Water allocation

Dams

Dams in the Auckland Region

In the Auckland Region dams have mainly been built for stock water or irrigation supply. In some places where groundwater yields are low and summer stream flows unreliable, dams may be the only secure supply of water for these purposes.

Recently there has been an increasing trend in the construction of dams for ornamental or aesthetic reasons, or for the creation of wildlife habitat.

Latest surveys show that there are approximately 4,500 dams in the Auckland Region.

Authorisation

All dams require appropriate authorisation by the ARC regardless of whether they were constructed long ago or more recently. This is the responsibility of the current land owner. This authorisation can be in the form of a Permitted Activity under rules 6.5.46 - 6.5.50 of the Proposed Auckland Regional Plan: Air, Land and Water (PARP:ALW) (which does not require a resource consent) subject to certain conditions. For proposed dams, you must notify the ARC of your intention to construct the dam using a Dam notification form at least 15 working days prior to construction.

If your dam does not comply with all of the conditions of the Permitted Activity rule (for example, the size of the catchment upstream of the dam is too large, or the dam wall is too high) you will need to apply for a Resource Consent to Dam Water. In order to do this you will need to complete an application form to Dam Water.

On Stream and Off stream dams

A dam built on a permanent stream is an ‘On Stream Dam'. A dam constructed on an intermittent stream is an ‘Off Stream Dam'. Follow the link to the definition of a permanent and intermittent stream. The definition of permanent and intermittent streams are set in the PARP:ALW, Chapter 12 Definitions and Abbreviations.

Most on stream dams constructed since October 2001 are non complying activities in terms of the Proposed Auckland Regional Plan: Air, Land and Water 2001. It is very difficult to obtain resource consent for an on stream dam built since October 2001 unless there is a clear environmental benefit and there are no realistic alternatives.

The Effects of Dams on the Environment

Damming water can alter stream hydrology and affect a stream's ability to deal with contaminants. This leads to poor water quality, and a reduction in available stream habitat. These factors not only affect our freshwater ecosystems but also affect the availability of water for downstream users.

Dams built across streams can block fish from accessing upstream habitat. The damming of water is likely to be a contributing factor in the decline of New Zealand's freshwater fish populations. For more information on native fish and fish passage refer to Technical Publication 131

The construction or existence of even the smallest dams can become a contentious issue between neighbours. It is important that dam structures and the water they impound are located well within property boundaries so that they will not affect adjacent, upstream or downstream properties in any way.

There is a risk that a dam will fail. This can damage the downstream environment, including freshwater ecosystems, property, people, communities and infrastructure.

Alternatives to Damming Water

Before planning to build a dam it is important to check that this is your only option. Where readily available, groundwater is a more favourable water supply option than damming water. You could also consider installing tanks to collect roof water.

Design, Dam Safety and Monitoring

When designing your dam it is important that you consider the Auckland Regional Council's Dam Safety Guidelines

If you have a resource consent for your dam you will automatically be added to the ARC's dam monitoring programme. However, with all dams it is always a responsibility of the dam owner to regularly inspect their dam for any signs of seepage through the embankment or along pipes, to control any vegetation growth and to maintain the dam and its spillways in good structural condition.

One of the most commonly encountered problems with dams is vegetation growing on or near the embankment. Vegetation prevents thorough visual inspections of the embankment condition and root systems can provide pathways for seepage through the embankment, which can lead to the dam breaching. No vegetation other than grass should be allowed to grow on or near the embankment and should be carefully removed with prior advice from an engineer or the ARC. Once large vegetation becomes established (i.e. trees) it may be safer to keep them in place and monitor them. Removing the tree could cause its roots to rot and water to seep through the resulting voids.

Building Act Dams

If you own a dam that impounds greater than 3 m depth of water and stores over 20,000 m³ of water, you may be affected by the new Building Act 2004. Learn more about how the new Building Act 2004 relates to large dams.

Stream Crossings

Most stream crossings involve placing a culvert in a stream and placing fill over it. These crossings can dam water if the culvert pipe is undersized or if the culvert pipe is not placed below the natural level of the stream bed.

A well constructed crossing involves placing the culvert pipe slightly below the stream bed to allow unimpeded stream flow under all flow conditions and free fish passage. Learn more about fish passage guidelines.

To avoid requiring authorisation for a dam, do not construct raised or perched culverts. If there is any increase in water level upstream of the culvert it is considered to be damming water.

Chapter 7 of the PARP:ALW has Permitted Activity Rules relating to culverts. If the conditions of these rules cannot be met you may require a resource consent from the ARC Sediment and Stormwater Team.

Is Your Flood Spillway Big Enough to Safely Pass a 100 Year Event

Flood spillways must be constructed in natural ground away from the dam embankment so that they can safely pass water during large flood events without the embankment overtopping, or the spillway eroding - both of which can cause catastrophic failure of the dam.

Adequate flood spillway sizing can be worked out using the size of the catchment upstream of the dam. If you need help working out your catchment area you can contact the ARC Water Allocation Team. For more information regarding a suitable flood spillway design, please refer to the ARC Dam Safety Guidelines.