. In general there is an interaction between average water level and current velocity on the one hand and waves on the other. The influence of water level and current on wave propagation is clearly due to the presence of terms depending on the water depth and the current in the action balance equation.
There is an influence in the other direction; waves exert a force on the average water motion, mainly in areas where the wave height veries strongly. Due to the strong dissipation in the surf zone it is mainly in this zone that there is wave setup and a longshore current.

The correct procedure for the modelling of regions with strong interaction is to make alternatively wave and flow computations, until the results converge.
The flow computations compute both the water level and the current velocity. The wave-induced forces are one of the external forcings that drive the flow model, more or less like the wind forcing. The water level and current velocity are written to a file that is used as input in the wave model.
In the subsequent wave computations one of the output quantities is FORCE, short for the wave-induced force, which then is used as input to the current model. FORCE is one of the output quantities for which the DHH-version of SWAN can make a (vectorial) plot.

Setup and wave-induced currents are considerable only in very narrow regions, because the forcing by the waves is substantial only in surf zones. It is therefore useless to compute these in large regions. Only in nested regions close to the coast this computation makes sense.


In some circumstances the computations of wave-induced current and setup can be decoupled. It is difficult to give the general conditions for this decoupling. If the wave-induced currents are relatively small the decoupling is justified. In such circumstances SWAN can compute the setup directly, without the need to use a current model. It is always an approximation.
In 1-d situations the decoupling is always correct.
The computation of the setup is done if the command SETUP is present. The result of the setup computation is shown in a table or a plot, since there is an output quantity called SETUP (this time SETUP is a keyword in an output command, not a separate command as in the previous case). The output quantity SETUP is an output quantity like HSIG or PER.
SWAN will take into account the influence of the setup on the water level, i.e. the depth is corrected with the value of the setup.

Note that it is incorrect to compute setup in SWAN if the interaction of waves and currents is computed in the way as suggested in the previous section, since the setup is already included in the water level computed by the flow model.

Checking results of the coupled model

Since the wave-induced flow and the setup have to be computed using two coupled models, checking whether the results are correct is more complicated than the two models separately. Also the transfer of data between the two models must be tested thoroughly. Questions involved are: 1. are the units of the quantities written by one model the same as the units of the quantities expected by the other model as input? 2.Are the vectors defined with respect to the same coordinate system?

Therefore the following advice: Never (I mean NEVER) start with a complicated case, always run a few simple cases first where you roughly know what result you should obtain.
For instance consider a 1-d case with roughly normally incident waves. It is known that the setup at the coast is between 7 and 10% of the incoming Hs. This you can use to verify whether the coupled system produces a good result. Also you can compare with the setup computed by Swan itself because in 1-d cases this setup is correct.
In a 1-d case with oblique incidence the coupled system should be able to compute a longshore current. For such situations there are cases to be found in literature that show the order of magnitude that you should get. And of course you can go to the beach and get a feeling for the order of magnitude of the current velocity.

© 2012: Nico Booij