This section describes how to:

• inspect ponds and dams, and identify potential causes of failure
• review ponds and dams periodically
• maintain ponds and dams.

The responsible person for an operation must take all practicable steps to eliminate, isolate or minimise hazards associated with ponds and dams. Safe systems of work should identify and control any risks to workers and anyone else who may be affected by activities associated with ponds and dams (including nearby landowners). This includes workers who need access to potentially hazardous areas for purposes such as carrying out inspections and cleaning out ponds or dams.

The responsible person must make sure a competent person examines ponds where workers are, or will be, before the start of each working shift and at suitable times during the shift. This practice should also be applied to dams.

If a principal hazard management plan (PHMP) for tips, ponds and voids at a mining operation requires regular inspections to be carried out, then the PHMP must specify how often inspections occur. Every accessible area of dams or ponds, including areas with barriers, should be inspected by a competent person at least once a week.

For more information on PHMPs, planning and design criteria, geotechnical assessments and construction of ponds and dams see Section 7.

On this page

• 13.1 Inspecting for signs of failure
• 13.2 Technical operational review
• 13.3 Cleaning out ponds and dams

13.1 Inspecting for signs of failure

Once a dam or pond has been constructed, regular monitoring (including routine visual inspections) and maintenance should be carried out to minimise the risk of failure.

The most common failures for a typical small earth dam or pond are surface erosion from overtopping, internal erosion (such as piping or seepage) and embankment slumping. These failures can arise from defects such as inadequate spillways, uncontrolled seepage, design and construction deficiencies, and a lack of maintenance.

If any of the following signs of distress or other unusual characteristics develop, take immediate action to ensure safety, and engage a technical expert to investigate, to make sure the dam or pond is safe and compliant with all necessary regulations.

Upstream slope

Inspect the upstream slope of an earth dam or pond for any signs of erosion, beaching or slumping. These may be caused by wave action, flooding, or a rapid drop in the water level.

A damaged upstream slope reduces the stability of the dam by limiting its ability to resist wave action and high water levels. Failure of the upstream slope can result from undercutting, erosion, depressions, and other signs of a possible slip or landslide.

Crest

Inspect the crest of a dam or pond for shape and cracks. A variation in levels across the top of the dam or pond may indicate abnormal settlement (vertical downward movement) or an underlying void and internal erosion that could cause the eventual failure of the dam or pond.

Dewatering and overflow channels

Check dewatering channels for weed growth and side collapses. Safety issues include edge collapse while inspecting, silt build-up in the channel, and vegetation hiding undermined edges.

Overflows can be decanting pipes, angled pipes, spillways or armoured channels. Inspect these regularly, particularly after substantial rainfall.

Check for blocked intakes of decant, or angled pipes clogged with vegetation or other debris. A significant hazard when clearing blocked intakes is the sudden release of water into the pipe, which can suck someone into the intake, causing injury or drowning. Blockages should only be cleared with machinery or tools that keep people away from the intake.

Clear partially blocked overflow channels quickly and safely. Put in place remedial measures to limit the amount of floating vegetation. Make sure armoured channels are not scoured when there is high water flow, as this can erode the crest and affect the stability of the embankment.

Downstream slope

Inspect for seepage when the water is at or near its highest level. Examine the downstream slope, downstream toe, abutments, and areas near spillways or outlets.

Seepage can be indicated by wet spots or muddy areas that are usually accompanied by the lush growth of tussock and other grasses. The use of piezometers will help you to detect seepage early and should be considered as a control measure. You may need to get specialist advice from a competent person about the type of piezometer to use and where to place them.

Small amounts of steady seepage (not concentrated flows) are not usually a serious concern, as long as there is controlled drainage and ponding does not occur. An area of known seepage that suddenly stops or significantly decreases may indicate an area of distress and should be investigated.

13.2 Technical operational review

A competent person should undertake regular technical operational reviews to make sure the dam or pond is operating as intended and meeting all regulatory requirements.

Technical operational reviews should:

• check that previous review recommendations have been actioned
• confirm appropriate responses have been given to any incidents or issues
• verify compliance with specifications (for example, inspection, monitoring and quality control) and regulatory requirements
• validate the continued use of the dam or pond design
• recommend any necessary operational or design modifications.

The type and level of information provided in the technical operational review should be consistent with your risk assessment for dams and ponds.

Keep a record of review outcomes indicating recommended actions and details of how issues were addressed or solutions implemented. Seek specialist advice from a competent person if necessary.

If the tip or pond is a dam under the Building Act 2004 and requires a dam safety assurance programme (DSAP) under the dam safety scheme, the DSAP will also include requirements for inspection and review of the dam. For more information on DSAPs and requirements, see the Building Performance website(external link)

13.3 Cleaning out ponds and dams

The main risks when cleaning out ponds and dams are created by undercutting and making the embankment unstable (particularly under water), or by mobile plant driving on too soft ground that cannot support the plant’s weight.

Settling ponds can be deceptive, as they can form a crust which appears stable but the silt beneath remains soft. Mobile plant should be kept back from the edge by at least a distance of 1.5 times the height of the face (see Figure 62).

Removing silt

Carry out a risk assessment to identify the appropriate methodology and plant to use to remove silt. The most common way is to use an excavator. Other methods include using dredges, suction pumps or vacuum pumps.

Operators using mobile plant, such as an excavator, should constantly monitor the crest for signs of slumping, cracking or instability. If there are signs of instability, all work should stop, workers and plant should be removed, and access to the area prohibited. Seek geotechnical advice if necessary.

The mobile plant operator should only remove silt as planned, and not excavate the pond or dam’s retaining structure. The edge of the silt pond should always be clearly marked, ideally by barriers such as a bund.

The mobile plant should be as far from the lagoon edge as operationally possible, and should be capable of obtaining the necessary depth of dig while maintaining this distance. The mobile plant’s tracks should be perpendicular (at a right angle) to the pond edge, so a safe, rapid exit from the area can be made if the slope suddenly becomes unstable.

The excavated silt should be removed as far from the crest of the pond or dam as possible to prevent loading of the crest, causing it to fail. Make sure any removed silt does not block the safe exit route of the mobile plant.

If excavated silt is loaded directly into a truck, the type of vehicle used should be risk assessed to ensure it is fit for purpose. Consider the type and moisture content of material being loaded, and the load shifting in the tray of the vehicle.

When not in use, all mobile plant should be parked safely away from the water’s edge.