intro story D-Flow FM


D-Flow Flexible Mesh

D-Flow Flexible Mesh (D-Flow FM) is the new software engine for hydrodynamical simulations on unstructured grids in 1D-2D-3D. Together with the familiar curvilinear meshes from Delft3D 4, the unstructured grid can consist of triangles, pentagons (etc.) and 1D channel networks, all in one single mesh. It combines proven technology from the hydrodynamic engines of Delft3D 4 and SOBEK 2 and adds flexible administration, resulting in:

  • Easier 1D-2D-3D model coupling, intuitive setup of boundary conditions and meteorological forcings (amongst others).
  • More flexible 2D gridding in delta regions, river junctions, harbours, intertidal flats and more.
  • High performance by smart use of multicore architectures, and grid computing clusters.
An overview of the current developments can be found here.
The D-Flow FM - team would be delighted if you would participate in discussions on the generation of meshes, the specification of boundary conditions, the running of computations, and all kinds of other relevant topics. Feel free to share your smart questions and/or brilliant solutions! 


We have launched a new website (still under construction so expect continuous improvements) and a new forum dedicated to Delft3D Flexible Mesh.

Please follow this link to the new forum: 

Post your questions, issues, suggestions, difficulties related to our Delft3D Flexible Mesh Suite on the new forum.





Sub groups
D-Flow Flexible Mesh
Cohesive sediments & muddy systems


Message Boards

Boundary conditions in macrotidal estuary

Federico Zabaleta, modified 3 Years ago.

Boundary conditions in macrotidal estuary

Youngling Posts: 1 Join Date: 12/22/16 Recent Posts
Hello everyone,

I am modelling a macrotidal estuary (with tidal amplitudes of 8 m), and I have a few doubts regarding the BC outside of the estuary.

I've attached a file with two models, and the two options for BC. The one in the left, has the west boundary with Water Level condition and the north and south boundaries with Neumann condition. The one in the right has only a water level condition for the ocean BC (I know that the size of the domains are quite different).

I would like to know if there is any difference in using one option or the other, and which values should I use for the Neumann condition in the first option.

Any advice or recomendation will be really appreciated.

Best regards,
Ben Williams, modified 3 Years ago.

RE: Boundary conditions in macrotidal estuary

Jedi Knight Posts: 114 Join Date: 3/23/11 Recent Posts
Hi Fredrico,

Those model domains are a work of art! They must have taken ages....

I didn't see a scale on those diagrams but I am guessing the offshore and lateral extents of your ocean boundary is order of 50km for the domain on the left and 10km for the domain on the right. From a practical approach both seem pretty reasonable. The choice will depend on the physics of your coast (i.e. the way in which the tide propagates along or across your coastline, and the way the tide resonates within your estuary), and if you have tidal constituent available at multiple locations.

Your stated (spring?) tidal range is 8m (at the ocean boundary?) and there is tidal amplification within the estuary, which suggests that tidal currents will be pretty large and therefore coriolis effect may alter your offshore tide by introducing a lateral gradient across the boundary. This lateral gradient can be quite appreciable for mid and high lattitudes. You don't mention if you have a single tide gauge or if you have data at multiple locations at your ocean boundary.

If single tide gauge only (and the lateral extent of your model is very small relative to the tidal wavelength) I would be of the opinion to use the domain on the right, as the lateral extent is probably small enough that spatial gradients in surface elevation can be ignored. On the other hand, if tidal resonance is at play you need to make sure that your boundaries are far enough away from the estuary to not interfere with the tidal propagation within the estuary and its interaction wit the adjacent coast. As a first approximation, if the ebb tidal jet is able to reach your ocean boundary without dissipating then your boundary is too close as the assumption of uniform or smoothly varying surface levels is violated.

It is more likely that the offshore boundary for the domain on the left is far enough away to not interfere with the tidal resonance within your estuary. However the ocean extent of your model is too large to assume a single water level across it, unless that what occurs in nature. If the tide propagates *along* your coast and water levels are constant in the cross shore dimension then you can assume neumann lateral boundaries with zero gradient through time. You will need to specify seperate tidal constituents at each end of your ocean boundary to capture the tidal wave as it propagates past your estuary.

If the tide propagates directly onshore then you'll need to specify tidal levels at both your onshore and offshore corners of your lateral boundaries, and neumann lateral boundaries would not be appropriate. If you really need to use neumann boundaries, for example if you will have a wave model and therefore currents driven by wave set up and radiation stresses, then you can have a water level boundary from the offshore to say 15m water depth and then a separate nuemann boundary from the 15m deoth to the shore. If the distance from shore to 10 or 15m contour is say 1 or 2km then assuming zero tidal gradient in the cross shore is pretty valid

You'll be able to tell if the tide propagates in the along or cross shore dimension by looking at a co-tidal chart of the area.

So, not exactly a clear answer but hope this helps a little. It looks a really interesting area and i wish you luck with your endeavours.

All the best,