Tuflow and estry manual Version 3

3.12.3Channels

A channel is defined by a length, a Manning’s n value, a table of hydraulic properties versus elevation and other parameters depending on the type of channel. Channels can also represent hydraulic structures such as bridges, culverts, weirs, etc. Section 3.13.1 describes the options for defining the hydraulic properties table.

Channels are specified as lines or polylines in a 1d_nwk GIS layer. To connect channels the ends of the channels must be snapped. (Note: As of Build 2002-08-AC, it is not a requirement that both ends of the channel must be snapped to a node – see Create Nodes). The channels and any digitised nodes may be in the same GIS layer or other layer(s).

Subsequent 1d_nwk layers are used to modify the network at individual objects. For example, if a culvert is to be upgraded in size, then rather than making a copy of the whole 1d_nwk layer, select the culvert channel, save it as another 1d_nwk layer and modify the channel to represent the upgraded culvert. Use Read MI Network twice to first read in the base 1d_nwk layer, then the 1d_nwk layer with the single channel representing the upgraded culvert. Provided the channel has the same ID and is snapped to the same nodes, it will override the original culvert channel. Using this approach minimises data duplication and, if executed logically and well documented, is a very effective approach to modelling.

The attributes required (see Table 4 .10) depend on the channel type (see Table 4 .9).

Channel flow direction is positive in the direction the line/polyline is digitised. This is visualised in the GIS using a line style that has arrows or other symbolism indicating the line direction.

If a channel has a very steep gradient, critical flow problems may be avoided by specifying that the section properties be calculated from the conditions at the upstream end of the channel. “S” channels test for the occurrence of upstream controlled flow and automatically switch between the two regimes.

The water level in a node is not permitted to fall below the nodes bed level, so if an adjoining channel has an effective invert below this level a permanent phantom flow would flow from the empty node. To prevent this happening the input geometry is checked and any occurrences are reported as an ERROR.
Table 4.9 1D Channel Types