Obstacle Cell¶

Overview¶

An Obstacle Cell completely blocks flow across all vertical layers from the riverbed to the water surface. Use this feature to represent solid structures that fully obstruct the flow, such as bridge piers, revetment foundations, and embankment cores.

Obstacle Top Elevation has been removed

The Obstacle Top Elevation setting available in previous versions has been removed. Obstacle cells now always block all vertical layers from bed to water surface.

For structures with a defined crest elevation that allows overflow (weirs, sills, abutments, etc.), use Structure BC instead.

When to Use: Obstacle Cell vs. Structure BC¶

	Obstacle Cell	Structure BC
Set in	Cell Attribute	Boundary Condition
Height control	Always full-block	Partial block up to crest elevation
Overflow over crest	Not possible	Possible
Culvert opening	Not possible	Possible
Typical use	Bridge piers, complete solid walls	Weirs, sills, abutments, culvert bodies

Setup Procedure in iRIC¶

Step 1: Select the Grid¶

Open the pre-processing screen in iRIC and select your simulation grid.

Step 2: Open Cell Attributes → Assign Obstacle Cells¶

From the menu or left panel, select Cell Attributes and choose Obstacle Cell. Draw a polygon over the target cells. In the dialog that appears, select Obstacle and click OK.

Obstacle Cell assignment dialog with highlighted obstacle cells (blue)

Value	Label	Meaning
`0`	Normal cell	Flow passes through freely
`1`	Obstacle	All vertical layers blocked

Step 3: Verify the Configuration¶

Use the 3D view to confirm that obstacle cells are correctly placed. All σ-layers at obstacle cells will be blocked.

How It Works in the Solver¶

For each obstacle cell, all vertical layers k = 1 (bottom) through k = kmax (surface) have velocity forced to zero (obst3d(i,j,k) = 1).
When a dry-cell condition (hs < 0) occurs, the water surface is snapped to the bed elevation eta(i,j).

Notes¶

Migrating from Obstacle Top Elevation

If your project file contains Obstacle Top Elevation settings from a previous version, those values will be ignored. To reproduce a partial-height structure, create a new Structure BC group.

Combining with Structure BC

If you need a structure that is fully solid at its base but allows overflow above a certain elevation, use Structure BC rather than Obstacle Cell.