Tsun2

Use the Imamuras's tsun2 code to model a tsunami inundation. Although the SWAN code has a parameter called flooding I don't know how to use it. Instead I use the tsun2 code to compute the coastal flooding simulations. In order to that these simulations are realistic, they have to be run over small areas described by very detailed DTMs that include both topography as well as bathymetry data. The so called far field codes are not appropriate to model coastal inundations and the shallow water models (like tsun2) use such a small grid spacing that for the sake of running time is jus not practicable to make them run since the earthquake epicenter until the coast. The strategy is than to use a far field model to compute the tsunami propagation using a coarse grid spacing (typically about 1 km) until near shore and than use this simulation to feed a shallow water code.

The tsun2 code thus runs on a very fine grid mesh - with grid spacing typically not superior to 50 meters - and must be feed at every grid node of one or two of its grids borders. The feeding procedure consists in providing a maregraph record for all grid nodes of the border(s) at which the external wave arrives at the finer grid. However, as mentioned above, this is impracticable because it would imply running an external feeding code (Swan in this case) at the same (very fine) grid spacing as the fine grid. The solution adopted was than to use finer grids that have grid node spacing that are sub-multiples of the coarse grid. For example, dx = 50 m equals 1 km / 20. That way we insure that every 20-th +1 finer grid node will coincide with one node of the coarser grid. To complicate a bit more (but safer against misalignments) it is desirable that the origin coordinates of the finer grid coincide with those of one node of the coarser grid. With these conditions meat, feeding all grid nodes of the finer grid is simply accomplishing by linear interpolation of the water height using data at nodes actually feed by the far field code (one every 21 in the example above). It is a user task to construct such a pair of coarse/fine grids using a cartesian coordinate system (e.g. UTM).

It is now that the Plot satations on grid borders option in Swan comes handy. By selecting this option the finer grid is loaded and all its grid nodes around the four borders will be plotted on the figure derived from the coarse grid (if you doubt it, zoom in until you are convinced). Here a previous knowledge of the direction from which the tsunami arrives is required. Remove the borders at which the tsunami cannot arrive (that is, without crossing the smaller area). That will leave you with at most two frontiers. Now run Swan and save the maregraph file. That file is the one you need to feed in the Tsun2 module, but before you can run it there is still one last step. Select Write params file in Tsun2 entry and save a new parameter file (this time for Tsun2). This file contains indexes information used to translate the maregraph file created by Swan into coordinates of the finer grid used in Tsun2. Selecting Compute pops up an option window very similar to the Swans options window. In fact it contains only one extra option, which is Jump initial. This is meant for when the wave will arrive significantly later than the first records in the maregraph file. The value entered means the number of seconds to jump before the start of the simulation. As an example, with a time increment of 0.25 sec a jump of 100 means jumping the first 400 records of the maregraphs file.

Copyright <2010>, <Joaquim Luis>

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