This tutorial shows how to run a basic Itzï simulation using freely available dataset.

It assumes that GRASS 7 and Itzï are properly installed on your machine and that you possess a basic knowledge of GRASS.

Get ready

Here we will use the GRASS North Carolina dataset. Please download the GRASS 7 version and extract it in your grassdata directory.

Then start GRASS in the PERMANENT mapset.

Surface modelling

Adjust the region

Fit the lidar elevation raster map and set a resolution of 5m:

$ g.region -p raster=elev_lid792_1m@PERMANENT res=5 save=lidar_5m

Resample the DEM

Please note that this step is not strictly necessary. The Itzï simulation will be carried out in any case on the defined computational region extent and resolution. However the bilinear interpolation smooth the surface, which prevent high slope values that could occur if using the GRASS default nearest-neighbour sampling.

$ r.resamp.interp input=elev_lid792_1m@PERMANENT output=elev_lid792_5m

Create a raster mask

Generate a drainage direction map and then create a watershed raster using the outlet point coordinates:

$ r.watershed elevation=elev_lid792_5m drainage=elev_lid792_5m_drainage
$ r.water.outlet input=elev_lid792_5m_drainage output=watershed coordinates=638888,220011

Create a raster mask to prevent calculation outside of the watershed:

$ r.mask rast=watershed

Create boundary condition maps

Create a vector map with the watershed outlet point:

$ echo '638888|220011' > watershed_out.txt
$ v.in.ascii input=watershed_out.txt output=watershed_out

Using this vector map, create two raster maps for the boundary conditions. The first with a value corresponding to the type of condition, here 4 corresponds to a fixed water depth inside the domain. The second being the value of the depth wanted, here 0.

$ v.to.rast input=watershed_out type=point output=bctype use=val value=4
$ v.to.rast input=watershed_out type=point output=bcvalue use=val value=0

Create rainfall and friction maps

Create maps of uniform rainfall and friction coefficient:

$ r.mapcalc exp='rain=100'
$ r.mapcalc exp='n=0.05'

Create a parameters file

Create a new parameter file and fill it with the ID of the created maps. It should look like the following:

duration = 02:00:00
record_step = 00:05:00

dem = elev_lid792_5m@PERMANENT
friction = n@PERMANENT
rain = rain@PERMANENT
bctype = bctype@PERMANENT
bcval = bcvalue@PERMANENT

prefix = nc_itzi_tutorial
values = h, wse, v, vdir, boundaries

stats_file = nc_itzi_tutorial.csv

Run the simulation

Run the simulation:

$ itzi run <parameter_file_name>

At the end of the simulation, Itzï should have generated five Space-Time Raster Dataset (STRDS) in the form:


The maps contained in those STDRS are following this naming convention:


Here is the example of the map nc_itzi_tutorial_h_0020:

NC depth

All the results can be processed using the GRASS tools for raster maps and / or space-time dataset. For instance, it is easy to generate an animation of the results using g.gui.animation.

Culvert modelling

New in version 17.7.

As you can notice in the image above, the flow accumulates at some points. One of this accumulation is due to a road that act like a dike and weir. It is better seen when displaying the streets and flow arrows:

Illustration of road blockage

One option to solve this problem is to model a culvert using the coupled modelling capacity of Itzï. Itzï is able to run the SWMM drainage model alongside the surface model, and model the interactions between the two.

In the present case, the first step is to decide where the culvert will be located, and annotate:

  • The coordinates of the the input and output node,
  • the altitude at those points.

Unfortunately, two issues limit the use for the modelling of culvert:

  • SWMM needs to have a connected outfall node in the network model
  • The Itzï coupling code is designed for manhole, not culvert entrance.

We can circumvent those limitations by first, adding an outlet at a higher elevation, linked to the rest of the network by a dummy pipe, and second, set the coupling surface to a large surface (here we’ll set it equal to the cell surface).

SWMM configuration file

The description of the drainage network is done in a classic SWMM configuration file. More information could be found in the SWMM user’s manual.


START_DATE and START_TIME are not taken into account during a coupled simulation. The drainage model always starts and stops at the same time than the surface model.

'Wake county culvert'

FLOW_UNITS           CMS
START_DATE           01/01/0001
START_TIME           00:00:00
REPORT_START_DATE    01/01/0001
END_DATE             01/01/0001
END_TIME             2:00:00
SWEEP_START          01/01
SWEEP_END            12/31
DRY_DAYS             0
REPORT_STEP          00:05:00
WET_STEP             00:00:05
DRY_STEP             01:00:00
ROUTING_STEP         2
MIN_SURFAREA         25
MIN_SLOPE            0

;;        Invert   Max.   Init.  Surcharge  Ponded
;;Name    Elev.    Depth  Depth  Depth      Area
;;------- -------- ------ ------ ---------- ------
J0        112      0.0    0      0          0
J1        111.4    0.0    0      0          0

;;         Invert    Outfall  Stage/Table    Tide
;;Name     Elev.     Type     Time Series    Gate
;;-------- --------- -------- -------------- ----
O2         1000      FREE                    NO

;;Node       X-Coord  Y-Coord
;;---------- -------  -------
J0           638752   220262
J1           638769   220233

;;       Inlet  Outlet         Manning Inlet  Outlet
;;Name   Node   Node    Length N       Offset Offset
;;------ ------ ------- ------ ------- ------ ------
C0       J0     J1      34     0.017   0      0
C1       J1     O2      100    0.017   0      0

;;Link    Shape      Geom1 Geom2 Geom3 Geom4 Barrels
;;------- ---------- ----- ----- ----- ----- -------
C0        CIRCULAR   1.5   0     0     0     2
C1        CIRCULAR   0.1   0     0     0     1

Here, J0 and J1 are the input and output nodes of the culvert, and C0 is the culvert itself. The latter is made of two pipes of 1.5m of diameter. The outfall O2 and the link C1 are added to comply with the SWMM rule needing them.

Update the Itzï’s parameter file

The parameter file of created in the precedent tutorial could be used and adapted by the addition of the [drainage] section, like so:

duration = 00:50:00
record_step = 00:05:00

dem = elev_lid792_5m@PERMANENT
friction = n@PERMANENT
rain = rain@PERMANENT
bctype = bctype@PERMANENT
bcval = bcvalue@PERMANENT

prefix = nc_itzi_tutorial_drainage
values = h, v, vdir

stats_file = nc_itzi_tutorial_drainage.csv

swmm_inp = tutorial_drainage.inp
output = nc_itzi_tutorial_drainage

cfl = 0.7
theta = 0.9
dtmax = .5

Where swmm_inp is the path to the SWMM configuration file and output is the name of the Space-Time Vector Dataset where the drainage data will be written.

Running the simulation

The simulation is ran the same way as the previous tutorial. Itzï will call SWMM that will in turn loads its own configuration file automatically.

The resulting water depth map is shown here:

Water depth with culvert

The area upstream the road is noticeably less flooded, with a maximum water depth coming down from 1.03m without culvert to 0.45m with culvert. You can use the temporal tools of GRASS to query the evolution in time of the drainage network values. For example, to get the evolution of the flow leaving the upstream node J0 of the culvert:

t.vect.db.select input=nc_itzi_tutorial_drainage@itzi_results columns=outflow where="node_id=='J0'"