[Igloo_79]

I think they are likely to be related to instability issues of the model in general. Sorry I cannot be more specific!

[Igloo_79]

You need to replicate the high/low chord data in the XS with Lids editor so that is looks similar to XS 516, with a rectangular bridge opening rather than a deck across the channel.

[Igloo_79]

To restrict flow to within the culvert like XS 516, you need to extend the geometry of the ‘lid’ to the left and right so that it covers the XS width and is greyed out like in 516.

[Igloo_79]

Hi – first you need to set up your model with standard open channel cross sections, including the internal XS of the culvert. To add XS with lids, select the XS at the upstream face of the culvert:

Open the geometry editor > Options > Add a Lid to XS > enter data for high and low chord plus tick ‘add Preissman slot’.

You will need to repeat this for every internal XS of the culvert up to and including the DS face. Don’t forget to adjust expansion/contraction coefficients too.

My understanding is that XS with Lids is designed for modelling long culverts with internal changes in XS geometry. I think that the high WSE in my previous posts were related to model instability, which has subsequently been resolved.

Good luck!

[Igloo_79]

No – I just raised the DS low chord of bridge 1 by ~0.3m, plus moved the ineffective flow areas a similar distance horizontally. At bridge 2, the structures and bounding XS were incorrectly skewed, so I had to correct that.

I eventually managed to get the model to run by raising the DS low chord only at Bridge 1. Am going to make the rest of the changes one by one, though, this is obviously becomes a time consuming method…

[Igloo_79]

Thanks for this Lonnie. Looks like a good approach!

I notice that your LS/2D Flow Areas do not align to the ends of your 1D XS. The same was true for some parts of my model, leading to ‘dry’ patches where the 1D and 2D domains were not connected. It is quite a pain to realign them to the 1D boundary (i.e. XS Interpolation Surface). Would appreciate your experience of this?

[Igloo_79]

My intention was to create a new 1D geometry file only containing XS for the channel sections where I want to modify the Terrain (not tried it yet, though).

Defining the elevation banks correctly is very important in 1D/2D model coupled by LS in order to have consistent results.
Can you give some more details please? I am happy the 1D bank stations are set appropriately and have aligned the LS/2D Flow Area boundary to the end of the XS and along the high ground separating the channel.

[Igloo_79]

Yes, I think you’re right – the WSE is below the terrain elevation at these locations(!) Thanks.

As it is a long river (8km), I am reluctant to modify the entire channel terrain as it will result in some loss of detail from the DTM, so I may just do this at locations where the channel appears ‘dry’ in Mapper.

[Igloo_79]

I would check the Lateral Structure editor. Might help to post screenshots of that, as well as the Geometry editor zooming in on 1D + 2D elements

[Igloo_79]

Hi – you should be able to find the answers in the official HEC-RAS manuals. However… regarding Q1, the DS reach lengths are the distance downstream to the next XS.

– Channel – distance between channel inverts, usually measured along the river centreline/reach line
– ROB/LOB – distance between L and R bank stations; you can measure these manually along the banklines (you need to turn them on in RAS Mapper), but if you have lots of XS then it can be automated by exporting the banklines as a shapefile and analysing in GIS.

Good luck!

[Igloo_79]

I use two methods for moving XS in the Geometry editor:

1. Edit > Move points – then move the start and end points of the cutline to your desired location (if there are lots of points it can be easier to delete them apart from the start and end points e.g. in the GIS Tools/XS Cutlines Table)

2. GIS Tools > XS Cutlines Table – paste in the new XY coordinates (you can get from the Measure Line tool]

Good luck!

[Igloo_79]

Yes, it is an arch culvert.

[Igloo_79]

Many thanks for the info. I have not used obstructions before. Presumably, the elevation should be set to an arbitrary level above which no water could flow? My feeling is that cropping the XS may be a ‘cleaner’ approach.

[Igloo_79]

That is useful to know @ldguethle

I have surveyed 1D cross sections (sample below) I want to connect to 2D flow areas. My intention is to draw the lateral structure (and 2D flow area) just outside the bank stations; thus would leave 10-15m of the XS overlapping with the 2D flow area. As there are >100 XS, this will be time consuming to crop each one – so I am wondering what effect this may have on the model in order to justify the extra model time? I would appreciate your thoughts.

[Igloo_79]

Thanks for your reply Cameron. This would mean that some of the 1D XS will overlap with the 2D Flow Area (and Lateral Structure). I’ve seen posts recommending avoiding such overlap, albeit without explaining the reasoning for doing so. (It will be time consuming to crop so many XS.) What is your view?

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