# Fixed grid monitoring¶

Warning

This feature is still under development and should be used with caution. The documentation is also still incomplete.

GeoClaw has the capability to monitor certain quantities on a specified “fixed grid” by interpolating from the AMR grids active at each time step, or at specified time increments. This is useful in particular to record the maximum flow depth observed at each point over the course of a computation, or the maximum flow velocity, momentum, or momentum flux. These quantities are often of interest in hazard modeling.

It is also possible to record the arrival time of a flow or wave at each point on the grid. The “grids” do not have to be rectangular grids aligned with the coordinate directions, but can consist of an arbitrary list of points that could also be points along a one-dimensional transect or points following a coastline, for example.

Each grid is specified by an input file in a specified form described below. The list of file names for desired grids is specified in the setrun function, see Fixed grid maximum monitoring / arrival times.

This is an improved version of the algorithms used in earlier versions of GeoClaw, and now correctly interpolates when a grid point lies near the junction of two grid patches, which was not always handled properly before. The earlier version can still be used for outputing results at intermediate times on a fixed grid (see Fixed grid output), but is not recommended for the purpose of monitoring maxima or arrival times.

## Input file specification¶

The input file describing a grid of points has the following form:

tstart_max
tend_max
dt_check
min_level_check
arrival_tol
npts
x1 y1
x2 y2
...


with a total of ‘npts’ pairs of x-y coordinates specified. These points need not line on a regular grid and can be specified in any order.

The output files will list values for the points in the same order as in the input file. See fgmax_processing for some hints on processing and plotting the results.

The other paramters in the input file are:

• tstart_max : float

starting time to monitor maximum

• tend_max : float

ending time to monitor maximum

• dt_check : float

time increment for monitoring maximum and arrivals. Interpolate to fixed grid and update values only if the time since the last updating exceeds this time increment. Set to 0 to monitor every time step.

• min_level_check : integer

Minimum AMR level to check for updating the maximum value observed and the arrival time. Care must be taken in selecting this value since the maximum observed when interpolating to a point from a coarse AMR level may be much larger than the value that would be seen on a fine grid that better resolves the topography at this point. Often AMR “regions” are used to specify that a fine grid at some level L should always be used in the region of interest over the time period from start_max to tend_max, and then it is natural to set min_level_check to L.

• arrival_tol : float

The time reported as the “arrival time” is the first time the value of the surface elevation is greater than sea_level + arrival_tol.

## Values to monitor¶

The values to be monitored are specified by the subroutine fgmax_values. The default subroutine found in the library \$CLAW/geoclaw/src/2d/shallow/fgmax_values.f90 monitors a single value defined roughly to be the surface elevation h + B in wet cells and some large negative number in dry cells.

Describe this better.

## Processing and plotting fgmax output¶

For an example see apps/tsunami/chile2010_fgmax in the Clawpack Applications repository.

Describe further.