4.3. Water system

To model the water system of a polder you will need different types of structures and perhaps a larger level of detail than available in your bathymetry files. For this purpose, 3Di offers channels and structures that can be linked to the 2D computational grid. We refer to them as the 1D components. This tutorial shows you the steps needed to add these elements to an existing model. More background information, can be found at 1D Flow.

The first thing you must do is activate 1D flow in the global settings. Also consider the following parameters in the global or numerical settings:

  • advection_1d
  • max_angle_1d_advection
  • max_degree
  • use_of_nested_newton

4.3.1. Channels

To add a channel or network of channels consider the following steps:

  1. First, load these tables from the spatialite database:

    1. v2_channel (line geometry)
    2. v2_connection_nodes (point geometry)
    3. v2_cross_section_location (point geometry)
    4. v2_cross_section_definition (no geometry)
  2. Channels are drawn in between connection nodes, so start by adding nodes on the start and end-points of your channels. If you wish to add structures later on, make sure to add enough nodes as they are also linked between connection nodes (and thus not placed on top of channels).

  3. After saving your edits in QGIS, the connection nodes id’s are filled automatically. You can fill out the other attributes of the connection nodes later on.

  4. Now draw your channels from connection node to connection node. Make sure you snap the start and end-points to the connection nodes and fill out the connection_node_start_id and connection_node_end_id for every channel feature. Then fill all the attribute fields for all channels except the id’s and save your edits.

  5. Every channels needs at least one cross-section. Start by adding one or multiple cross-section definitions in the table v2_cross_section_definition. You have several options to define you profile, check the Database overview for more details. One definition can be used on multiple locations. Note that the v2_cross_section_definition id’s are filled after you save your edits.

  6. You can place cross-sections on channels using the v2_cross_section_location. You may place multiple cross-sections on one channel. When placing locations, consider the following:

    1. Cross-section locations must be placed on a channel vertex. If you have no vertex available on you channel, add one.
    2. Cross-section locations may not be placed on the start- or end-point of a channel
    3. When placing multiple cross-section locations on one channel consider the distance between your computation points. Make sure you have sufficient computation points on your channel, to take this variations into account.
    4. Refer to the correct channel id in the attribute field channel_id.
    5. Refer to the correct definition in the attribute field definition_id
    6. If it is a connected or double connected channel, make sure to fill in the bank_level. This is the level, that determines the exchange between the water in the channel and that on the 2D surface.

After these steps your channel is complete. If you wish you can fill out the initial water level on the connection nodes. In case there are added computational points in the channel, these get the initial water level of the start connection node. Embedded channels require some additional steps, these are described below.

4.3.2. Structures

Structures in 3Di are defined as a connection between two connection nodes. 3Di supports four types of structures (Structures):

  1. Pump station
  2. Weir
  3. Orifice
  4. Culvert

Check out the Database overview for a short description of the requirements of the structure attributes. However, here are some notes to help you:

  • The shape of the weir, orifice and culvert is stored in the cross-section definition table. So make sure you have some available before you start adding these structures. (Weir)
  • Culverts are the only structure type that has a geometry, it is a line. This means it can be curved. The culvert length is derived from its geometry. All other structures are defined only as a link between nodes. (Orifice and Culvert)
  • To add a structure, make sure you have two connection nodes available at the ends of two channels. Fill in the correct start and end connection node id in the sqlite tables. You must work in the v2_structure table. Editing in view tables is not supported.
  • The pump station pumps from the start node to the end node. You can choose how it is controlled using the type attribute. (Pump station)
  • If you wish to model several structures that are only connected to each other, for instance a culvert followed by a weir. You must add a small storage area to the connection node. Normally, the storage area is derived from the cross-section of the adjoining channel, but when there is no channel connected to a connection node, it has by default no storage. So add it manually, or your model won’t work.
  • Finally, you must make sure that one of the cross-section’s reference levels near the structure is below the start, crest or invert level of the adjoining structure.

4.3.3. 1D boundary condition

Boundary conditions for the 1D system are placed on connection nodes. They can be placed on connection nodes that are connected to a single isolated channel or structure, so not on an embedded or connected channel. Check the different types of boundary conditions available in the Database overview.

The time series field in the spatialite database can only be filled by copy-pasting your time serie into the spatialite as QGIS does not allow you to enter a newline. You may use this example:

0,0.000000 15,1.000000 30,2.000000 45,3.000000 60,2.000000 9999,2.000000

4.3.4. Embedded channels

Embedded channels are usefull when you wish to add more detailed profiles to a course raster-file. Also, they are the most efficient way to add channels since they don’t add to the number of computational points. In fact, the volume in the channel is integrated with that of the 2D computational cell. When modelling embedded channels, consider the following:

  • The water level in the embedded channel is always equal to the water level in the underlaying 2D grid cell.
  • Embedded channels add extra connections between 2D grid cells, but ignore obstacles and levees.
  • Make sure the embedded channel profile always partially lays below the surface level in you DEM (you can’t have floating embedded channels).
  • Make sure no more than one channel vertice falls inside a single raster-file pixel.
  • Embedded channels only function when they connect several 2D grid cells, so make sure no embedded channel falls completely inside one 2D grid cell,
  • All connection nodes connected to an embedded channel become embedded, so make sure structures or channels of other types that are connected to these connection node cross at least one 2D grid cell boundary, and
  • Do not place boundary conditions directly on embedded channels.