Safety and maintenance

Additional factors which should be considered during the preparation of the design include:

Fencing requirements

For public safety the need for fencing of all structures should be considered. For example, all inlet and outlet structures in the proposed system will require fencing. A fence should also be included in the design for any other structures which have a vertical drop exceeding 750mm and are accessible to the public.

Conventional fencing shouldn't be used to cross floodways as debris will tend to build up against them. This may worsen flooding upstream and could create a potential downstream flood wave if sudden failure was to occur. In some unique situations (subject to our approval) floodway panels may be used to cross floodways where security is required. Floodway panels are designed to break away at their base under pressure and fall back as the flow reduces.


If entry to an existing underground drain is required, it's necessary to contact us to arrange inspection. Entry into any underground drain must only be undertaken by persons appropriately qualified in Confined Space entry.

Various other safety hazards exist in underground drains. These include:

  • drops
  • silt traps
  • ventilation

Grates should be provided to inlets to deter entry, but grates shouldn't be used on outlets. Fencing should be used to provide security to outlets. Designers should consider the safety of the public during flooding. If persons could be washed into a drain then they should be able to be washed out. Depending on the situation, it may be appropriate to design for the invert of the outlet to be permanently drowned out to deter public access.


A safety bar should be installed across the drain immediately upstream of a drop structure. Step irons should be built into the make up sections for hand-holds at drops and transitions.

Silt traps

A potential hazard exists if silt traps are designed to be included within the underground drain. These can be difficult to clean and dangerous gases will emit from the build up of organic silt. This problem can be avoided by designing silt traps that are included in the outlet structures where they can be more easily cleaned.

Dangerous gases

To minimise the possibility of a build up of dangerous gases in underground drains, Gatic Type BV (or similar) covers must be used on all drains that are to be vested in us. These covers are a vented version of a Type B cover.

Maintenance requirements

This section details the maintenance requirements for:

  • underground drains
  • channels

The designer should consult us about its maintenance requirements for each job for open channels. Access for maintenance vehicles must be considered.

Underground drains

Maintenance factors must be considered in the design of underground drains which include:

  • manhole spacing
  • manhole location
  • manhole openings
  • inlets

Manhole spacing

Normally manholes should be separated by a distance of 80 to 100D (nominal internal diameter) with 200m as the maximum. The spacing between manholes should reduce to a maximum of 50m in heavy industrial areas. For these cases, it's necessary to contact us for the required spacing.

When the drain is located at the back of kerb and the side entry pits are constructed on the main drain then the frequency of side entry pits required for road drainage may dictate manhole spacing.

Manhole location

Manholes should be designed at the safest location. The following is a list of preferred sites, in order of preference:

  • nature strip - within 2m of the road pavement
  • nature strip - other locations
  • side entry pit
  • median strip
  • footpath
  • roadway (this should be avoided where possible and traffic volumes should always be considered)

A manhole must be located at our drainage limit where practicable. This limit shouldn't be defined at a location where the manhole is in a carriageway carrying heavy traffic.

Manhole openings

For drains where siltation may be a problem and a silt trap is to be constructed, the size of the manhole opening should be increased to a minimum of 750 x 1200mm. This will allow cleaning equipment to enter the drain. As noted elsewhere, the covers of the manhole must be vented.

Minor drain connections and inlets

Provision for council drain connections should be included in the design. Refer to Standard Drawing No. 7251/8/307 for details.


Channels must meet operational and maintenance requirements as well as being designed in an environmentally sensitive manner.

The following features should be considered when preparing a channel design:

  • Slope mowers can't operate on slopes steeper than 1v:5h.
  • Operational and maintenance aspects should be discussed with us for all channel works. This should be done as early as possible during the functional design stage as it may affect reserve widths. The type of grass-cutting machinery that's likely to be used is an important factor that will affect the grade of the grassed batters.
  • Ramps that allow access to the invert are essential and should be spaced about 400m apart. The ramp should be 3m wide at a maximum grade of 1v:8h.
  • Reversing distances for vehicles should be less than 200m. This can be achieved by providing turning bays or exit ramps. The radius of the minimum turning circle for a truck is 15m, while the maximum grade that they can turn on is 1v:6h for dry conditions or 1v:15h for wet conditions.
  • Provide 4.5 metre wide maintenance tracks or 4 metre wide berms, on one or both sides of the channel. Sections of the maintenance track up to 100m in length, may be reduced to 3.5m width, if absolutely essential. Allow for back stays on chainmesh fences which will encroach 600mm onto the track.
  • Use sloping drops in the invert rather than vertical drops to facilitate maintenance vehicle movement.
  • If the channel isn't to have a berm or maintenance track, it's desirable to design the channel batters with a grade no steeper than 1v:4h. This increases the flow capacity and improves public safety.
  • Grassed channel batters with a grade of 1v:5h or flatter can be maintained with a slasher mounted on a tractor. The maximum allowable grades the tractor can operate on are 1v:4h (dry conditions) and 1v:10h(wet conditions).
  • Batters with a grade of 1v:4h or steeper are maintained with slope mowers. These will operate from either the berm or invert. For batters to be maintained from the invert, it'll be necessary to increase the invert width to 3m to allow for the slope mower. The maximum allowable grades the slope mower can operate on are 1v:10h (dry conditions) and 1v:40h (wet conditions).
  • Where batter slopes are steeper than 1v:5h or aren't easily accessible for regular maintenance, the designer should consider planting the batters out with low-lying shrubs rather than grass.
  • Inverts of grassed floodways should be dished with a minimum side grade of 1v:20h.
  • When a silt trap is provided within a channel, adequate access must be provided for maintenance and area set aside for drying out the wet sediment.


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