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

Z-model & Sigma-model

Mirko Stefani, modified 1 Year ago.

Z-model & Sigma-model

Youngling Posts: 2 Join Date: 3/29/19 Recent Posts

Hi everyone,


I'm doing a project on a real lake using Delft3D but I still don't understand the difference between a Z-model and a Sigma-model. I understood how to implement it into the program but I can't apprehend the theory behind these two models. Hope someone can help me explaining it.


Best regards,


Richard Measures, modified 1 Year ago.

RE: Z-model & Sigma-model

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

Hi Mirko,

In a standard sigma model the vertical layering is based on a proportion of the depth in each cell, thus the elevation of the divisions between layers varies between cells (due to varying bed levels and water surface levels) and also varies with time (as the water level varies).

In a Z-model the vertical layers are fixed at specific elevations i.e. the elevation of the boundaries between different layers do not change in space or time (except for tracking the water surface elevation).

In general sigma layers tend to be more stable in my experience but for specific problems (e.g. modelling stratified lakes where you want to avoid adding too much numerical diffusion) Z-layers may be preferable.

Does that make sense?