5.5 Open Channels
5.5.1 Inertial Channels
An open channel that includes the inertia term is specified as a series of lines in one or more 1d_nwk GIS layers with an attribute type of ‘S’, S signifying a sloping channel that can handle steep, super-critical flows. S channels are typically all natural channels and artificial channels such as concrete lined open drains. They automatically test for the occurrence of upstream controlled flow and automatically switch between the two regimes, and is the preferred type for all open channels. Other open channel types, Normal (type “blank”) and Gradient (type “G”) are kept for backward compatibility and discussed in Section 5.8.4. Table 5.3 lists the 1d_nwk attributes that are required for open channels.
The hydraulic properties table for open channels is typically provided in the form of cross-sectional data referenced within a 1d_xs GIS layer and using the command Read GIS Table Links but can also specified from external sources (see Section 5.6).
Lines within the 1d_xs GIS layer may be digitised midway along the channel or snapped to the channel ends. The treatment of the cross-sectional data is different depending on the digitisation. It is also possible to automatically create interpolated cross-sections. The attributes of the 1d_xs GIS layer is described in Table 5.4 and further information on cross-sections is provided in Section 5.6.
Refer to TUFLOW Tutorial Module 11 for a model demonstration.
5.5.2 Non-Inertial Channels
To bypass the Courant stability condition, a special channel flag (N) is included, known as non-inertial channel or a friction-controlled channel. This is valid for the S (open channel) and the superseded blank and G gradient channels. To apply this to an S type channel the channel type is SN.
For a non-inertial channel, the inertia terms are ignored (eliminating inertial effects) and the stability control procedure is automatically applied. Although rarely required, the suppression of the inertia terms can be useful for stabilising very short S channels with high velocities.
| No. | Default GIS Attribute Name | Description | Type |
|---|---|---|---|
| 1 | ID | Unique identifier up to 12 characters in length. It may contain any character except for quotes and commas, and cannot be blank. As a general rule, spaces and special characters (e.g. “\”) should be avoided, although they are accepted. The same ID can be used for a channel and a node, but no two nodes and no two channels can have the same ID. | Char(12) |
| 2 | Type |
As described in Table 5.1. S: Steep ChannelG: Gradient Channel Blank: Normal Channel |
Char(4) |
| 3 | Ignore | If a “T”, “t”, “Y” or “y” is specified, the object will be ignored (T for True and Y for Yes). Any other entry, including a blank field, will treat the object as active. | Char(1) |
| 4 |
UCS (Use Channel Storage at nodes). |
If left blank or set to Yes (“Y” or “y”) or True (“T” or “t”), the storage based on the width of the channel over half the channel length is assigned to the upstream and downstream nodes connected to the channel. If set to No (“N” or “n”) or False (“F” or “f”), the channel width and length does not contribute to the node’s storage. See Section 5.12.1.1 for further discussion. | Char(1) |
| 5 | Len_or_ANA |
If greater than zero, sets the length of the channel in metres. If the length is less than zero, except for the special values below, the length of the line is used. Note, not used to specify the length of a pit channel (which is assumed to have zero length). |
Float |
| 6 | n_nF_Cd |
The Manning’s n or Manning’s n multiplier for the channel. If not using materials or Manning’s values in the cross-section the Manning’s n value is specified using this attribute. If using materials or Manning’s n to define the bed resistance from XZ tables (see Sections 5.6.1.1.2 and Section 5.6.1.1.3), n_nF_Cd is a multiplier and is typically set to one (1) as it becomes a multiplication factor of the materials’ Manning’s n values. It may be adjusted as part of the calibration process. |
Float |
| 7 | US_Invert |
G, S Channel Type: Sets the upstream and downstream inverts. Note that the invert is taken as the maximum of the US_Invert and the DS_Invert attributes. Use -99999 to use the bed of the cross-section as the invert. |
Float |
| 8 | DS_Invert | Sets the downstream invert of the channel using the same rules as described for the US_Invert attribute above. | Float |
| 9 | Form_Loss |
Additional form losses (factor of dynamic head) due to bends, bridge piers, etc. This method is preferred instead of increasing Manning’s n to account for losses. For S channels, this only applies when not in upstream controlled friction mode. |
Float |
| 10 | pBlockage | Not used. | Float |
| 11 | Inlet_Type | Not used unless using the legacy feature that accesses MIKE 11 cross-section data - see the 2018 TUFLOW Manual for details. | Char |
| 12 | Conn_1D_2D |
If using Flood Modeller .dat files, the cross-section ID. This is used to link to the cross-section details within the cross-section database (specified via the XS Database command). If using the legacy feature that accesses MIKE 11 cross-section data - see the 2018 TUFLOW Manual for details. Otherwise, not used. |
Char |
| 13 | Conn_No | Not used unless using the legacy feature that accesses MIKE 11 cross-section data - see the 2018 TUFLOW Manual for details. | Integer |
| 14 | Width_or_Dia | Not used. | Float |
| 15 | Height_or_WF | Not used. | Float |
| 16 | Number_of | Not used. | Integer |
| 17 | HConF_or_WC | Not used. | Float |
| 18 | WConF_or_WEx | Not used. | Float |
| 19 | EntryC_or_WSa | Not used. | Float |
| 20 | ExitC_or_WSb | Can be used to apply a form loss coefficient per unit length of channel for an S type channel. For example, a value of 0.0001 v2/g / metre or (0.1 per km) for a 200m channel, the extra form loss would be 0.0001 x 200 or 0.02 v2/2g. This can be used to account for irregularities in the bed form not accounted for by Manning’s n value (eg. submerged rock ledges and obstructions, or large boulders). If using it is recommended that this is calibrated. | Float |