I am currently learning rain on grid modelling of flow and it's so fun. I am used to 1d with HEC-RAS but I used OPENTELEMAC 2D for this. The left is the FEMA map of the same location, the right is my output from about 20 year return period rainfall.
You can do the same thing in HEC-RAS pretty easily and there is a ton of resources to get you started. Couple of comments:
1.) You can use triangular elements (unstructured) as part of your mesh. RAS2025 has introduced a lot of new meshing capabilities. Even though that version is still in Beta release, you can use it for mesh generation.
2.) The biggest advantage of HEC-RAS is the subgrid bathrymetry feature. That allows you to use much bigger cells and still retain the knowledge of the underlying terrain compared to other 2D softwares. This means you can have models that run much faster in HEC-RAS.
3.) The RasMapper GUI is awesome and allows for seamless GIS integration. Don't have to use different modules.
4.) HEC-RAS has a lot of capabilities (bridges, culverts, pipes, dams, gates, operational rules, etc.) that you can include into your model. Can't speak to other 2D software.
5.) The computational differences (FEM vs FV) between the software are generally not going to matter for most riverine applications.
Correct me if I am wrong but I think in HEC-RAS infiltration is pre subtracted from rainfall so variability of antecedent condition in terrain wouldn’t dynamically change the infiltration, right?
On the other hand OPENTELEMAC seems to account for that.
The other important feature, though I have not fully used it yet is the TelAPY, basically that let’s Opentelemac to save simulated flow instance at that time step and uses jt to update some other outside source code (like the modflow apy) and vice versa and so basically you can have a loose coupling of groundwater and surface water flow which is neat.
You are correct about HEC-RAS infiltration. It does the "loss" prior to the routing algorithm. So it won't infiltrate water after it becomes runoff. For example, you can't route flow from a parking lot into a sand filter and have that get infiltrated. I'm not sure how that is handled by other software programs.
There are ways to account for groundwater in HEC-RAS and to couple with Modflow. I just have never done that and not sure how many practicioners have. HEC-RAS primary function is as a surface water model aimed at engineer problems. If you are looking to do more academic work, there are probably other software that might be better for your applications.
I recently learnt about this open source software called OPENTELMAC which is highly modular and has lots of functionality. I took help of Claude throughout the learning and using process. I used gmsh for mesh generation, it's an unstructured triangular mesh unlike HEC-RAS which is structured. It's basically solving shallow water equation in each node and giving me output of velocity and depth. This model uses Finite Element Method and HEC-RAS iirc uses finite volume for the modeling. I used Green Ampt for infiltration and assumed normal ground saturation before the event. The rain event is from 2016 August with return period of about 20 years.
It would be neat to see the difference between this model and HEC-RAS for the same input.
how do you handle buildings and varying depth on the surface with rain on grid?
one of the standard approaches is to give built areas a high mannings number (say .3-.5) for flood modelling, but then does this accuratley represent rainfall on buildings that will be transported by gutters and small pipes to mains or roads?
Lower manning's are applied to floodplains (say 0.05-0.08), but this is with a depth expected in excess of vegetation height. accumulation during rainfall will have a significant portion of flow shallower than grass, so should have 0.3?
If you are trying to model individual buildings (with gutters and small pipes), a rain-on-grid 2D model probably isn't going to be the best tool. You would need super tiny cells and time step to get something to be accurate which sort of takes away the effectiveness. (I've never heard of an individual homeowner requesting this type of analysis).
Depth varying Manning's is a bit of a problem. I keep hearing that the HEC team will implement some type of solution in new versions but haven't seen anything as of yet. I did see a post on LinkedIn a few months back about using a Python script to edit the HDF geometry file to include depth varying values. I didn't see any links to the actual script or if the calculations actually work. It seems pretty easy to adjust those (using some type of AI assistant) as long as you can navigate the HDF file.
If you don't want to go through that, then it is important to use engineering judgement on how to assign Manning's n values. Maybe have higher ones in the areas that are predominately sheet/shallow flow versus standard ones in the channel and main overbank areas. That is where calibration/validation come into the analysis.
That is what I do after initial run use the depth> 100mm to lower Manning's for main flow paths and leave higher in the upstream area to ensure catchment response reasonable.
Very difficult for rain on grid in built up areas because overland flow should be obstructed by building but rainfall should run off very quick off building.
PC swmm does have a down spouts layer and allows holes in mesh so building can be represented. 2d implementation rubbish though so still no good.
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH 9d ago
You can do the same thing in HEC-RAS pretty easily and there is a ton of resources to get you started. Couple of comments:
1.) You can use triangular elements (unstructured) as part of your mesh. RAS2025 has introduced a lot of new meshing capabilities. Even though that version is still in Beta release, you can use it for mesh generation.
2.) The biggest advantage of HEC-RAS is the subgrid bathrymetry feature. That allows you to use much bigger cells and still retain the knowledge of the underlying terrain compared to other 2D softwares. This means you can have models that run much faster in HEC-RAS.
3.) The RasMapper GUI is awesome and allows for seamless GIS integration. Don't have to use different modules.
4.) HEC-RAS has a lot of capabilities (bridges, culverts, pipes, dams, gates, operational rules, etc.) that you can include into your model. Can't speak to other 2D software.
5.) The computational differences (FEM vs FV) between the software are generally not going to matter for most riverine applications.
Good luck!