8.3 1D Boundaries (1d_bc Layers)

Boundary conditions for 1D domains are defined using one or more 1d_bc GIS layers. The different types of boundaries and links are described in Table 8.3.

GIS 1d_bc layer(s) generally contain points that are snapped to the 1D node in a 1d_nwk layer. Each point has several attributes, as described in Table 8.4.

In addition to point GIS objects, regions can be specified for 1D QT boundaries. If a region is used, the QT hydrograph is equally distributed to all nodes falling within the region that are not assigned a water level (H) boundary (this includes nodes connected to the 2D domain via a 2D SX). If no suitable nodes are found within a QT region an ERROR occurs.

Note: It is not possible to assign both a flow type boundary and water level type boundary to the same 1D node. For example, a HT and QT boundary on the same node.

Models using 1d_bc layers are provided in the Boundary Condition Options Example Model Dataset on the TUFLOW Wiki.

Table 8.3: **1D Boundary Condition and Link Types**
Type	Description	Comments
Water Level
HQ	Water Level (Head) versus Flow (m)	Assigns a water level to the node based on the flow entering the node. This boundary is normally applied at the downstream end of a model. The water level versus flow (HQ) curve can be: Manually assigned using the 1d_bc GIS layer ““Name”” attribute (Table 8.4) and the boundary database (Section 8.5). An automatically generated HQ curve based on the cross-sectional properties and a specified friction slope. Specify “slope=friction_slope” (e.g. slope=0.01) for the “Name” attribute in the 1d_bc point layer (Table 8.4) defining the HQ boundary. As long as your boundary is sufficiently far from your area of interest, the friction slope can be estimated from the two nearest cross-sections to the downstream boundary. This feature requires a single channel connected to the node and does not work with channels using HW cross-sections with varying roughness. This functionality was added in the 2023-03-AD release.
HT	Water Level (Head) versus Time (m)	Assigns a water level to the node based on a water level versus time curve using the 1d_bc GIS layer ““Name”” attribute (Table 8.4) and the boundary database (Section 8.5). If other HT or HS boundaries are applied to the node, the water level is set to the sum of the water level boundaries.
Treatment		1D nodes can be wet or dry. If the water level is below the bed, computationally the bed level is assigned as the water level to the node. As the water level in the node is defined by the boundary, the node’s storage has no bearing on the results.
Combinations		Any number of 1D water level boundaries can be assigned to the same node. The sum of the water levels is assigned as the boundary. For example, a storm tide may be specified as a combination of a tidal HS boundary, a HT boundary of the storm surge and another HT boundary of the wave set up. The HS boundary would be water elevations and the two HT boundaries water depths. If you have a 2D SX boundary connected to a node and also have a HT and/or HS boundary at the same node, the 2D SX boundary prevails and no warning is given.
Flow
QH	Flow vs Water Level (Head) (m³/s)	Assigns a flow to the node based on the water level of the node at the previous half timestep using the 1d_bc GIS layer ““Name”” attribute (Table 8.4) and the boundary database (Section 8.5).
QT	Flow versus Time (m³/s)	Assigns a flow into the node based on a flow versus time curve using the 1d_bc GIS layer ““Name”” attribute (Table 8.4) and the boundary database (Section 8.5). A negative flow extracts water from the node. A region (polygon) can be used to equally distribute the flow hydrograph to all nodes that fall within the region. Any nodes that are an H boundary or are connected to a 2D SX cell are not included. Where pits are connected to the 2D domain, flow is applied to the bottom of the pit (bottom 1D node). The limit on the number of 1D QT regions assigned to a 1D node is ten (10).
Treatment		The 1D node can be wet or dry. The storage of the 1D node influences the results. If the node storage is made excessively large, the flow hydrograph is attenuated. Conversely, if it is under-sized the node is likely to be unstable.
Combinations		Any number of 1D flow boundaries can be assigned to the same node. The final flow is the sum of the flows assigned. A connection to a 2D HX boundary (automatically sets as a QX boundary at the node) can be applied in conjunction with other 1D Q boundaries.

Table 8.4: **1D Boundary Conditions (1d_bc) Attribute Descriptions**
No.	Default GIS Attribute Name	Description	Type
1	Type	The type of BC using one of the two letter flags described in Table 8.3.	Char(2)
2	Flags	Available flags are: S Apply a cubic spline fit to the boundary values (HT, QT, HQ and QH only). Useful for simulating tidal HT boundaries. The flags below are available for QT regions. Any combination of C, O and P can be used. If C, O or P are not specified, the flow is distributed to all non-H boundaries (1D H boundaries are any boundary starting with H or any 2D SX connections). Note, specifying CO is the same as specifying no flags. QT regions only: C Only direct flow into closed nodes. Closed nodes are those that only have culverts/pipes (C, R or I channels) connected. Pits may or may not be connected to a closed node. O Only direct flow into nodes that have at least one open channel connected. An open channel is any channel besides C, R or I channels. P Only direct flow into the bottom of pits, i.e. all nodes with a pit connected.	Char(6)
3	Name	The name of the BC in the BC Database (see Section 8.5). An error will be generated if no name is specified. Note for automatic calculation of HQ boundary a slope can be specified with keyword “slope=slope in m/m”. Refer to Table 8.3.	Char(50)
4	Description	Optional field for entering comments. Not used.	Char(250)