Dear EFDC Team,

I have some questions about EFDC .

1. What is “L” in the cell information ?

2. During calibration In hydrodynamic and sediment transport , I get the value of Bottom roughness , horizontal eddy viscosity and vertical eddy viscosity , horizontal and vertical momentum diffusivity molecular diffusivity .

I have read the theory and computation volume 1 ( Hydrodynamics and mass transport ) and volume 2 ( Sediment and contaminant Transport and Fate ) but I do not understand . I think the bottom roughness to compute the vertical shear stress on the vertical boundary conditions . “Horizontal eddy viscosity and vertical eddy viscosity” are coeffisien to solve the momentum equation in this case to compute the shear stress . “Horizontal momentum diffusivity and vertical molecular diffusivity” is used to solve the transport equation . In the theory it is said that “vertical eddy viscosity and diffusivity” are calculated using methods developed by Mellor and Yamada . So why in Hydrodynamics tab : turbulent diffusion option I have to include the value of eddy viscosity and diffusivity vertical? .

3. When I choose “active HMD with smagorinsky coeffisient” then if AHD > o EE will apply the background as well as smagorinsky HMD . Whether this will result in a fixed coefficient value smagorinsky to any content AHD ( AHD value for any > 0 ) ?

4. what is the function of " time advance filter " ?

5. What is the purpose of the “use constant " in " background or constant eddy viscosity and diffusivity " in " turbulent diffusion option” ?

6. How important it is to change the value of “maximum magnitude for diffusivity terms” ?

7. I experienced the value of " time step " obtained in the calibration process does not guarantee subsequent simulation runs smoothly . I used the " time step " calibration for several simulations by changing the downstream boundary conditions , it turns out there are some simulations that produce " floating divided by zero " though " time step < CFL delta " and " courant < 1 " . Whether this was due to changes in the downstream boundary conditions ?

Thanks to answer my question.

Sincerely,

DYAH ARI

I have some questions about EFDC .

1. What is "L" in the cell information ?

In EFDC "L" is used to represent (I, J) in a grid cell. Each cell is assigned a unique "L" variable.

2. During calibration In hydrodynamic and sediment transport , I get the value of Bottom roughness , horizontal eddy viscosity and vertical eddy viscosity , horizontal and vertical momentum diffusivity molecular diffusivity .

I have read the theory and computation volume 1 ( Hydrodynamics and mass transport ) and volume 2 ( Sediment and contaminant Transport and Fate ) but I do not understand . I think the bottom roughness to compute the vertical shear stress on the vertical boundary conditions . "Horizontal eddy viscosity and vertical eddy viscosity" are coeffisien to solve the momentum equation in this case to compute the shear stress . "Horizontal momentum diffusivity and vertical molecular diffusivity" is used to solve the transport equation . In the theory it is said that "vertical eddy viscosity and diffusivity" are calculated using methods developed by Mellor and Yamada . So why in Hydrodynamics tab : turbulent diffusion option I have to include the value of eddy viscosity and diffusivity vertical? .

# In EFDC model, subroutine CALAVB calculates vertical viscosity and diffusivity. There are three options available to calculate the vertical viscosity and diffusivity. The three methods are :

a) Galerpin

IF(ISTOPT(0) == 1)THEN

SFAV0= 0.392010

SFAV1= 7.760050

SFAV2=34.676440

SFAV3= 6.127200

SFAB0= 0.493928

SFAB1=34.676440

RIQMIN=-0.999/SFAB1

ENDIF

b) Kantha and Clayson (1994)

IF(ISTOPT(0) == 2)THEN

SFAV0= 0.392010

SFAV1= 8.679790

SFAV2=30.192000

SFAV3= 6.127200

SFAB0= 0.493928

SFAB1=30.192000

RIQMIN=-0.999/SFAB1

ENDIF

c) Kantha (2003)

IF(ISTOPT(0) == 3)THEN

SFAV0= 0.392010

SFAV1=14.509100

SFAV2=24.388300

SFAV3= 3.236400

SFAB0= 0.490025

SFAB1=24.388300

RIQMIN=-0.999/SFAB1

ENDIF

You can see all of these options in EFDC explorer and they are derived by using Modified Mellor's and Yamada approach.

Please take a look at source code in detail.

3. When I choose "active HMD with smagorinsky coeffisient" then if AHD > o EE will apply the background as well as smagorinsky HMD . Whether this will result in a fixed coefficient value smagorinsky to any content AHD ( AHD value for any > 0 ) ?

# Please look at source code subroutine "CALHDMF.f90" which shows the implementation of smagorinsky coefficient. Using this subroutine "AH" is calculated using SMAGORINSKY'S SUBGRID SCALE FORMULATION PLUS A CONSTANT AHO.

You can find that AH will change in each time step from the viewing options in EFDC Explorer (internal vars).

4. what is the function of " time advance filter " ?

# In most of the applications I have used "Average" option. You can download some of the sample projects from efdcexplorer.com and look what parameters are being used.

5. What is the purpose of the "use constant " in " background or constant eddy viscosity and diffusivity " in " turbulent diffusion option" ?

# EFDC uses that background value (AHO) also to compute the smagorinsky coefficient (See subroutine CALHDMF.f90")

Here is a code from CALHDMF showing AHD and AHO both being used to calculate AH

IF(ahd > 0.0)THEN

241 ! *** CALCULATE SMAGORINSKY HORIZONTAL VISCOSITY

242 DO k=1,kc

243 DO l=lf,ll

244 tmpval=ahd*dxp(l)*dyp(l)

245 dsqr=dxu1(l,k)*dxu1(l,k)+dyv1(l,k)*dyv1(l,k)+sxy(l,k)*sxy(l,k)/2.

246 ah(l,k)=aho+tmpval*sqrt(dsqr)

247 ENDDO

248 ENDDO

249 ELSEIF(n < 10 .OR. iswave == 2 .OR. iswave==4)THEN

250 ! *** ONLY NEED TO ASSIGN INITIALLY

251 DO k=1,kc

252 DO l=lf,ll

253 ah(l,k)=aho

254 ENDDO

255 ENDDO

256 ENDIF

257

6. How important it is to change the value of "maximum magnitude for diffusivity terms" ?

# If you are not sure about the parameters in this section, I suggest not to change.

7. I experienced the value of " time step " obtained in the calibration process does not guarantee subsequent simulation runs smoothly . I used the " time step " calibration for several simulations by changing the downstream boundary conditions , it turns out there are some simulations that produce " floating divided by zero " though " time step < CFL delta " and " courant < 1 " . Whether this was due to changes in the downstream boundary conditions ?

# EFDC is little buggy sometimes. You just need to make sure you have appropriate boundary conditions. Sometimes it is hard to tell why the model crashed.

Well, the answer given may not be the exact you are hoping for. But I hope this will give your more ideas how turbulence , diffusivity and other parameters such as smagorinsky coefficient are handled in EFDC model.

I hope to write one blog post to explain more in detail soon.

**I suggest asking only one question in one post.**

Best,

Janesh Devkota

Thanks EFDC Team for the explanation, this is enlightening for me.