Tutorial: Virtual Tide Gauges

A virtual tide gauge is a point where an NSWING run records the water height through time, like a real tide gauge (maregraph) would. You place the gauges with the mouse, on the bathymetry the run uses. NSWING takes them from memory: there is no positions file to write or keep.

This tutorial assumes a window already set up for NSWING: bathymetry, an Okada source and, optionally, nested levels. See Running NSWING. The pictures come from the set-up of that tutorial (the source SW of Cape São Vicente, levels down to the Tejo estuary), run with Max level to use = 2.

1. Place the gauges

In the Geophysics menu, switch to the Tsunamis discipline (Geophysics › → Tsunamis), then pick Plot virtual tide gauges. The cursor becomes a cross: click once where the gauge goes. Each pick of the menu places one gauge. Pick it again for the next one.

Each gauge is a circle named Tide gauge 1, Tide gauge 2, … in Scene Objects.

Here five gauges sit in and around the mouth of the Tejo: (1) off Costa da Caparica, (2) in the channel at Belém, (3) at Alcântara, (4) off Carcavelos and (5) at Terreiro do Paço. All of them are inside the nested levels (the orange rectangles), in water 15 to 31 m deep.

Five virtual tide gauges in the Tejo mouth

The gauges in Scene Objects

Where to put them

Put gauges in the water, and inside the finest nested grid that covers them. NSWING computes the series on the innermost grid. A gauge outside it is skipped, with a warning in the console. A gauge on land records nothing useful.

2. Edit them

A gauge is an ordinary symbol, edited like any other:

Move: double-click the gauge (a yellow square appears in it), then drag it.
Size, colour, outline: right-click the gauge, or use its properties in Scene Objects.
Leave one out of a run: uncheck its row in Scene Objects. Only visible gauges are used.
Delete: Remove in its Scene Objects row.

3. Open the NSWING dialog

Pick NSWING tsunami… from the same Tsunamis menu. When the window has visible gauges, the Maregraphs box of the dialog is set up for them:

Field	What it shows
Maregraphs	Ticked.
In file	In memory location, grayed out. The gauges are passed to NSWING directly, so there is no file to choose. Its tooltip says how many gauges were found.
Out file	`maregs.dat`, as a full path, in the folder of the bathymetry file. When the bathymetry has no file of its own, it uses the folder of the session file the window was saved to or loaded from. You can change it with the ... button.
Saving step	One sample every that many time steps (1 = every step).

The Maregraphs box with the gauges in memory

The dialog reads the gauges again each time it comes to the front, so you can add, move or remove gauges while it is open. With no visible gauges left, In file becomes an editable file field again.

4. Run

Press RUN. The gauge positions go to NSWING with the grids, and the series is written to the Out file when the run ends.

With Auto load simulation ticked, two windows open when the run finishes:

the simulation, in a new iGMT window with the Aquamoto viewer;
an X,Y Plot of the gauges: time in seconds on X, one curve per gauge, named after it.

The five gauges after the run

The wave reaches the gauges outside the estuary first: Costa da Caparica (gauge 1) at about 2500 s, Carcavelos (gauge 4) at about 2800 s. Belém and Alcântara (2, 3) follow at 3400–3600 s, and Terreiro do Paço (5), furthest up the estuary, only starts to rise as the 63-minute run ends.

The X,Y Plot belongs to the simulation window. Closing it or minimising it parks it as a row in that window's Scene Objects (bottom of the panel below), and a double-click on the row brings it back.

The parked gauge plot in the simulation window's Scene Objects

Save files & RUN writes the gauges as tide_gauges.dat next to the other saved files, and the command line it shows names that file. It is then a normal lon lat name positions file, usable as the In file of a later run.

5. The output file

maregs.dat is a plain text table that GMT, Mirone and the X,Y Plot tool read directly. It starts with six header rows, then a short note (written by iGMT) on what they mean, then the data after a >XY line:

#	Tide_gauge_1	Tide_gauge_2	Tide_gauge_3	Tide_gauge_4	Tide_gauge_5
#		-9.27000	-9.20000	-9.16000	-9.32000	-9.13000
#		38.63000	38.68467	38.69000	38.67000	38.70467
#		14.9	30.9	22.5	17.7	17.9
# X		-9.27000	-9.20000	-9.16000	-9.32000	-9.13000
# Y		38.63000	38.68467	38.69000	38.67000	38.70467
# --- What the header rows above mean (written by InteractiveGMT) ---
# ...
>XY
1.879	0.01126	0.01090	0.01068	0.01160	0.01053
5.636	0.01126	0.01090	0.01068	0.01160	0.01053
...

Gauge 2 was placed at 38.685 N; row 3 says 38.68467, the nearest node of the grid it was computed on.

Row	Content
1	The gauge names, one per column.
2	X (longitude) of the grid node where each gauge is computed. A gauge is moved to the nearest node of the grid it is computed on, so this can differ slightly from where you clicked.
3	Y (latitude) of that node.
4	Water depth at that node, in metres, positive down (negative = land).
5, 6	`# X` / `# Y`: the same coordinates again, in the form Mirone reads.
data	Column 1: time in seconds since the start of the simulation. Then one column per gauge: the water surface height in metres, in the order of the header columns.

6. Sessions

The gauges are saved with the window by File → Save Session…. Loading the session brings them back as gauges: still draggable, and found again by the NSWING dialog.

Running NSWING: the full run set-up.
Nested Grids: the levels the gauges should sit inside.
X,Y Plot Tool: working with the gauge curves.