## Bottom drag coefficient

MOHID Water forum. Questions and discussion around MOHID Water
xavierbarbara@yahoo.com
MOHID Beginner
Posts: 6
Joined: Thu Jan 01, 1970 00:00

### Bottom drag coefficient

Hello!

I would like to know which equation is used as the bottom drag coefficient in the model I'm developing (2D model with curvilinear bathymetry).
Is it the one normally used for 2D with the Manning coefficient or the one with von Karman coefficient?

Bárbara

Posts: 349
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Tecnico - Lisbon - Portugal
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Hi Barbara,

It depends on the options you've selected in the InterfaceSedimentWater input data file. If you post the options in your file I can tell which model are you using.

Cheers,
Luis

xavierbarbara@yahoo.com
MOHID Beginner
Posts: 6
Joined: Thu Jan 01, 1970 00:00

### Interface sediment water

This is the only thing I have in the Interface SedimentWater

<begin_rugosity>
INITIALIZATION_METHOD : CONSTANT
DEFAULTVALUE : 0.0025
<end_rugosity>

Bárbara

Posts: 349
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Tecnico - Lisbon - Portugal
Contact:
Hi Barbara,

You've used the logarithmic law of the wall.

Best regards,
Luis

xavierbarbara@yahoo.com
MOHID Beginner
Posts: 6
Joined: Thu Jan 01, 1970 00:00

### rugosity

Thanks!

In this case (wall law), which are the typical rugosity values for estuaries?

In order to use the Manning one, what would I have to do?

Best Regards,

Bárbara

Posts: 349
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Tecnico - Lisbon - Portugal
Contact:
Hi Bárbara,

Instead of defining the rugosity like this:

<begin_rugosity>
[see ModuleFillMatrix for instructions]
<end_rugosity>

define the Manning coefficient like this:

<begin_manning>
[see ModuleFillMatrix for instructions]
<end_manning>

For typical values in estuaries you should look at literature, but we normal use the 0.0025m (2.5mm), the same as in the ocean. I would say the value ranges from 0.01 to 0.001m, and it dependent on calibration of the model.

Hope it helps,
Luis

mfernand@fing.edu.uy
MOHID User
Posts: 30
Joined: Thu Jan 01, 1970 00:00
Location: Montevideo, Uruguay
Hi,
I am working with a 2D model and I have been imposing the rugosity as an absolute value (zo), using the logaritmic law.
However, I wonder how can the model use a logaritmic law in a 2D configuration. A 2D model calculates the vertical integrated velocity, does it use this velocity to calculate the bottom stress?
As I don't really understand what the model is doing when I impose an absolute rugosity value in a 2D configuration, I am now using a Manning value. However, I have not found how is the Manning coeficient applied to calculate the bottom stress.
Regards,
Mariana

Posts: 349
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Tecnico - Lisbon - Portugal
Contact:
Hi Mariana,

You can use a Manning coefficient by defining the following block in the InterfaceSedimentWater input data file:

<begin_manning>
DEFAULTVALUE : 0.022
<end_manning>

you can also define a grid data file or a box file to setup different coefficients in different areas by using the normal http://www.mohid.com/wiki/index.php?title=Module_FillMatrix rules.

Hope this helps,
Luis

mfernand@fing.edu.uy
MOHID User
Posts: 30
Joined: Thu Jan 01, 1970 00:00
Location: Montevideo, Uruguay
Hi Luis,
I am alredy running the model using a Manning coefficient.
But what I would like to know is how is the Manning coefficient used for calculating the bottom stress.
I would imagine is something like:
T=rho.g.n^2.u^2/h^1/3
where, T is the bottom stress, n is the Manning coefficient, u is the horizontal velocity and h is the water depth.
Could you please confirm this expression?
Finally, is there any way I could impose a constant drag coefficient (Cf) for calculating the bottom stress. Something like: T=Cf.rho.u^2/2, where Cf is independent of the water depth.
Thank you very much for your help.
Kind regards,
Mariana