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

How to decide the Reflection parameter alpha value? (flow)

Esther Espinoza, modified 3 Years ago.

How to decide the Reflection parameter alpha value? (flow)

Padawan Posts: 25 Join Date: 4/6/17 Recent Posts
Hi, I'm modelling water level for an estuary area (1 month) and I would like to know if you can explain me a little bit about the reflection parameter alpha that it's needed in the bounday section of the flow module. I've read the manual, but I'm not sure how to decide it's value or how big is the influence of that parameter on the results of the water level simulation.

Thank you
Bert Jagers, modified 3 Years ago.

RE: How to decide the Reflection parameter alpha value? (flow)

Jedi Knight Posts: 201 Join Date: 12/22/10 Recent Posts
Hi Esther,

For an estuary I would expect to implement an alpha value on the downstream (sea) water level boundary.

The alpha value suggested in the manual equation (9.76) for a water level boundary equals Td * sqrt(H/g) where Td is the time it takes for a free surface wave to travel from the left boundary to the right boundary of the model area, H is the water depth, and g is the gravitational acceleration (about 9.8).

Here the words "left" and "right" boundary refer to the typical upstream and downstream boundaries, but shorter characteristic distances that may reflect free surface waves may be appropriate instead. A free surface wave travels with U +/- sqrt(g*H) which is for small average velocity U well approximated by sqrt(g*H). The time Td then equals L / sqrt(g*H) where L is the aforementioned characteristic length. If we insert this approximation for Td in (9.76) the expression for alpha at the water level boundary becomes: L / g, that is the characteristic length divided by about 10.

For an estuary with characteristic length scale of 20 km we would thus preferably use an alpha value of 20000/10 = 2000 to avoid boundary reflections inside the domain. The larger the estuary, the large the alpha value. Now, the question arises: can we increase alpha without complications? Unfortunately, no.

By increasing alpha, the boundary starts also to dampen any variations that itself tries to send in. I believe that values up to about 1000 are usually safe, but I recommend to compare the the imposed boundary forcing signal with the observed signal at the boundary and check whether it corresponds satisfactorily for the alpha value that you would like to use. Note that it will not match perfectly if the observation point is inside the grid whereas the forcing is imposed in a virtual grid cell just outside the grid.

Hope this helps,

Esther Espinoza, modified 2 Years ago.

RE: How to decide the Reflection parameter alpha value? (flow)

Padawan Posts: 25 Join Date: 4/6/17 Recent Posts
Thank you so much Bert, it was really helpful!