Frequently asked questions

No. The layer parameters are read when the run starts, so they cannot be changed while the solver is paused; you have to stop the run, change the settings, and start again.

Before rerunning, revise the bed-material layer settings under the calculation conditions:

• Thickness of bedload layer (exchange layer): a value around the maximum grain size dmax or d90 is a common choice.
• Thickness of deposited layer: about 2 dmax, for example.
• Maximum number of deposited layers: increase it if thick deposition is expected.

Thicker layers or a larger maximum number of layers prevent the limit from being reached. The same guidance is given in the iRIC help FAQ on setting the grain size distribution of the exchange and deposited layers.

In current iRIC (4.x) the riv file creator macro is no longer needed and can be skipped. Cross-section survey data can be imported directly: prepare a list of the XY coordinates of the left-bank and right-bank ends of each survey station together with the cross-section data, and iRIC will read each section from it. The file format and procedure are described in the iRIC User's Manual under "Related files > River survey data > MLIT longitudinal/cross-section survey data" (iric-gui-user-manual.readthedocs.io, section 06/01_riversurvey).

It has not been removed; the menu label is different from the manual wording. Right-click on the pre-processor screen with the river survey data selected and choose Generate Cross-Section Data. This is the command the manual calls "Create River Survey Data" — "cross-section data" and "river survey data" refer to the same thing, and the English manual simply uses a different translation.

iRIC itself does not export calculation results as a raster. The recommended route is:

1. Export the results from the post-processor as ESRI shapefiles (the same export used for transects / point data).
2. Convert the shapefile to a raster in a GIS application such as QGIS or ArcGIS (e.g. rasterize the point or polygon attributes onto a grid of the desired resolution).

Alternatively, export the results as topography files (.tpo), CSV or VTK, which are point-data formats that can also be gridded in GIS or other analysis tools.

The standard Nays2DH distributed with iRIC does not provide a feature for specifying inflow discharges in arbitrary cells along the bank lines. A tributary can be represented by including a confluence channel in the computational grid (the Nays2DH manual and a tutorial video cover this), but that does not give multiple point inflows as lateral boundary conditions. According to a forum reply, this kind of lateral inflow is planned for the successor solver that is under development, so waiting for that solver or modifying the open-source Nays2DH code are the current options.

For a flat or nearly flat bed this result is physically correct, not an error. The water surface super-elevates on the outer bank to drive the flow around the bend; through the hydrostatic coupling, this produces a steeper streamwise water-surface slope near the inner bank and a flatter one near the outer bank, so the flow accelerates along the inner bank at the bend apex. Higher outer-bank velocities in real rivers arise from the point bar / pool topography and from secondary flow, which a 2D depth-averaged model like Nays2DH cannot reproduce fully; enabling bed deformation or using a 3D solver brings the result closer to observations. The theoretical background is covered in iRIC Academy.

Enable the bank erosion model in the Nays2DH calculation conditions; the grid then moves with the eroding banks. The settings are described on page 26 of the Nays2DH solver manual. Note that the grid "reconnection" (neck cut-off shortcut) seen in some demonstration videos is not available in the released Nays2DH or other solvers, so simulations run until a cut-off would occur, not beyond.

Yes. Use the CSV export of calculation results in the post-processor (see the iRIC 4 GUI manual, "Calculation result > Export"). CSV export writes one file per time step; each file lists the values for all grid nodes and cells, arranged by index (i, j), so the files can be opened directly in Excel. A short tutorial video on exporting results is linked from the iRIC forum.

Not with the standard Nays2DH included in iRIC: water levels can only be prescribed at the upstream and downstream boundaries, not at an arbitrary interior point. If this is essential, Nays2DH is open source and can be modified; the source code is available in the iRICsolvers GitHub repository (v4_Nays2DH). Otherwise, adjust the downstream water-surface boundary condition and roughness so that the simulated depth at the point of interest matches the measured value.

One known cause is the grain size distribution itself: the size classes must not start with a 0 mm class. A zero-diameter class produces numerical instabilities when bed deformation begins. Remove the 0 mm entry so that every class has a positive diameter and check that the cumulative distribution is monotonic. If the run still fails, review the bedload-layer and deposited-layer thicknesses and the time step, but the zero class is the first thing to check.

These columns appear in the grid file exported from a Nays2DH project and mean the following:

• Z: not used by the solver; it can be ignored.
• N_Elevation: the bed elevation defined at each grid node (the movable bed used for the calculation).
• N_Elevation_zb: the fixed-bed elevation defined at each grid node (the level below which the bed cannot erode).

At the time of the thread these meanings were not documented in the manual, which is why they are listed here.

iRIC can display a longitudinal profile at a chosen time step using the 1D graph (chart) feature in the post-processing window. Select the time step of interest, then create a 1D graph along the desired grid line to see the water surface profile; the graph data can also be exported for use in a spreadsheet. The procedure is described in the iRIC GUI user manual under the chart section: https://iric-gui-user-manual.readthedocs.io/en/latest/05/02_chart.html

This happens with the "Create grid from cross-section data" algorithm when no transverse division points have been defined, so each cross-section line is treated as a single cell width. You can add them in the Grid Creating Condition view:

1. Click the cross-section line you want to subdivide to select it.
2. Choose "Add Division Points" from the right-click menu.
3. Enter the number of divisions and click OK.
4. Regenerate the grid.

The iRIC GUI user manual describes this grid creating algorithm in detail (section on creating a 2D grid from river survey data).