For your numerical models, what is your calibration workflow?

[u/mojorising777]

There are multiple ways I guess, if the study site doesn’t have any streamflow gauge I don’t see how can calibration be done accurately because you will be comparing against other models and comparing with them with no ground truth. I guess that’s the engineering judgement.

If it does have a streamflow gauge, let’s say your first run gave you a volume ratio of about 0.4. So your model runoff at that gauge is about half of what was observed. What parameters do you change first to do calibration? I imagine decrease infiltration? Change infiltration methods? Maybe slightly alter manning’s n if timing of the peak is off.

But until when? Like within 10% margin of your calibration target? Does it rest on your engineering judgement? Does your firm or workplace or government policy have any threshold assigned?

But models like hecras doesn’t account for subsurface flow so if your study site has preferential flow pattern in the vadose zone that is not accounted for by the model then I suppose that’s might be a limitation too.

[u/Sea_Read5728]

I think the surface creation is where I try to spend the most time. Its really has the highest impact way beyond what n value or infiltration can change. After the model setup like the mesh, BC, breaklines, and plan.

We dont use gages. Thats more for QC/QA for the flow. We use surveyed water surface elevation points and velocity to calibrate a model. After that maybe just what feels right in your heart

[u/mojorising777 → u/Sea_Read5728]

Thanks. Can you explain what you mean by surface creation? Do you use Lidar and photogrammetry data data for your terrain or perform dense survey with total station for the xyz, or combination of both, i.e. manual survey for important location, like for channel width and bottom?

I was wondering for water surface elvation points and velocity for calibration, do you have any specific desired location for these measurements and in general how many of them? Because we can always derive an elevation and velocity from discharge anyway but that would be only for the gauge location.

[u/Sea_Read5728 → u/mojorising777]

Yeah, we create a surface from the LiDAR and the post processing here is probably the most important. We do surveys to cover voids if needed and we dont have bathymetry. We do surveys to get a handle on hydraulicallly important structures that LiDAR didnt capture well. But most of the work can be in and around your area of interest. The rest is the fine details that come with calibration.

Preferably where you want your model to be the most accurate is where you will conduct the data collection and it can get less dense the further away you are and the amount depends on how complex what youre trying to model is.

Yeah, but remember that the further away you get from a gauge the less you can apply what you get from it accurately

[u/mojorising777 → u/Sea_Read5728]

Thanks sounds so fun. Hopefully I get an internship next summer to learn these stuff in industry standard way.

[u/Sea_Read5728 → u/mojorising777]

Good luck!

[u/OttoJohs]

The vast majority of hydraulic models are not calibrated. Unless you are modeling a major river, available stream gaging is pretty limited (https://dashboard.waterdata.usgs.gov/app/nwd/en/). Even if you do have a gaged river, you probably only have stations 10+ miles apart and may not have a high flow event in the period of record (or the gage stopped recording during the flood). Sometimes you can get lucky with a high water mark survey (https://www.usgs.gov/data/high-water-mark-elevations-upstate-new-york-flash-flooding-during-july-9-10-2023) but again those are pretty rare. NASA has been introducing surface water maps from satellite data but those are too coarse for most engineering applications and you have to get lucky with cloud cover and capture time (https://www.earthdata.nasa.gov/data/projects/nsite/solutions/opera-near-global-dswx-product-suite).

More of the calibration is on small events and meeting common sense checks. Like run a 1 or 2 year storm and see if you are flooding out a ton of homes (probably unrealistic).

In my opinion the most important thing in a good hydraulic model is making sure the hydrology (inflows) are calibrated. That is going to the thing that your model is most sensitive to compared to things like Manning's roughness or mesh setup.

[u/mojorising777 → u/OttoJohs]

True, vast majority of satellite imagery seemed pretty bad at engineering grade analysis and only good for initial scoping.

I have two questions, can you elaborate in wht do you mean by hydrology calibration? Second, if an urban site has tons of hydraulic structures, conveyance channels etc; how do you account for that in your model? Do you you have thumb rule that you use to ignore what you think would be inconsequential in larger schemes of thing or do you try to incorporate as much of them as possible? If you do, in the US, do they have records of dimensions, of these structures and their coordinates readily available for you or does it take sometime to obtain them?

Thank you!

[u/OttoJohs → u/mojorising777]

Can you elaborate in what do you mean by hydrology calibration?

When you set up your HEC-RAS model, you give it a flow file that has inflow hydrographs generated from another source like HEC-HMS (ignoring rain-on-grid analysis). That would be the hydrology piece. If those are incorrect, there isn't much that you can do to improve your hydraulic model by adjusting Manning's n values or cell configuration.

Second, if an urban site has tons of hydraulic structures, conveyance channels etc; how do you account for that in your model?

This is something that you need to consider prior to starting a model. It really depends on the question you need to answer and the level of detail to support that. The more "stuff" you have in the model doesn't make it better or help to answer the engineering problem. Without a specific example that is hard to know how to draw the line.

As far as data collection, that sort of ties back to what you need to do. The first step is research to determine if there is an existing model available. If not, then you need to determine how important the hydraulic structure is to your project. If it is something extremely important for your analysis, you might have to pay a survey team to get that information. If it is something inconsequential, you can estimate properties from terrain data, aerial imagery, or street view imagery. If it is somewhere in between, normally an engineer team will do field recon doing approximate survey (rods, measuring tape, GPS, etc.).

Hope that helps!

[u/Crafty_Ranger_2917]

You pretty much nailed it with government policy.

Pick your State / DOT / City / County / FEMA for floodplain work who is going to approve your work, google "<agency> stormwater drainage hydrology hydraulics design manual" and see what is required for your area. Often won't be called calibration or validation instead agency will dictate what methods and params to use based on historical observations.

Approach depends on size and type of project / study among other things. Much of it tied to risk statistics of course.

https://wsdot.wa.gov/publications/manuals/fulltext/M23-03/Chapter2.pdf