Methodology

This chapter describes in more detail the steps FM2PROF takes to go from a 2D representation of reality to a 1D representation of reality. Conceptually, FM2PROF works in three distinct steps: (1) initialisation, (2) building cross-sections and (3) finalisation.

Initialisation

Image caption

The initialisation step involves parsing (i.e. reading and making available for further analysis) the input. In this step control volumes and sections are defined as well. This step may take some time to complete, but this preprocessing greatly reduces the computation times in the next step.

Region polygon file

The region polygon file (Dutch: gebiedsvakken) is provided in the configuration and should be a valid MultiPolygon geojson file. If a polygon file is provided, FM2PROF will perform an inpolygon algorithm during initialisation and write the results to a .region_cache.json file which is placed in the same directory as the 2D map file. This can take up to 30 minutes for a sizable river. If a cache file is already present and valid, it will be read instead. A cache file is invalidated if the 2D input has changed since the cache was created.

Note

There are special conditions for the geojson file, see MultiPolygon.

The region file is used to map each point (node, edge or face) in the 2D input grid to one unique region. They are used to determine what 2D points are assigned to which cross-section, see Classification of volumes

Section polygon file

The section polygon file (Dutch: gebiedsvakken) is provided in the configuration and should be a valid MultiPolygon geojson file. If a polygon file is provided, FM2PROF will perform an inpolygon algorithm during initialisation and write the results to a .section_cache.json file which is placed in the same directory as the 2D map file. This can take up to 30 minutes for a sizable river. If a cache file is already present and valid, it will be read instead. A cache file is invalidated if the 2D input has changed since the cache was created.

Sections are used to divide the cross-section between floodplain and main channel (e.g. the 'floodplain' section and the 'main channel' section). This distinction is only used to assign different roughness values to each section.

Tip

The default section is "floodplain1". To reduce initialisation time, provide only a section polygon covering the main channel (so a polygon with attribute section set to main). By default, all points not covered by the main channel will be floodplain1.

Import 2D data

Parsing 2D data

Dflow2d uses a staggered grid to solve the (hydrostatic) flow equations. Because of this staggered approach, there is not a single 2D point that has all information. Flow information (flow velocity, discharge) is stored in flow links (or edges), while geometry (bed level) is stored in cell faces. FM2PROF needs both information from the faces, as from the links.

The dflow2d staggered grid.

Classification of control volumes

Control volumes are used to define which 2D datepoints are linked to which 1D cross-section. This is done in the following steps:

• Each 2D point (node, edge and face) is assigned a Region. If a region polygon is provided, each 2D point is assigned the same region as the polygon they are in. If no region polygon is provided, each point will be assigned to the same default region.
• Each cross-section is assigned a region following the same principle.
• For each region seperately, we perform nearest neighbour classification to uniquely identify each 2D point to a 1D cross-section. Only 2D points that have the same region as the cross-section can be assigned to a cross-section.

Classification of sections

Sections are used to output a different roughness function for the main channel and the floodplains. The purpose of the classification is to determine whether a 2D point belongs to the main channel section, or to the floodplain section (see warning below).

Two methods are implemented:

• Variance based classification<section_classification_variance>{.interpreted-text role="ref"}
• Polygon-based classification

Build Cross-Section

The dflow2d staggered grid.

Once initialisation is complete, will loop over each cross-section location<Cross-section location>{.interpreted-text role="term"}. In each iteration, the program takes two steps: (1) building the geometry and (2) building the roughness tables.

Warning

No cross-section will be generated for locations that have no 2D data assigned or have less than 10 faces assigned. This may happen if a location lies outside the 2D grid, or if there are many cross-section closely together. If this happens, an error is raised by FM2PROF. The user should check the cross-section location input file to resolve the problem.

Build Geometry

In each loop, a number of steps is taken based on the 2D data that is uniquely assigned to that cross-section:

• Lakes are identified using the identify_lakes
• Flow volume and Storage volume are seperated using the distinguish_storage
• The water level dependent geometry<Water level (in)dependent geometry>{.interpreted-text role="term"} is computed using wl_dependent_css{.interpreted-text role="ref"}
• The water level independent geometry<Water level (in)dependent geometry>{.interpreted-text role="term"} is computed using wl_independent_css
• The parameters for Summerdikes are defined using the sd_optimisation
• Finally, the cross-section is simplified using the Visvalingam-Whyatt method of poly-line vertex reduction simplify_css

Build roughness

At each cross-section point, a roughness look-up table is constructed that relates water level (in m + NAP) to a Chézy roughness coefficient. This is done in three steps:

• For each section, a roughness table is constructed by averaging the 2D points

Finalisation

The dflow2d staggered grid.

Interpolate friction tables

Some output formats (e.g. SOBEK 3) require that the friction tables are uniform for each branch: each point on the branch must have the same dimensions. This means that if friction is defined as a function of water level, that each point on a branch must have the same water level dimension.

During cross-section generation, it is not yet known what the water levels are for each point. During finalisation, a uniform water level dimension is defined. For each point on the branch, the friction value is interpolated to the uniform dimension.

Export output files

Output files are written to the required 1D format. Optionally, additional files are written as well. These optional output files can be enabled in the debug section of the configuration file.