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Im doing a hydrodynamic simulation in a mangrove region using Telemac2d and currently im working on the multiple mesh density and bathymetry on BK.

Can anyone suggest or recommend some reference guidelines so that i could insert the friction coefficient value into BK at specific area of my study site that have mangrove trees(Rhizophora species) which might influence the strength and flow of the current dynamic in that region.

Im also wondering how could we determine the friction coefficient of the mangrove area both computationally and field base.

Thank you in advance for the help,

Don

Hi Don,

For your 1st question, are you looking for ways to vary the bottom friction in your domain or you just want to simulate the mangrove trees by increasing the bottom friction?

If you just want to simulate the trees, probably you can have a look at this post/discussion (and a few others) on how to implement the drag force in your domain. Though the discussions are mainly for implementing structure in the model, I think they should be quite similar to what you are trying to do.

As for your 2nd question:

to determine bottom friction computationally, there are a few roughness formula that you can choose from in Telemac. You then need to specify the appropriate coefficient for the model. Next you will need to validate your model using field data. To surmise, your coefficient values MUST be calibrated and validated using measurement data, as that's the only way to make sure the paramaters and values chosen for your model are "correct" and robust.

Hope this help

Anas

Hi Anas,
How to give the appropriate friction values(how come we can judge what could be the appropriate value, based on depth or any other factor)!!! You mean, we have to specify the zones and the respective value in the domain ( heterogeneous friction values over the domain).

Then how we can use the field data to refine the coefficient!!
you mean, this can be done automatically by telemac or we have to validate by trail and error.

Thanks,
Sathish

Hello Sathish,

1. For appropriate friction values, usually I will start with values suggested from the literature. The values are not based on the depth, but actually a representation of the seabed. There are ranges of acceptable values that you can start with, and I would suggest you do some study beforehand to understand how the bottom friction works.

2. So far the value of my bottom friction is applied for the whole domain (homogenous value). Nonetheless, if you know a way to implement it based on zones (heterogenous), please let me know!

Note: from my experience, each numerical models behaves differently (obviously).So if you found any suggested friction value that is relevant for your study, BUT using different software, I would suggest you give it a try, and then proceed with calibration and validation process.

3. Using the field data to refine the coefficient

you need to extract the outputs from your model (velocity, depth etc) and then compare them with the measurement data. so it's basically a trial and error

Anas

Hey Anas,
I did trail and error only that is why I wanted to know is it possible to automatically, to save the time as well as to improve the accuracy.

Heterogeneous Bottom Friction:

1) I did split my domain in to five zones ( You can use ArcGIS or AutoCAD). It should be a closed polyline.
2) I give different elevation ( basically Chezy's friciton coeeficient) values using BK or ArcGIS
3) Then Keep all the zones in their receptive place
4) Make it as a single polygon
5) Create a variable in the geometry file using BK - "BOTTOM FRICTION"
6) Use the single polygon for this variable.
7)Now you have a geo file with ur bahty (FOND) and friction coefficient (BOTTOM FRICTION)

This is how I did...

Can anyone suggest is there any other easy way to do this!!!!

Thanks,
Sathish

Hello Sathish:

You can create multiple polygons (or zones) under the same file (.i2s) in BlueKenue. Right-click your (.i2s) file in your Data Items, and select "Add Closed Line". To change the value of each polygon/zone, right-click the feature in the 2D view, click "Edit...".
I would try to avoid using multiple programs and hence avoid the import/export problem between ArcGIS/AutoCAD and BlueKenue.

Once the zones are defined, it just a matter of changing the values in each zones during the calibration process - unless of course you want to modify the zones.

For automation, you could use a combination of Python and Telemac python libraries. There are some examples in v7p0r1\examples\python27.

Cheers, Julien

Hi Julien,

Can you elaborate a little bit more on how to implement heterogenuous bottom friction in a domain?

As far as I know, if I create a new closed line and assign a value to it, the value will be used as the distance between the nodes on the newly created closed line when I put it under density tab in mesh generator

I have been trying to vary bottom friction values for different
zones with no success so far

Thanks

Hi Anas,

What you describe is one way of specifying node density when generating a mesh.

To define a spatially varying friction field you will start with an existing mesh and instead of mapping bathymetry to the nodes you will map your desired friction values. See Sections 1.6.8 and 2.4.2.1 in the Blue Kenue manual.

Cheers... Martin

Thanks Martin for your guide.

just to confirm your comments (and also to clarify Sections 1.6.8 and 2.4.2.1 in the Blue Kenue manual.)

1. Generate a mesh for my domain
2. create a closed line/polygon at the interested zone, and assign a value (for example if I would like to use Chezy 44 for the specific zone, would I give the line a value of botton friction = 0.005 or Chezy 44?)
3. map the mesh with the closed line/polygon
4. only then I map my domain mesh with the bathymetry.

so in the steering CAS file, when I choose law of bottom friction = Chezy, and FRICTION COEFFICIENT FOR THE BOTTOM = 60, my whole domain will have a bottom friction value of Chezy 60, except at the domain where the value would be 44 as assigned?

Anas

The values defined in external files should overwrite what you specify in case file. To confirm, you can append W (stands for friction coefficient) to VARIABLES FOR GRAPHIC PRINTOUTS instructing telemac to write friction coefficient to result file.