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RE: positive and negative values below reference plane

Marina Diers, modified 4 Years ago.

positive and negative values below reference plane

Youngling Posts: 6 Join Date: 10/19/20 Recent Posts
Hi everyone,I'm using Delft3d-FM for a university project and during studying the Manuels for Flow and Wave I recognized a discrepancy. Flow is measuring the bed below the reference plane as negative, so e.g. land above the water surface -> positive, bed level in the middle of the lake ->negative.
In the Wave manual is written that the bed level is measured downwards positive.Is that only valid if I do both programs standalone? If I want to couple them and I say I want to use the Bed level from Flow, is the program converting the values by itself, or can't I use the values automatically and I need a separate bathymetry?I hope you understand my struggle and that someone can help me or share their experiences?
Thanks and kind regards,
Marina
Sebastian Martijena, modified 4 Years ago.

RE: positive and negative values below reference plane

Youngling Posts: 11 Join Date: 11/18/15 Recent Posts
Hi Marina!In Delft3D-FM all levels are measured with a positive vertical axis "pointing up", i.e. oposed to the vector "g" (gravity).
Hope it helps.Sebastian
Marina Diers, modified 4 Years ago.

RE: positive and negative values below reference plane

Youngling Posts: 6 Join Date: 10/19/20 Recent Posts
Hei Sebastian, thanks for your response, but in the manual is written:
Flow: 5.4.4 Bed level
When you double click on Bed level in the Project window or select Bed level from the dropdown box in the spatial editor section of the Map ribbon, the spatial editor is activated (Figure 5.34). This editor can be used to generate a bathymetry for your computational grid. How to work with the spatial editor is described in Appendix H. Be aware that the bathymetry in D-Flow FM is de?ned as the bed level (e.g. positive upward), implying that all bed levels below the reference plane are negative. By default the bed levels are de?ned on the net nodes.

Wave: page 35:
During the computations, D-Waves determines the water depth from the bottom level, the water level and the water level correction. Bottom levels are de?ned as the level of the bottom relative to some horizontal datum level (e.g. a still water level), positive downward. Water levels are de?ned with respect to the same datum as the bottom; the water level is positive upward.

If I get it right, two different directions (negative or positive) of the axis are explained?
Dmitry Kushnir, modified 4 Years ago.

RE: positive and negative values below reference plane

Padawan Posts: 25 Join Date: 11/9/12 Recent Posts
Hi Marina, That's right. In D-Flow FM (unstructured grids), the sign convention for bathymetry is positive upwards. In D-Waves (curvilinear grids) depth is positive downward. 
Regards, Dmytro
Marina Diers, modified 4 Years ago.

RE: positive and negative values below reference plane

Youngling Posts: 6 Join Date: 10/19/20 Recent Posts
Thanks Dmitry,do you know the reason for that?And if I want to couple them and I say I want to use the Bed level from Flow, is the program converting the values by itself, or can't I use the values automatically and I need a separate bathymetry?
Dmitry Kushnir, modified 4 Years ago.

RE: positive and negative values below reference plane

Padawan Posts: 25 Join Date: 11/9/12 Recent Posts
Hi Marina,
I don't know the exact reason for this. My guess is that D-Waves can handle only structured grids (recti- or curvilinear), which where initially used by Delft3D-FLOW and Delt3D-WAVE (structured). This implies that one have to prepare separate set of files for D-Waves (including grids), to perform coupled simulation. Check out example 12_dflowfm, which contains test case "e100_f02_c02-FriesianInlet_schematic_FM_wave" for the coupled FM + Wave simulation. To answer your question about converting bed level from FM to Wave notation:  the simplest way to do that is to export D-Flow FM bathymetry to .xyz-file, load it into Waves model and then multiply samples by -1 using "Set value" - "Multiply" in Delta Shell spatial editor tab (screen-grad attached).  

Regards, Dmitry