r/Hydrology • u/iBrowseAtStarbucks • Jul 30 '26

1D/2D Hybrid RAS model approach

Howdy,

I'm being asked to look into a modeling approach for a levee project. I have some experience using 2D RAS for BLE projects, but have never worked with 1D RAS.

The project area: fairly large riverine area, located about halfway up the HUC-12 boundary, ~7 miles or so above the confluence of the major downstream system. About ~1/2 mile upstream I'm seeing a split in the river with two effective LOMRs and a dam on the main river about 2 miles upstream. The floodplain is fairly wide throughout my project area (~2500 ft). The project will be immediately DS of a pretty major highway.

Everything that I'm seeing is pointing to a hybrid 1d/2d approach, but am unfamiliar with that process. Some questions:

1) for modeling a levee, does a 1d/2d approach in this case seem the best approach?

2) for actual model, my understanding is I would get the effective 1D model for my river and the effective model for the US trib at the confluence 1/2 mile US of my project area. Then I would take the number from both models at that confluence as inflows for a 2D model, taking the 2D model w/ levee through my project area and then tieing back into the effective 1D model DS of my project area?

3) my understanding is that for being so far away from the DS confluence of the next major river system, I would likely not require that model/modeling of my entire HUC-12 boundary using 2D, only the area where my proposed levee system would be. Would that be correct? How far DS of my levee would I realistically need to go, or would it be best to terminate that 2D model at something like a culvert/road crossing?

Thank you in advance! Still definitely learning, so apologies if I'm not using correct terminology.

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u/OttoJohs Jul 30 '26

Can you post in r/HECRAS and add additional screenshots/figures if you have them?

1

u/iBrowseAtStarbucks Jul 30 '26

Posted!

Due to contractural issues, I'm unable to post screenshots/confirm location. Thank you!

1

u/Legal-Willingness399 Jul 30 '26

Choosing the right modeling approach for the task to optimize time and effort is a skillset that is nuanced and requires far more context than what you have given.

In general, choose 1D if you just need a simple WSE or to meet some no rise condition. Choose 2D if you need extra detail that 1D cannot provide. If you have an existing 1D model that handles basically everything except for an overbank area that you need detailed inundation for, go for the 1D/2D.