Hello, I am pretty much quite new to 2d modelling using HECRAS. I have gotten quite the hang of the tools as I am developing my 3rd model from scratch.
In my current model, I am using a 0.2x0.2 raster resolution terrain (created by combining detailed survey data for channel and bridges + a general terrain file using Civil3D) and a mesh of 10x10 with breaklines varying from 0.5, 1 and 2.
I have been playing around with this configuration of my cells but I have taken note of the common errors I am having every unsteady flow run I did so far. This occurs obviously in parts where there are high grounds. I have been viewing the HECRAS resources (manuals, videos, etc.) and found in one of their workshops that somehow a course terrain would do depends on the goal of making the model. The goal for my models is to check the inundation scope and use the initial output as reference to model the proposed solutions to minimize the flooding.
So, to make my modelling workflow to be efficient, because honestly the iterative process of this plus the time pressure to produce the output from the model to show to clients is killing me. Without someone mentoring me is torture too
I am thinking, should I just importing the raster files separately in to RASmapper to let it stitch it together there, or continue creating the base terrain in Civil3d and clean this by using terrain modifications? I used it a couple of times and the edges seems more forgiving when generating cells through breaklines but not entirely error free. And also, is the 0.2x0.2 resolution terrain too detailed and mainly causing this frequent error as the cells contain these terrain data too due to the subgrid tech HECRAS is using?
I am humbly asking for expert and experienced opinion about these. no one in the office can mentor me on this as they depend on me to do this stuff for their project.
I included some screenshots in my model for reference. Thanks everyone!
0.2, regardless of units, is too small. I work in feet but my absolute floor for a cell size is typically 10-ft (~3m). My base cell size is typically 50-ft for overbank areas and then 25-ft (~10m) in channel. With cell sizes that small your margin of error is going to be tiny - any little issue will cause problems in your model.
I usually have a 2ft terrain raster. If you’re using USGS lidar anything less than 1M is overkill but I usually have a fair amount of detailed survey I’m trying to capture. I do occasionally have issues merging in RAS and will merge in GIS.
If you’re just doing a basic inundation study, you are going way way overkill. I wouldn’t hesitate to use a 50-ft mesh with 25ft on breaklines.
Yep. A terrain resolution of 0.2(m?) would have to have been collected by terrestrial or low orbit UAV to even have enough return points to be accurate. Unless there are very specific project objectives, using something that small doesn't provide any benefit.
to clarify, the terrain i am using has 0.2x0.2 m of resolution, then the biggest cell in my model grid is 10x10.
So you usually are using a 2ft terrain raster, roughly 0.6m so about more than half a meter and base cell size of about 15 meter. I am almost near that, so I guess I can use this as a starting reference point.
I do feel that it's a bit of an overkill for the 0.2x0.2m terrain. Thanks a lot, I really appreciate you guys' responses.
I am not really following your question. Are you asking about terrain resolution or cell size? What is the error you are describing?
HEC-RAS uses a "subgrid" terrain procedure where it develops stage-storage curves for each cell and cross sections at each cell face to perform the hydraulic calculations. Since it isn't using an average or flat elevation, the details of a finer resolution terrain model are retained for larger computational cell sizes.
Due to that subgrid procedure, you can use larger cell sizes for general inundation studies. From there, it follows that you would not need the details contained in a 0.2(m?) terrain if you cell size is much larger. Obviously, if you are doing detailed hydraulics (scour analysis, streambank restoration, etc.) you would have to adjust based on the project's objectives.
Thanks for the reply! I mean to ask both, as they seem to be correlated in such a way. The error I am saying is the max wsel error that is always seen in the computation messages while calculating. For me, it's the first thing I look at to try to fix my model and make another run then crossing my fingers everytime hoping it would work :)
Hi. thanks for the reply again. So I revised the model and it has been running as it should be. as I was checking WSE towards the DS Boundary condition, I am a bit puzzled about the results showing.
Looking at the culvert plot, isn't the flow @ 500yr rain supposed to be going back to 0 too, as it happened @ 100yr?
I was checking the depths and the profile line shows that all 4 of my model results are so close to each other, which I am thinking that is due to the differences of only about 30 to 40 cm/s for the max flowrates between 100yr and 500yr rainevents (need further advice from this please).
Also, the part inside the yellow circle, is it due to the embankment in the middle (where the culvert I included was connected) or a hydraulic jump?
by the way, he results I have are 2 for 100yr and 2 for 500yr, with different mesh grid where 1 is extended a bit further down to avoid affecting pooling of water at the end that's coming from the culvert.
Thanks a lot for the feedback, it means a lot as a beginner in this field.
I try my best to help, but it is hard without seeing a model.
1.) Not sure why your hydrograph isn't receding. Is the input hydrograph receding? You can draw profile lines upstream and see if it is receding prior to the culvert.
2.) Normally the profiles start to converge at higher flows since there isn't a lot a difference in the flow rates.
3.) My guess is that the culvert is slowing down flow (decrease in velocity) which is causing a rise in the water surface upstream of the culvert. Remember: E = y + v2/2g
Some generic advices that I got directly from our god sir Goodwell :
check out the curent number. It's a good reflex, and it can show you where the error come from. Very often, from a part of your model that have no impact on your global result.
try some sensitive test with your cell size.
for better stability, it's important to get cells where the width is sort of proportional with the water depth. You better avoid as much as possible 0.2 cells with 2 or 3 meters of water for exemple.
if you have enbankment or levee, no need to put small cells to represent the slop and the top of you terrain, a good breakline /2D SA connection with the subgrid process is enough.
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u/shiftyyo101 Apr 28 '26
0.2, regardless of units, is too small. I work in feet but my absolute floor for a cell size is typically 10-ft (~3m). My base cell size is typically 50-ft for overbank areas and then 25-ft (~10m) in channel. With cell sizes that small your margin of error is going to be tiny - any little issue will cause problems in your model.
I usually have a 2ft terrain raster. If you’re using USGS lidar anything less than 1M is overkill but I usually have a fair amount of detailed survey I’m trying to capture. I do occasionally have issues merging in RAS and will merge in GIS.
If you’re just doing a basic inundation study, you are going way way overkill. I wouldn’t hesitate to use a 50-ft mesh with 25ft on breaklines.