Manning´s n values

[JensPK]

Hello everyone,

I have some questions about the different ways to apply Manning´s n values in HEC RAS 5.0.

1. What influence does the default value for 2D Flow Areas have if you use a Land Cover Layer with spatially varied Manning´s n values as well?
When I kept the values in the Layer constant and changed the default roughness, it still changed the results.
2. When I added a 2D Manning n Region (polygon) and defined a high Manning value for the polygon, there has been no effect on the results. How is that possible?
Can you give only one roughness to a 2D Manning n Region or does it correspond to the table from the Land Cover Layer so you can give it several Manning values divided by the boundaries in the layer?

Thanks in advance!

[Anonymous]

Try to open the 2D flow area editor and click “force mesh recomputation” after you associate your land cover layer. Just make sure you don’t regenerate or you will lose any edits you made to the mesh. This has worked for me in the past

[JensPK]

I used “force mesh recomputation”, but what do you mean with regenerating?

[Lonnie A]

In Rasmapper verify your changes are being updated on the cell faces (right click on the cell face and select property table).

What I have to do some times is within the geometry window in 2d area edit mode modify a n-value and save the file. Then modify it back to what you want in order for RAS to recognize a change has occurred and needs to reprocess the cell faces. So then select force mesh recomputation and back in RASmapper select the compute 2D hydraulics table. After it has ran verify the changes are applied to the cell face.

I find when you change the land classification RAS likes to revert back to the default n-value across the entire 2D area and there is no way to know this without digging into it in mapper.

[cameron]

I agree with Lonnie and check that the correct manning values are being used. I have run into multiple occasions when HEC-RAS would revert to using a single roughness value for no reason and it is almost impossible to track down why.

[iacopo]

Hello,
I have tried to follow your instructions but I didn’t obtain any good results. I created “2D Area Mann n regions” and I assigned the right values for every region but it is like they aren’t considered by Hec-ras. By using 2D hydraulics table I realised that Hec-ras considers just the default ones. Has anyone of you some suggestions more about how to solve this problem?
Many thanks!

[Author]

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