5.4 Channels Overview

1d_nwk channels can represent open channels, hydraulic structures such as bridges, culverts/pipes, weirs, and operational structures (e.g. pumps and gates), and other flow controls such as a user defined flow matrix.

A channel is digitised as a line. To connect channels the ends of the channels must be snapped. Channel flow direction is positive in the direction the line is digitised. This is best visualised in the GIS using a line style that has arrows or other symbolism indicating the line direction.

A channel is defined by a length, a Manning’s n value, a table of hydraulic properties (wetted perimeter, flow area, hydraulic radius) versus elevation and other parameters depending on the type of channel. Table 5.1 below lists the available channel types and Table 5.2 lists the additional options that may be appended to selected channel types.

The hydraulic properties table for channels can be defined at a cross section positioned midway along the channel (for some structures and open channels) or can be derived from cross-sections located at the channel ends (for open channels only). The exceptions are:

  • For culverts (C and R types) the attribute information supplied within a 1d_nwk layer (i.e. diameter, width, etc.) is sufficient to define the hydraulic properties (i.e. no cross-section properties table is required).
  • For weirs (W), if no cross-section or hydraulic properties table is specified, and a Diameter_or_Width attribute value greater than 0.01 is specified, the weir is defined as being a rectangular section 10 metres high based on the invert and width values.

Tables of cross-section profiles, cross-section hydraulic properties and bridge loss coefficients are accessed using links within 1d_xs and 1d_bg GIS layers. Tables can also be used to define nodal surface areas (refer to Section 5.12.2.1). This allows these data to be entered in a comma delimited format using .csv files that can be managed and edited in spreadsheet software such as Microsoft Excel.

Modellers often keep the different data sets separate as numerous .csv files are often needed. Separate folders underneath the model folder (same level as the gis folder) are often used to store all the .csv files and the GIS layer. For example:

  • 1d_xs for XZ cross-section profiles in a model\xs folder
  • 1d_bg for bridge loss coefficient tables in a model\bg folder

The Read GIS Table Links command is used for linking tabular data to channels. The method for linking cross-sectional and bridge losses is as follows:

  • Lines are linked to channels. The method depends on whether the object has two or more vertices. The logic is:
    • For lines with two points (the start and end – no intermediate vertices) the line only needs to cross a channel – it does not have to snap to a vertex on the channel line. If the two-point line crosses more than one channel, the channel that is closest to the mid-point of the line is selected.
    • Lines with three or more vertices must have one of the vertices snap to a vertice on the channel line. If both types are specified, the snapped sections are given preference over any two-point line that crosses the channel line.
  • Other objects (regions and points) are not used.

The attributes and the method for determining the data to extract from the source file is outlined in Table 5.4 for 1d_xs and Table 5.12 for 1d_bg. Using the Column_1 attribute, several tables can be located in the one source file if desired.

Table 5.1: 1D Channel (Line) Types
Channel/Node Type Description
Open Channels
Open Channel S

Open channel that incorporates all flow regimes. Supercedes Normal (Blank) and Gradient (G) channels, as S channels switch into upstream controlled, friction only mode (i.e. no inertia terms) for higher Froude numbers (see Froude Check). This allows steep flow regimes such as super-critical flow to be represented. See also Froude Depth Adjustment.

This is the preferred open channel type as it incorporates all flow regimes, and use this channel in preference to Normal (Blank) and G channels.

Upstream and downstream bed invert attributes must be specified to define the slope of the channel, or the inverts can be taken from the channel’s cross-sections by specifying -99999 for the inverts.
Structures
Bridge
Section 5.7.2 		B Bridge structure – energy loss coefficients supplied by the user.
BB Bridge structure (introduced for Build 2016-03-AA) – only pier loss and submerged deck loss coefficients required (all other losses automatically calculated). In the future BB bridges will also recognise bridge definition inputs in a similar manner to BArch bridges to automatically generate loss coefficients.
Arch Bridge
Section 5.7.3 		BArch Arch Bridge structure (Build 2023-03-AA and onwards). Allows users to specify a .csv defining the properties of the arch bridge.
Culverts
Section 5.7.1 		C Pipe or Circular culvert.
I Irregular shaped culvert.
R Box or Rectangular culvert.
Gates
Section 5.7.6 		SG Sluice Gate.
Pump
Section 5.9.2 		P Pump.
Spillways
Section 5.7.5 and 5.9.6 		SP Gated or ungated spillways.
Weirs
Section 5.7.4 		W Weir structure (original weir channel).
WB Broad-crested weir.
WC Crump weir
WD User-defined weir.
WO Ogee-crested weir.
WR Rectangular weir (sharp-crested).
WT Trapezoidal / Cippoletti weir.
WV V-notch weir
WW Similar to the original W weir channel, but with more user options.
Special Channels
Normal (leave blank)

Normal flow channel defined by its length, bed resistance and hydraulic properties. The channel can wet and dry, however, for overbank areas (e.g. tidal flats or floodplains) gradient (G) or S channels should be used. For steep channels that may experience supercritical flow, use S channels.

Note: For all open channels it is recommended to use the S Type.
Gradient G

Similar to a Normal channel, except when the water level at one end of the channel falls below the channel bed, the channel invokes a free-overfall algorithm that keeps water flowing without using negative depths. The algorithm takes into account both the channel’s bed resistance and upstream controlled weir flow at the downstream end.

Gradient channels are designed for overbank areas such as tidal flats and floodplains. The upstream and downstream bed invert attributes must be specified to define the slope of the channel.

Note: For all open channels it is recommended to use the S Type.
Matrix Flow Channel M User defined flow channel using a flow matrix. The flow through the structure is dependent on the water levels upstream and downstream.
Depth-Discharge Channel Q User-defined stage discharge channel. The flow through the structure is only dependent on the upstream conditions, such as user defined spillways. If downstream levels are influential then an M channel (see above) may be required.
Connector X

Connects the end of one channel to another. This is particularly useful for connecting a side tributary or pipe into the main flow path. It also allows a different end cross-section or WLL to be specified for the side channel, rather than using the end cross-section on the main channel. The direction of the connector line is important.

Note: The line must start at the side channel and end at the main channel.

If two or more connectors are used at the same location (i.e. to connect two or more side channels to a main channel) their ends must all snap to the same main channel.
Operational Channels
Piping Failure PF Operational channel to model the “pipe failure” process (when water seepages through an embankment forming a small flow path), see Section 5.9.8. Introduced in the 2020-10-AA build.
Dam Failure DF Operational channel to model a “dam failure” process (when a dam/levee breaks), see Section 5.9.8. Introduced in the 2020-10-AA build.
Table 5.2: 1D Channel (Line) Types - Additional/Optional Channel Flags
Options Flag Description Applicable Channel Types
Adjust Structure Losses A Forces the adjust losses approach using the equations and methodology in Section 5.7.7 to adjust the inlet and outlet losses of a culvert or bridge channel according to the approach and departure velocities. This flag overrides Structure Losses if set to FIX. For example, to adjust the losses for a rectangular culvert specify a Type attribute of “RA”. Culverts (R, C, I)
Fix Structure Losses F

Forces the fix losses approach so as not to adjust the inlet and outlet losses of a culvert or bridge channel according to the approach and departure velocities. This flag overrides the Structure Losses setting if set to ADJUST (the default). See Section 5.7.7.

For example, to fix the losses for a circular culvert specify a Type attribute of “CF”. 		Culverts (R, C, I)
Downstream Controlled D For culverts, limits the flow regimes to the downstream controlled ones (see Table 5.6), unless it is a zero length channel (i.e. channel length less than 0.01m). Culverts (R, C, I)
Weir over the Top W If a “W” is specified in conjunction with a B, C or R channel (e.g. BW, CW or RW), a weir channel is automatically inserted to represent the flow overtopping the structure. This saves having to digitise the weir separately. To use this option requires adding the 10 optional attributes to the 1d_nwk layer as detailed in Table 5.18. Some of these attributes are used to specify the weir parameters. Culverts (R, C) and Bridges (B)
Energy E For structures specifies the use of energy level for the flow calculations. The default is to use energy (E), unless the global .ecf command Structure Flow Levels == WATER is used, in which case the default is to use water level (H). See H below. All Weirs except for ‘W’ type, Spillways (SP), Gates (SG) and Dam Failure Channels (DF).
Energy Upstream EH Introduced in the 2023-03-AC build, uses the energy level at the upstream node and water level at the downstream node of 1D structures for flux calculation. This option can also be applied globally using the .ecf command Structure Flow Levels == ENERGY UPSTREAM. All Weirs except for ‘W’ type, Spillways (SP), Gates (SG) and Dam Failure Channels (DF).
Water Surface H

For structures specifies use of water level for the flow calculations. The default is to use energy level unless Structure Flow Levels == WATER has been specified.

For example, if a broad-crested weir is to use water surface levels rather than energy levels, specify WBH (a space can be used so “WB H” may be preferred for clarity). 		All Weirs except for ‘W’ type, Spillways (SP), Gates (SG) and Dam Failure Channels (DF).
Non-inertial channel N Open Channel (S), Normal (blank) Gradient (G) and channels can be specified as non-inertial by including an “N” in the Type attribute. A non-inertial channel has the inertia term suppressed from the momentum equation. Open Channel (S, G ,blank)
Variable Geometry V

Normal and gradient channel cross-sections can vary over time by using a variable channel definition. Include a “V” in the Type attribute and see Section 5.7.4.6 for more details.

Note that prior to the 2013-12 release, a variable weir channel was specified as a WV channel type. As of the 2013-12 release, WV channels are processed as a V-notch weir. Variable weir channels must be specified as type “VW”. 		Open Channel (S, G ,blank) and W type Weir
Operational Control O “O” flag is required for structures that are to be operated using an operating control definition (see Section 5.8). For example, an operated pump would have a Type attribute of “PO” (or “OP”) see Section 5.8
Uni-directional (all channels) U Any channel can be defined as uni-directional by including a “U” in the Type attribute. Water will only flow in the positive direction of the channel (from upstream to downstream). For example a “RU” channel could be used to represent a flap gated rectangular culvert. All channel types