# Grid resolution vs deep basin modeling - Lake - Delft3D

## intro story Lake

## LakeLakes and reservoirs are important resources for communities around the globe. They provide food, drinking and irrigation water and can also offer spots for recreation. Under certain conditions lakes can also represent a threat for local communities, such as flooding or limnic eruptions. The ecological state of shallow lakes may be hampered by suspended sediments due to wind driven currents and waves. In deep lakes, stratification can cause undesirable anoxic conditions in the deeper areas. Understanding these systems is crucial to enable a sound management of these water bodies. Delft3D is the state-of-the-art modelling framework for hydrodynamics, water quality, ecology, waves and morphology. Here, in this space, we share knowledge & experiences, and discuss issues related to modelling of lakes.
| |

## Message Boards

### Grid resolution vs deep basin modeling

##### Marina Amadori, modified 2 Years ago.

#### Grid resolution vs deep basin modeling

Youngling Posts: 2 Join Date: 8/7/18 Recent PostsHi,

I am working on stationary wind-induced velocity profile (Coriolis force neglected) in un-stratified box domains, constant turbulence model and no thermal fluxes. I am comparing the numerical results from boxes with different depths with the theoretical solution for the steady state (Heaps 1984).

I verified that equilibrium is achieved (I am performing 60 days simulations) and I found good agreement for shallow boxes (H = 10 m, 30 m, 50 m, see Fig.v_res100m.png in the attachments). A mismatch between numerical and theoretical solution arises when I simulate deep boxes (H = 300 m, H = 500 m, always in Fig.v_res100m.png).

As a first attempt I thought it was a time step problem and I reduced it to meet the conditions in Table 10.1 (FLOW manual) but I didn’t get any improvement.Then I tried changing the horizontal grid resolution and I got the results in Fig.v_allres.png (in the attachments), where I found that finer horizontal resolutions (DX = 20 m, 50 m, 100 m, square cells) lead to worse results. I got the best match with DX = 2000 m but still I am not there. What if I choose DX = 5000 m?

Now, my questions are:

1. Why are coarser resolutions giving better result?

2. If the answer is the "ADI accuracy" in Table 10.1, why is reducing time step not providing any change in the solution?

3. Is there a clear/formulated criterion for choosing the proper grid size according to the depth of the basin?

4. How is it this criterion related to the choice of the background vertical eddy viscosity (I am modeling with constant turbulence model, nuz = 0.0373 m2/s)?

Thank you!

Marina

### Attachments:

##### Richard Measures, modified 2 Years ago.

#### RE: Grid resolution vs deep basin modeling

Jedi Knight Posts: 178 Join Date: 3/23/11 Recent PostsHi Marina,

Interesting question - Well explained and I really like your graphs.

Would you be able to give some more information about your vertical resolution e.g. how many layers do you have? Do you have the same number of layers when you increase the depth or do you have more vertical layers for the deeper models so the layer thickness is the same?

I don't have a lot of experience with constant vertical eddy viscosity but from my experience calibrating constant horizontal eddy viscosity I expect it could be quite sensitive to vertical resolution (the selection of appropriate horizontal eddy viscosity is sensitive to horizontal resolution). I expect that thinner vertical layers would require a smaller eddy viscosity to get the same result?

That doesn't help explain how horizontal resolution is influencing the vertical profile though! Have you checked to see if there are boundary effects propagating into your model horizontally? These could be affected by horizontal resolution and would be more significant for deeper domains. Maybe your horizontal eddy coefficient is too large for the models where you have a small cell size (I find the default horizontal eddy viscosity value is only suitable for very large cell sizes). As you are focused on vertical profiles I would try setting the horizontal eddy viscosity very small (but non-zero as that can cause weird instabilities) e.g. 0.001. Also you could try increasing the spatial size of the domain to see if that has any effect (i.e. reducing any horizontal boundary effect).

Good luck, I look forward to finding out what the problem was when you do solve it!

Cheers,

Richard

##### john smith, modified 1 Year ago.

#### RE: Grid resolution vs deep basin modeling

Youngling Posts: 1 Join Date: 10/2/19 Recent Posts<p>Hi, </p>

<p>I am working on stationary wind-induced velocity profile (Coriolis

force neglected) in un-stratified box domains, constant turbulence

model and no thermal fluxes. I am comparing the numerical

results from boxes with different depths with the theoretical solution

for the steady state (Heaps 1984). </p>

<p>I verified that equilibrium is achieved (I am performing 60 days

simulations) and I found good agreement for shallow boxes (H = 10 m,

30 m, 50 m, see Fig.v_res100m.png in the attachments). A mismatch

between numerical and theoretical solution arises when I simulate deep

boxes (H = 300 m, H = 500 m, always in Fig.v_res100m.png). </p>

<p>As a first attempt I thought it was a time step problem and I reduced

it to meet the conditions in Table 10.1 (FLOW manual) but I didn’t get

any improvement.Then I tried changing the horizontal grid resolution

and I got the results in Fig.v_allres.png (in the attachments), where

I found that finer horizontal resolutions (DX = 20 m, 50 m, 100 m,

square cells) lead to worse results. I got the best match with DX =

2000 m but still I am not there. What if I choose DX = 5000 m?</p>

<p>Now, my questions are:</p>

<p>1. Why are coarser resolutions giving better result?</p>

<p>2. If the answer is the "ADI accuracy" in Table 10.1, why

is reducing time step not providing any change in the solution?</p>

<p>3. Is there a clear/formulated criterion for choosing the proper grid

size according to the depth of the basin?</p>

<p>4. How is it this criterion related to the choice of the background

vertical eddy viscosity (I am modeling with constant turbulence model,

nuz = 0.0373 m2/s)?</p>

<p> </p>

<p>Thank you!</p>

<p>Marina<br />

<br />

<br /> </p>