Important concepts

Control volume

A control volume is a bounded area that describes a section of a 2D model that is assigned to a 1D cross-section. The figure below shows a schematical overview.

Control volumes are a conceptual concept and not defined in their own right. Instead, all relevant 2D data, such as bed levels, flow velocities and roughness information is assigned to a cross-section by k-Nearest Neighbour classification. As such, the bounds of the control volumes are not uniquely defined by a polygon but rather by a collection of points.

Control volumes use the 2D coordinates of each cross-section as specified in the CrossSectionLocationFile during the nearest-neighbour classification. The 2D input data used during classification is filtered by region in the following way: a cross-section within a certain region can only be assigned 2D information from that region.

The CrossSectionLocationFile additionally contains information on the 1D coordinates (branch and chainage) and the length. The figure below shows how cross-section length is defined. We recommend that this file is generated using the GenerateCrossSectionLocations tool.

An approximation of the control volumes is generated by the program during the finalisation step. This is a convex hull approximation, which is usefull for debugging input errors (see Troubleshooting)

Note

Control volumes are the equivalent of WAQ2PROF's sobekvakken

A control volume

Regions

Regions are used to have some finer control over which 2D model output is assigned to which 1D cross-section. If no region are defined, 2D model output is assigned to cross-section using k-nearest neighbour. This is not always a good approach, for example if a tributary or retention area. In the figure a section of the River Meuse is plotted near the Blitterswijck retention area. The retention area is demarcated from the main river by levees. Cross-sections generated for the retention area should therefore not 'eat out of' the area of the main channel - which could results in a small cross-section non-physical constriction of the flow.

Region polygons are used to prevent cross-sections generated in the retention area to 'eat out of' the main channel. Within each region polygon (red borders) nearest neighour is used to assign 2D points to cross-sections. Points with the same color are associated with the same 1D cross-section

Sections

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.

Water level (in)dependent geometry

It is often not possible to start the 2D computation from a completely dry bed - instead some initial water level is present in the model. This initial condition divides the 1D geometry in water level dependent part and a water level independent part. Below the initial condition, we cannot take advantage of the 2D model to tell us which cells are part of the conveyance and which cells are wet. Instead, the water level is artificially lowered in a number of steps to estimate the volume below the initial water levels.

Summerdikes

Summerdikes are a Dutch term for levees that are designed to be flooded with higher discharges, but not with relatively low floods (i.e.: they withstand summer floods). They contrast with 'winterdikes', which are designed to not flood at all. Summerdikes effectively comparimentalise the floodplain. They can have a profound effect on stage-discharge relationships: as these levees overflow the compartments start flowing which leads to a retention effect. Such an effect cannot be modelled using regulare cross-sections. SOBEK therefore has a 'summerdike' functionality.

Lakes

Lakes are water bodies that are not hydraulically connected to the main channel in the first few timesteps of the 2D model computation. They do not contribute to the volume present in the control volume until they connect with the rest of the river and will not feature in the water level independent computation. Water bodies that are connected to the main channel in the first few timesteps do count as volume. However, as these likely do not contribute to conveyance, they will be flagged as 'storage' instead.

Total volume

The Total volume refers to the volume of water [in m\^3] within a Control volume for a given water level at the Cross-section location{.interpreted-text role="term"}. The total volume is the sum of the Flow volume and the Storage volume.

Flow volume

The Flow volume is defined as the volume of water [in m\^3] for which the conditions for flowing water are met. This volume is considered to be available for the conveyance of water through the Control volume.

See distinguish_storage

Storage volume

The Storage volume is defined as the volume of water [in m\^3] for which the conditions for flowing water are not met. Storage volume does not contribute to conveyance, but serves only for water retention. Examples include groyne fields and

See distinguish_storage

Total width

See Total volume

Flow width

See Flow volume