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 ?

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.