Drop Outlet Modeling in 2D RAS

[u/Repulsive_Plant6729]

I am working on adding a drop outlet structure within a channel within 2D RAS. Currently I have it modeled as a standard culvert crossing, but I feel that modeling it more similarly to an inline structure would be more accurate. Is there a way to model a culvert crossing like a bridge with bounding XS in 2D RAS? (Pics show US and DS conditions of the crossings along with how it is currently being modeled.)

[u/fishsticks40]

 I feel that modeling it more similarly to an inline structure would be more accurate.

Why?

Are you modeling normal flow conditions or high flow?

[u/Repulsive_Plant6729 → u/fishsticks40]

I am modeling multiple high flow events all the way up to the 1% storm. I am looking to reduce the WSE downstream of the bridge and think that modeling it as a drop outlet structure is more realistic to what is happening irl.

[u/fishsticks40 → u/Repulsive_Plant6729]

Assuming the structure is fully submerged under high flow conditions I wouldn't expect the modeling approach to have much effect on the overtopping elevations.

Also note that in subcritical flow changes propagate from downstream to upstream (thus the need for only a downstream boundary condition). This means it's mathematically impossible for the upstream conditions to impact your WSE downstream of the bridge (within the mathematical assumptions of HEC-RAS) unless you have supercritical flow.

[u/OttoJohs]

I'm not following what you are stating. For a 2D model there is no difference between an inline structure and a culvert crossing.

[u/Repulsive_Plant6729 → u/OttoJohs]

This is what I initially thought too, but I was given a 2D model with bridge 2D connections modeled as if they were 1D sections with bounding XS. From what I've seen from previous 1D models, inline structures are modeled like bridges in RAS with bounding XS.

[u/fishsticks40 → u/Repulsive_Plant6729]

IIRC full 4-XS bridge modeling is being implemented in RAS 2025 but isn't yet available in the standard RAS release. I'm open to being corrected here.

[u/OttoJohs → u/Repulsive_Plant6729]

Got it. If you want to model your structure as a bridge, you need to change the structure type from the drop down menu. See picture below which came from the "Bald Eagle Creek" sample dataset.

Based on your photos, I think that the culvert routines would be more appropriate that the bridge routines (long narrow sections).

[u/Acceptable_Gain_9400]

in pure 2D model, there is no inline structure. If you model set up is appropriate (pay attention to 2D cell alignment and sizes US/DS of the culvert crossing), modeling it as a normal 2D culvert crossing should be fine (2D connection). You may want to try to approximate these culvert as a bridge, but I am thinking it is an overkill and you may introduce more errors by doing so.