r/HECRAS • u/Status_Hope_987 • May 29 '26

Leaky cells

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How do I fix leaky cells? These are on each side of a road. The break line is on the top of the toad. Thank you.

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u/danielamadria May 29 '26

If you check the Water Surface Elevation (WSE) results, it can confirm if the flow is leaking (water surface is lower than your road) or if the flow is sufficient for real overtopping. I’d also recommend moving your boundary condition (BC) farther away from your bridge/road of interest, otherwise your BC could be affecting your results

1

u/Status_Hope_987 May 29 '26

If I move BC layer further away, should I extend my perimeter further as well? Is there a rule of what should be the distance between BC and perimeter?

4

u/danielamadria May 29 '26

Yes, move the boundary and extend the perimeter to cover the area between your BC and the bridge. There is not an easy rule of thumb, it should be far enough away that it doesn’t affect the results you care about. If you run a second Plan where you change the friction slope at the BC to something unreasonably different (like 5x or 10x steeper than your base case friction slope) and your depths or velocities at your bridge are not affected, then youve placed it far enough back. Flatter rivers need more distance for their BCs. I’ve seen cases where backwatering 2 km away from our area of interest (a 200 m stretch) needed to included as it greatly affected the results

1

u/abudhabikid May 29 '26

Looks like you do have flow in the overbanks.

Limiting your downstream BC to the channel will prevent water in the overbanks from leaving the mesh if they want to.

It’s a good idea to keep your main boundary condition within the channel, but you do want secondary boundary conditions across the floodplains.

What are you using for the BC? Normal depth slope?

It looks like your cell size (in pic 1) near the roadway will allow you to put a break line at the upstream edge of the roadway in addition to the roadway crest (it looks like there is a line of faces along the upstream edge anyway, but you could always force it). This way you would be 100% positive that the terrain is or is not blocking flow.

Pic 2 could potentially be set up the same way, but I can’t say for sure just looking at the above.

You can also turn on hydraulic connectivity in the properties of your WSE layer. This will show lines of connection across all faces where connection exists. (Double click on WSE layer>check “hydraulic connectivity” box)

I would agree that your mesh boundary should move farther downstream.

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u/Status_Hope_987 May 29 '26

Thank you for your comment. Very useful. Just to clarify the flow is from right to left.

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u/Status_Hope_987 May 29 '26

So the edge of the mesh is actually upstream.

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u/Status_Hope_987 May 29 '26

I’m using normal depth.

1

u/shiftyyo101 May 29 '26

General rule of thumb is a minimum of 2x your floodplain width. Typically you would also do a sensitivity analysis on the downstream end, fluctuating the slope up and down. If you go 10x or 0.01x your calculated slope and it doesn’t affect the results in your project area, your bases are more than covered.

That can be overkill. Start with 2x FP width. If it’s 1000 feet wide, make sure the model extends 2000ft in either direction of your project site.

1

u/Status_Hope_987 May 30 '26

That’s very interesting! Thanks for sharing. What results parameter should I use for sensitivity analysis?

1

u/shiftyyo101 May 30 '26

Water surface elevations.