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  Services » Regional Services » Consents and compliance » Dams, safety requirements and consents » What you need to know

What you need to know

What is the relationship between regional councils and dams?

All councils that wish to fulfil building consent functions are required to be accredited by The Ministry of Business, Innovation and Employment (MBIE) as a Building Consent Authority (BCA).

The Building Act 2004 (the Act) introduced changes that affect the construction of dams. The intent of the Act is to encourage better practice in building design, construction, and regulatory building control.

A key change implemented by the Act is the transfer of responsibility for building consent matters, regarding dams, from local councils on to regional councils. To prevent duplication of services and to provide greater efficiencies a transfer agreement has been reached between the Waikato Regional Council Auckland Council and other North Island regional councils.

This means that the regional councils have the regulatory responsibility to ensure large dams are well built, and the potential risks to people and property are minimised.

If you own a dam or canal, or are planning to construct, modify or remove a dam, canal or a structure associated with a dam, then this applies to you.

What applications can Waikato Regional Council process as a BCA?

WRC are responsible for performing functions under the Act relating to a “large dam” only. Refer Section 14(external link)

Do I need a resource consent or building consent to build a dam?

Resource consents

Resource consents are often required for dams as they involve the taking, using, damming and diverting of water and controlling of the quantity, level and flow of water. Also, the works associated with the dam construction itself can often trigger the requirement for resource consent.

Building consents

A building consent is required for all structures that meet the definition of a ‘large dam’ (see figures 1 and 2), as well as those structures that form part of the large dam structure itself — such as appurtenant structures.

The construction of any dam, that is not a large dam is ’exempt”(external link) building work, and does not require a building consent, nevertheless the Act requires all building work on dams to comply with the Building Code irrespective of the size of the dam. Refer Section 17(external link)

This applies to the construction of new dams, the modifications of existing dams and also applies to appurtenant structures of dams.

Once details of your project are known you should discuss your development with your relevant regional and local council to identify if any resource or building consent will be required.

How are dams defined?(section 7(external link))

Section 7 of the Act provides a definition of dams and large dams.

A dam:

  1. means an artificial barrier, and any connected structures, that –
    1. is constructed to hold back water or other fluid under constant pressure so as to form a reservoir
    2. is used for the storage, control, or diversion of water or other fluid
  2. includes:-
    1. flood control dam,
    2. a natural feature that has been significantly modified to function as a dam
    3. a canal; but
  3. does not include a stopbank designed to control flood waters.

What is an “appurtenant structure” (section 7(external link))

In relation to a dam, means a structure that is integral to the safe functioning of the dam as a structure for retaining water or other fluid.

What is a “large dam” (section 7(external link))

Means a dam that has a height of 4 or more metres and holds 20,000 or more cubic metres volume of water or other fluid.

Cross section of a large dam

Volume of dam

MBIE have referenced the “holding capacity” of a dam as being the “volume or peak flow of the flood that would cause the reservoir level to reach the crest of the dam with full use of the spillway(s) of the dam.

Measurement of dams (section 133B)(external link)

The height of a dam is to be measured as the vertical distance from the crest of the dam, as follows:

  • in the case of a dam across a stream - the height is to be measured from the natural bed of the stream at the lowest downstream outside limit of the dam; and
  • in the case of a dam that is not across a stream - the height is to be measured from the lowest elevation at the outside limit of the dam; and in the case of a canal - the height is to be measured from the invert of the canal.

Crest of dam (section 7)(external link)

The crest of a dam is the uppermost surface of the dam, not taking into account any camber allowed for settlement, or any curbs, parapets, guard rails, or other structures that are not part of the water-retaining structure; and for the avoidance of doubt, any freeboard is to be considered as part of the water-retaining structure.

Guidelines for rural owners of dams

For the purpose of defining whether a dam meets the large dam threshold, the volume shall be based on a reservoir level based on one of the following:-

  1. Where there is only an outlet pipe (and no overflow flood spillway, or the overflow flood spillway is less than 3.0 metres wide) – the reservoir level at the dam crest.
  2. Where there is a free overflow flood spillway (minimum 3.0 metres width) and the design hydrology is not known or current (typical of rural dams), the reservoir at the spillway crest level plus 300mm of spillway operation.
  3. Where the hydrology is current and spillway capacity meets NZSOLD Dam Safety Guidelines based on the dam impact classification (PIC) – the reservoir level corresponding to the dam operating at its design flood level;

What expert input is required?

Owners should consider obtaining suitably qualified professional advice to assist them with the requirements for the construction and ongoing monitoring of dam structures.

This advice may consist of, but not be limited to:

  • preparation of the necessary building consent and resource consent applications
  • preparation of the necessary drawings and engineering calculations
  • assessment of the dam against regulatory standards

Engineering details

The building code is non-specific in regard to dam design. For this reason, all dam applications will be assessed using the New Zealand Society of Large dams (NZSOLD Dam Safety Guidelines(external link) as an alternative solution.

Alternative solution

An alternative solution is a design solution which differs from the acceptable solutions given in the approved documents - yet complies with the New Zealand Building Code. It can include a material, component or construction method that differs completely or partially from those described in the approved documents.

For further information, refer to: MBIE website(external link)

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