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?
Thanks for your help.
Bárbara
Bottom drag coefficient

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Interface sediment water
This is the only thing I have in the Interface SedimentWater
<begin_rugosity>
INITIALIZATION_METHOD : CONSTANT
DEFAULTVALUE : 0.0025
<end_rugosity>
Thanks for your help,
Bárbara
<begin_rugosity>
INITIALIZATION_METHOD : CONSTANT
DEFAULTVALUE : 0.0025
<end_rugosity>
Thanks for your help,
Bárbara

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 Posts: 349
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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?
Thank you for your attention
Best Regards,
Bárbara
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?
Thank you for your attention
Best Regards,
Bárbara

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Hi Bárbara,
Instead of defining the rugosity like this:
define the Manning coefficient like this:
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
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

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 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.
Could you please help me answer these questions?
Thank you in advance for your help.
Regards,
Mariana
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.
Could you please help me answer these questions?
Thank you in advance for your help.
Regards,
Mariana

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

 MOHID User
 Posts: 30
 Joined: Thu Jan 01, 1970 00:00
 Location: Montevideo, Uruguay
Hi Luis,
Thank you for your reply.
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
Thank you for your reply.
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
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