Open channels

Land development reference 7.17

'As constructed' information for open channels is relatively easy to obtain due to the accessibility, but can be dangerous due to slope and smoothness of surfaces, especially if the area is wet. Whilst every effort should be made to fulfill the 'as constructed' information requirements, these mustn't place any person at risk nor infringe on Occupational Health and Safety requirements.

Location of channel with respect to title

Channel centrelines must be located in relation to title boundaries by offsets and ties, as well as MGA94(z55) co-ordinates. Offset distances to title boundaries are the primary measurements used to relate the Channel alignments and tangent points to title boundaries.

MGA94(z55) co-ordinates of both asset and title provide a means of verifying the positional accuracy of the asset in relation to VicMap Property.

To enable us to verify works have been constructed in accordance to design and to assist the surveyor in annotating the design plans, 'as constructed' cross sections locating the channel toes, channel invert, channel top of cuts and even adjoining levee banks, are to be taken co-incident with the design cross sections. This will enable a direct comparison to be undertaken. Cross sections are usually in the order of 30 to 50 metres apart, but must be taken at tangent points, any changes in horizontal direction, changes in grade, start and end of transitions and ends of constructed works.

The 'as constructed' cross sectional information (i.e. offset and level text) is to be annotated on the design cross sections. If the constructed open channel isn't in accordance with design, then the annotated design plan must clearly depict the difference by superimposing the 'as constructed' cross sections over the design cross sections. The title boundary should also be depicted on the cross section, with the appropriate offset, where relevant.

If the alignment of the channel is completely re-aligned, then new alignment plans and cross sectional plans will need to be submitted. These plan numbers should be incrementally increased from the last sheet number of the design set.

Channel inverts

The channel invert may be at the centre of the channel, at one of the toes or even from one toe to the other. Where the invert is offset from the centreline, chainages to those inverts is based on the centreline chainage that is square off.

Invert levels are to be annotated on the longitudinal section with associated running chainage and on cross sections with associated offset.


A running 'as constructed' chainage of the entire open channel must be annotated on the longitudinal section (with associated invert levels). Chainages should also be annotated on the alignment plan at any point where the chainage varies from design by more than several metres. Chainages must be specifically taken at the following points:

  • commencement and completion of works

  • tangent points

  • all changes in vertical grade

  • all changes in horizontal direction

  • at all cross sections

The running chainage is based on the channel centreline, which may or may not be where the invert is located.

Channel grade

The surveyor must calculate the new grades and where significantly different (greater than 15 per cent variation in grade), then the actual grade must be annotated on the design plan longitudinal section.

Channel material

The surveyor must confirm the channel's material (i.e. earthen, grassed, concrete lined, embedded rock work, etc). Where the constructed works aren't in accordance with design, the actual material(s) must be annotated on the design plan.

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