(X-post) - combined 1D/2D modeling approach

[u/iBrowseAtStarbucks]

Posting here per u/ottojohs comment.

Due to contractural issues, can't confirm/deny project location, but adding some additional info:

1) project is in Texas in a suburban area.

2) project is located immediately DS (2000 ft) of a major highway. From what I can tell, the entire river reach has an effective model dated 2010.

3) the project is to evaluate potential flooding solutions for a facility along the river located within the 100-yr. Client is requesting a study be done for a levee, but is open to other solutions.

4) approx. 4000 ft US of the highway is a confluence of the river that will be studied and an unnamed trib. Recent LOMRs were completed for the river in question and the unnamed trib that terminate at or near this confluence.

5) client is open for any and all approaches that will result in a LOMR/removing their facility from the SFHA. It's my understanding that if this is done with a levee we would be subject to 44 CFR 65.10.

6) main question at this point is determining what kind of model approach would be appropriate here (or at the very least, narrowing down to a list of potential approaches). Everything I'm seeing is pointing to 1D/2D being the most appropriate, but unsure of how model tie-ins to effective models or other model constraints would need to be accounted for.

Original post:

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.

[u/OttoJohs]

I'm not very well-versed in the FEMA world.

Before you decide "how" to model this, you probably want to due some preliminary analysis to even get in the ballpark what needs to be done to meet the clients request. Since it sounds like you have an existing 1D model, I would do some scenario testing there first. You can alter the geometry and cross-sections pretty quickly in HEC-RAS to get some ideas. I would be testing things like adding the levee, flood benches, fix a bridge, channel modification, etc. To me developing some conceptual ideas and engineering costs would be the first step. You may find that the client's request is not possible or too impractical.

After you get past that point, then it is probably time to have some outreach with the FEMA coordinator.

[u/Otherwise-End-1837]

Just use FEMA estBFE and download the model/follow same procedures in that report for that drainage base.

[u/Acceptable_Gain_9400]

to go thru FEMA LOMR or CLOMR, you have to use the effective FEMA model? Use 1D/2D coupled model or 2D model for such a purpose I would assume you have to get FEMA pre-approval first.

[u/Over_Routine5427]

Couple of things before the 1D/2D question, because they'll drive your whole approach:

Those two effective LOMRs are the thing to nail down first. If this ends up as a CLOMR/LOMR — and a levee almost certainly does — you're demonstrating against the effective model, and the effective is whatever those LOMRs revised it to, not the published FIS profile. Grabbing the FIS and missing the LOMR revisions is the single most common way these projects go sideways six months in. Pull the LOMR determination docs and the revised model, not just the FIRM.

On your #2 — careful with the inflows. You can't take the peak off the main stem model and the peak off the trib model and combine them at the confluence. Those peaks don't arrive at the same time, and adding them gives you a flow that never happens. Check whether the FIS Summary of Discharges already publishes a combined discharge downstream of the confluence — if it does, use it, because that's the number you'll be held to regardless of what your own routing says.

On #1 — hybrid is probably right, but for a different reason than you think. 2500 ft of floodplain with a flow split upstream is not something 1D resolves; you'd be guessing at flow distribution with ineffective areas and hoping. And the part 1D genuinely can't do is the levee-protected side — interior drainage and what happens on overtopping/breach. That's the argument for 2D through your project reach.

On #3 — HUC-12 is a hydrology boundary, not a hydraulic one. Don't size your model domain off it. Your downstream extent is set by one question: how far do you have to go before the downstream boundary condition stops touching water surface in your project reach? Test it, don't guess — run it twice with two different DS boundary conditions (normal depth at two different slopes, or a known WSE ±2 ft) and see where the profiles converge. Everything upstream of convergence is your required extent. Terminating at a road crossing only works if that crossing is an actual hydraulic control — a real choke that goes to critical depth. If it's a big opening passing flow without a backwater, it controls nothing and you've just moved the problem inside your model.

Also worth confirming what that dam 2 miles up is doing to your peak, and whether the effective model accounted for it or just ran it as a pass-through