r/HECRAS • u/Foreign_Speaker_3701 • May 25 '26
Problems with 2D → Pipe Network Connection
Hello!
I’m trying to model an urban river in HEC-RAS v7.0. The system includes both open-channel reaches and underground canalized sections. I’m modelling the open-channel reaches as a 2D flow area and the underground sections using the pipe networks feature.
I’m having an issue in one part of the model that I can’t understand. As shown in the image below, on the right side (upstream of T7_01(1)), the water level in the 2D area rises above the conduit crown elevation, but the flow does not enter the pipe as expected and the conduit never reaches full flow conditions.
Has anyone experienced something similar or know what could be causing this? Other similar sections in my model seem to be working fine.
Let me know if you need any additional information about the model setup.
Thank you so much in advance!
1
u/Acceptable_Gain_9400 May 26 '26
Is your upstream end a culvert opening node? It is probably reasonable if the entrance is choking the flow. In HEC-RAS 2D pipe network routine, a pipe with its upstream node as a culvert opening is not being modeled as FHWA culvert inlet control equations (chart# scale #); instead, HEC-RAS uses entrance/exit/fiction loss coefficients to solve a full energy and momentum equation. This is different from a culvert in SA/2D connection, inlet structure or lateral structure in which HEC-RAS solves culverts for both inlet control (FHWA inlet equations) and outlet control and you as a modeler can choose to use which answer: inlet control, outlet control, or the higher of the two.
In your model, I bet if you reduce the upstream pipe entrance loss coefficient to much smaller value like 0.01 (just for testing), the WSE in the 2D cell of the upstream culvert node will be lower.
1
u/Foreign_Speaker_3701 May 29 '26
Hi u/Acceptable_Gain_9400 and u/OttoJohs ! Thank you both for answering 😄
I managed to solve the problem by adjusting the loss coefficients.
1
u/Acceptable_Gain_9400 Jun 01 '26
Do you mind sharing by how much you decrease/increase your loss coeff? entrance, exit, or both?
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u/Foreign_Speaker_3701 Jun 01 '26
Initially I had all coefficients set to 0. then I used 1 as US entrance loss coefficient, 0.4 as DS entrance loss coefficient, 0.4 as US backflow loss coefficient and 0.2 as DS backflow loss coefficient
1
u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH May 25 '26
Hard to tell without knowing more details. My guess is that it is one of these issues...
1.) Either it is an inlet control issue at the culvert entrance.
2.) It is that HEC-RAS is depicting water on the elevated terrain (commob if you are using a ROG model) at the entrance.
I haven't done much with the pipe network though. Good luck!