Culvert – 2D blocked Culvert

[oryanas]

I have a detailed terrain data set that was flown. I want to model the culverts, but because some of the inverts are partially buried and the terrain reflects this, I get the error that the “culvert is lower than the cell it is connected to”. I can manually adjust my terrain data set in ArcMap to reflect this invert below existing terrain, but I was wondering if there is a way to adjust this in HEC-RAS. If I adjust the terrain downward to reflect the buried invert then I get additional overland capacity there too.

Logically I would think the connected cell elevation should reflect the invert plus the depth blocked (eg, Invert of 1200 + 0.5ft blocked = Cell Elevation of 1200.5) but RAS does not agree with me. There may not be an adequate solution to this, but I thought I would ask. Thanks.

[Anonymous]

unfortunately no, not in this version

[Lonnie A]

There are two options. You can use the “depth blocked” function in the culvert editor. The blocked depth will become the minimum culvert elevation that is checked against the terrain. The other option I use most frequently is to edit the terrain data using a GIS tool called ARIS. It lets me edit individual pixel elevations so I don’t have to make large terrain edits to allow for actual culvert invert placement.

[oryanas]

I thought I had tried adjusting the depth blocked but I will give it another go. Thanks for the response.

[Victor Padilha]

Is there a standard value to include in the blocked depth for the culvert to solve this issue?

I am having this problem and I used 0.1 – 0.2 in a few culverts, but I am concerned with the results…

Thanks!!

[oryanas]

The issue is that HEC-RAS cannot have a culvert invert below the lowest elevation of the cell it is connected to. This makes sense because if the culvert invert is below the minimum cell elevation then the culvert is buried. By adding a blocked depth to the culvert you are effectively modifying what HEC-RAS sees as the lowest culvert elevation by burying the culvert.

There is not a standard way of adding blocked depths as it is all dependent on your data. I have fixed this issue by both of the ways suggested earlier in the thread, depending on the situation.

1. If I have culvert invert survey that is a touch lower than the terrain elevation I have simply added a small blocked depth so that [culvert invert + blocked depth] ≥ [terrain elevation]. For very small blocked depths it does not have a significant impact on the results.

2. If the culvert invert is significantly lower than the terrain then I have gone in and modified the terrain elevation in GIS.

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