BG_Flood is a hydrodynamics model for simulating 2D depth-averaged flow. The following manual describes how to use the model but does not go into details about the model equations.

This manual is under revision and not up-to-date with the last version of the code. Please, use the Parameters and Forcings list as a reference

Input Parameters

Model controls

BG_Flood model reads an instruction file (or param file) that tells the model what to do and what forcing files to read. when instructions are not explicitely given the model will use the default value. How to use the param file is important for making BG_Flood do what you want it to do.

By default this instruction file is called BG_param.txt.

BG_param.txt

All the model inputs are controlled by the BG_param.txt file. It is a simple text file that contains the parameters of the model that the user wishes to change. While it is easy to build such file manually but also it is easy to use script and/or programming language to generate and modify text file to automatically to build and run many simulations.

How to use the BG_param.txt file

Model parameters can be modified in the file by specifying the parameter name, the equal sign, the desired value(s) for the parameter and optionally a semi-column ;

#My Dummy BG_param files
#Any lines starting with the Hash tag will be ignored
#You can also leave blank lines
 
# Parameter order does not really matter but if you specify a parameter twice the first value will be overwritten
 
# You can only put one parameter per line
bathy = mybathyfile.nc
 
#Any number of leading space or space between the parameter name and the equal sign will be accepted. Tabs are not acceptable
    theta       =      1.0
#Obviously you have to put the right name down (spelling and not case sensitive) otherwise the line will be ignored
tteettaa = 1.22
 
#If you want to add a comment at the end of a line with a parameter you can do it after putting a semi column e.g.:
CFL = 9; Was 0.5 but I don't know what I'm doing
 
#The model has only one required parameter and that is the (level 0) bathymetry grid name

General comment about input files

Some parameters expect a filename (e.g. bathy). While the model will accept any extension there are a couple of requirement. For spatially varying inputs, the extension controls how the file is read. You can use ESRI ascii grids .asc, mid-depth files .md or NetCDF .nc. NetCDF files are recommended and expected to have the .nc extension. Since NetCDF files can have several variables you can enforce (recommended) the variable being read by appending ?var to the file name (e.g. bathy = mybathyfile.nc?z).

For spatially and temporally varying input (e.g. wind or atmospheric pressure) a/multiple NetCDF file with a 3D variable is expected (see here).

Where the file input is a timeserie (e.g. boundary forcing), then the file extension is ignored but a text file is expected.

List of Parameters

Remember that the only required parameter to run the model is the bathymetry file. But if you want to run something fun you will need to specify boundary conditions, initial conditions and/or some forcings.

Full list of the parameters

Bathymetry/topography files

Input bathymetry data as a regular grid using .asc, .nc, .md. This is a required parameter that ultimately also defines the extend of the model domain and model resolution (if not defined by the user).

bathy = My_bathy_file.asc

The correct way of reading the file will be dedicated by the file extension. For .nc (netcdf) files you can specify the name of the variable in the file as: mybathyfile.nc?myvar (will look for zb if none are specified).

This input file is critical as it defines the extent of the model grid and the base resolution in the model.

Note: Different files can be provided to the code (using the instruction line multiple times). The code will use the last one having information at a given location when interpolating the data to create the bottom elevation (zb) variable.

Conputational mesh

BG-Flood generates its own mesh. By default, it is a quad regular mesh based on the DEM (Digital Elevation Model) extend and resolution. The extend of the mesh and resolution of the mesh can be modify by the user. The mesh can also be refined/coarsen in areas or following patterns prescribed by the user.

Masking

Parts of the input bathymetry can be masked and excluded from the computation. The model extent (and locations of side boundaries) will remain the same but entire blocks can be removed from the computational memory. These area will appear as NaN in the output file. The input grid is first devided in blocks of 16x16. If all the value within a block exceed the mask value (9999 as default) then that block is excluded from memory and no computation will occur there. An "area of interest" (AOI) can also be used to select a part of the domain. If none of the cells of a block is located in this area, that block will be excluded from memory. The AOI is prefered other the mask method as the later can create waterfalls on the borders of the domain, specially if the rain-on-grid method is used.

There are no fancy treatment of the boundaries of masked blocks so it is safer to select a mask threshold (mask) to keep most of the masked are dry. If water tries to cross to a masked block, The boundary of blocks are treated as Neumann (no gradient) boundaries so the bathymetry of the 2 cells adjacent to a masked block are set to the same value (the value of the second cell removed from the masked block).

Boundaries

Four type of boundaries can be applied at the edge of the model. By default neumann (no perpendicular gradient) is applied.

0: No slip (wall)
1: Neumann
2: dirichlets (water level )
3: Absorbing (only 1D/perpendicular absorbion is implemented)
(4: ~~ 2D Absorbing~~ (Not implemented yet))

For Boundary type 2 and 3 (Dirichlet and Absorbing) the level at the boundary level is imposed from a file so a file needs to be sepcified:

left = 1;
right = mybndfile.txt,3;
top = mybndfile.txt,3;

Boundary file (for type 2 or 3)

Water level boundary file are needed to type 2 and 3 boundaries. The files are 2 (or more) columns, one with time in the first column and water level is the other(s). Note that the time step in the file doesn't need to be constant. The model will linearly interpolated between steps in the file. The file can be either comma separated or tab separated. This is automatically detected.

Uniform boundary file

For uniform boundary condition (along the boundary axis) the files needs to be 2 column:

# Water level boundary
0  1.2
0  1.2
0  1.22
0  1.25
0  1.24

Variable boundary file

For Variable boundary condition (along the boundary axis) the files needs to be 3+ columns. The boundary axis is then divided by the number of boundary column and the water level is interpolated between location. for example:

# Water level boundary
0  1.2 0.0
0  1.2 0.0
0  1.22 0.0
0  1.25 0.0
0  1.24 0.0

Here the the water level on the near-end of the boundary (left for bottom and top bnds; bottom for left and right bnds) axis will vary between 1.2 and 1.25 but the far-end (right for bottom and top bnd and top for left and right bnds) will remain constant at 0.0.

Here is an example with 3 water level column:

 # Water level boundary
0 0.0 1.2 0.0
0 0.0 1.2 0.0
0 0.0 1.2 0.0
0 0.0 1.2 0.0
0 0.0 1.2 0.0 

In this case both near and far end of the boundary axis will remain zero and the center of the boundary axis will be 1.2m.

There is no restriction in the number of columns. These values from each column will be forced uniformly spaced on the boundary and forcing in between will be linearly interpolated.

Bottom friction

Bottom friction is applied implicitly in the model (applied to velocities after momentum and continuity equations are solved). There are 3 friction equations implemented defined in BG_param.txt as frictionmodel =:

0 Quadratic
1 Smart
-1 Mannings Quadratic friction is the default, with a uniform friction coefficient:
frictionmodel = 0

cf = 0.001

If a uniform friction is required add cf= to your BG_param.txt with the desired value. cf keyword is also used for the $z0$ of Smart formulation and $n$ of the Manning formulation.

For non-uniform friction parameter use the keyword cfmap or roughnessmap and assign an .asc or .nc file. For nc files you may need to supply the netcdf variable name: e.g. cfmap=final_rough.nc?zo. The roughness grid does not need to match the model grid dimension and coarser friction grid will be interpolated to the model grid cells and model cells outside of the forcing domain will be extrapolated (nearest value).

Rivers and Area discharge

At this stage river can only be added to the model as a vertical discharge where the water is added to a rectangle on the model with no velocity. To add rivers add a line per river with the parameters: river = Fluxfile,xstart,xend,ystart,yend; where Fluxfile is a 2 column text file containing time and dicharge in m3/s; xstart is the left coordinate of the square where the vertical discharge is applied, xend is the right coordinate of the square, ystart is the bottom coordinate of the square and yend is the top coordinate of the square. Example:

river = Votualevu_R.txt,1867430,1867455,3914065,3914090;

river = Mulomulo_R.txt,1867052,1867072,3911853,3911873;

Wind atm pressure forcing

Wind forcing (may contain bugs)

The hydrodynamics can be forced using a linear wind drag. the linear drag can be influenced with the keyword Cd. Wind input is defined with the keyword windfiles=. There are several ways to use the keyword.

spatially uniform txt file:

windfiles=mywind.txt

where mywind.txt is a text file with 3 column (time (in s), wind speed (m/s) and wind direction(degree North)). If the grid is in acoordinate system rotated from the north and a grdalpha is specified, the wind will be automatically rotated to the grid orientation.

Spatially and time varying input

windfiles=mywind.nc?uw,mywind.nc?vw

Here two arguments separated with a comma are expected. The first argument is the netcdf file and variable name containing the U component of the wind (along the X axis) and the second argument is the netcdf file and variable name containing the V component of the wind (along the Y axis). Both can be in the same netcdf file as in the example or in separate netcdf files (add the variable name with a ? similarly to other netcdf input options). The dimension of the wind forcing grid does not need to match the model grid dimension and coarser forcing will be interpolated to the model grid cells and model cells outside of the forcing domain will be extrapolated (nearest value).

Atmospheric pressure forcing

Spatially constant atmospheric pressure forcing is not relevant so only spatially varying forcing is feasable. Like for the wind this is done through a netcdf file:

atmpfile=myncfile.nc?atmpres

The forcing pressure is expected to be in Pa and the effect of the atmospheric pressure gradient is calculated as the difference to a reference pressure Paref=101300.0 converted to a height using Pa2m=0.00009916. If using hPa your will need to also change the reference pressure to Paref=1013.0 and the conversion parameter to Pa2m=0.009916. As with the wind forcing, the forcing grid does not need to match the model grid dimension and coarser forcing will be interpolated to the model grid cells and model cells outside of the forcing domain will be extrapolated (nearest value).

Outputs

There is two types of outputs:

map outputs of 2D variables regularly through time.
time-series output of basic values, at a chosen position, at each time step.

Map outputs

These maps are output as a nc file, with information on coordinates and blocks.

The map output can be modify by:

defining a timestep (in s) for these outputs:
outputtimestep = 3600.0;
changing the set of variables in the output file (from the list given in the manual)
outvars = zs,h,u,v,zb,hmax,Umax,hUmax,twet;
changing the name of the output file:
outfile = Results_tuto_basicRun.nc;
choosing one or more zones to outputs (by default, the full domain is output):
outzone=MyZoneName.nc,x1,x2,y1,y2;

outzone=MyZoneNameb.nc,x1b,x2b,y1b,y2b;
saving the output as float (variables are saved as short integer by default.):
smallnc = 0;

By default, the variables outputs are the one listed in the following paragraph: Default snapshot outputs.

Default snapshot outputs

Parameter	Definition	Unit
u	U velocity (at cell center) zonal velocity positive right	[m/s]
v	V velocity (at cell center) meridional velocity positive right	[m/s]
h	water depth at cell center	[m]
zs	Water level elevation above datum	[m]
zb	Topography elevation above datum	[m]

Complementary variables

Parameter	Definition	Unit
vort	Vorticity	[rotation/s]
cf	Bottom friction coefficient (Manning n or z0)	varies with model used

Mean/averaged output between output steps

This is for averaging variables in between output steps, useful for mean tidal flow calculation that averages out vortices. The average time is outtimestep.

Parameter	Definition	Unit
umean	Averaged u velocity (at cell center) zonal velocity positive right	[m/s]
vmean	Averaged v velocity (at cell center) meridional velocity positive right	[m/s]
hmean	Averaged water depth at cell center	[m]
zsmean	Averaged Water level elevation above datum	[m]

Max output

The max can be calculated for the overall simulation (default) or between output steps ( if resetmax = 1;)

Parameter	Definition	Unit
umax	Maximum u velocity (at cell center) zonal velocity positive right	[m/s]
vmax	Maximum v velocity (at cell center) meridional velocity positive right	[m/s]
hmax	Maximum water depth at cell center	[m]
zsmax	Maximum Water level elevation above datum	[m]

Risk assesment related output

These variables are used to evaluate the damage resulting from the innundation (as a complement to hmax for example).

Parameter	Definition	Unit
hUmax	Maximum of h time the velocity (U for amplitude of (u,v))	[m2/s]
Umax	Maximum of the velocity (U for amplitude of (u,v))	[m/s]
twet	Duration innundation of the cell in s (h>0.1m)	[s]

Model related outputs

These outputs will be produce only if the associated model/forcing is used.

If an atmospheric forcing is used:

Parameter	Definition	Unit
Patm	Atmospheric pressure	[Pa]
datmpdx/datmpdy	Gradients of atmospheric pressure	[Pa/m]

If the infiltration model (ILCL) is used, the quantity of water that infiltrate in the ground is saved, as a cumulated value in hgw.

Parameter	Definition	Unit
il	Initial loss coefficient	[mm]
cl	Continuous loss coefficient	[mm/hr]
hgw	Cumulated height of infiltrated water in the ground	[m]

Other gradients and intermediate terms of the equations

Terms of the equation can also been output such as the gradients (for error tracking mainly):

Parameter	Definition	Unit
dhdx / dhdy	Gradient of water elevation (h) in the x and y direction respectively	[]
dzsdx / dzsdy	Gradient of the water surface (zs) in the x and y direction respectively	[]
dudx / dudy	Gradient of x-velocity (u) in the x and y direction respectively	[s-1]
dvdx / dvdy	Gradient of y-velocity (v) in the x and y direction respectively	[s-1]
Fhu / Fhv	Flux of h time u in the x and y direction respectively	[m2/s2]
Fqux / Fqvx	XXXXXXXXXXXX	[XXX]
Su / Sv	Topographic source term	[XXX]
dh	Variation in elevation	[m]
du / dv	Variation of the x- and y-velocity respectively	[m/s]

Point or Time-Serie output

For each Time-Serie output needed, a line with the destination file and the postition is needed:

TSnodesout=Offshore.txt,xloc,yloc;

The file contains 5 colums $(t, zs, h, u,v)$ with the value at the nearest grid point (to the position defined by the user).

Adaptative grid

At the stage of development, the code will adapt the grid only before the computation but not along the calcul.

The code is based on a Block-uniform quadtree mesh. Each block, actually a 16 by 16 cells, is one unit of computation in the GPU. These blocks can have different resolutions (but the resolution does not change during the computation at this stage).

By default, the initial resolution of the grid is the resolution of the bathymetry/topographic data. To refine or coarsen the grid, you can weather use the "dx" key word and choose a new resolution for the whole domain; wether use different levels of resolution. The reference level, correponding to the bathymetry resolution or "dx" if defined by the user, will be the level 0. Levels of resolution are then defined in relation to the reference levels using positive integers to increase the resolution or refine and negative integer to coarsen the grid by a multiple of two. For a given level $n$ , the resolution $dx_n$ will be: $$dx_n=\frac{dx_0}{2^n}$$

with $dx_0$ the resolution at level 0.

When refinning using the level implementation, different key words are expected:

Initlevel: level used to create the first mesh created by the code in the mesh refinement process (only a technical information)
Maxlevel: maximim level of refinement (over-ruling other commands)
Minlevel: minimum level of refinement (over-ruling other commands)

The grid can also be unregular with an adaptition of the grid to the model (variables at initialisation step or user-defined refinement map). In this case, the cells will be devided in 4 cells for refinement, or 4 cells merged in one for coarsening. The code will ensure a progressive change of resolution (no cell should have a neighbour with more than 1 level of resolution of difference.)

The different methods of refinement available in the code are called using the key word "Adaptation". The refinement can be based on a classical input variable or a variable calculated during the initialisation:

'Threshold': impose a threshold for a different level of resolution
'Inrange': impose a range for a different level of resolution
'Targetlevel': the levels of resolution will be targeted but will be overruled by the maxlevel, minlevel entrance.

For example, for the adaptation with targeted levels:

initlevel = init ;
maxlevel =  max ;
minlevel = min ;
Adaptation = Targetlevel,MyLevelMap.nc?levels ;

Where max and min represent the range of level expected, and init is a number in this range (it is advice to use the min level). MyLevelMap is a netcdf 2D map of levels, that can have a different resolution and dimension from the computational grid. The amplitude of the levels on the map can also be larger than than min/max. All these levels are positive or negative integer.

For a bathymetry map of 10m resolution ( or $ dx=10m$), we can use $min=-3$, $max=2$ and $init=-3$ to create a grid where coarser cell will be $10/2^-3=80m$ and the thinner $10/2=2.5m$. The level file would contains a 2D map with integer values from -3 to 2.