Table of contents
- The purpose of this document
- The issue
- Definitions of dams
- Elements that need to be considered for a structure to be a dam for the purposes of the Act
- Determination - structures to be considered for declaration
1 Purpose of this document
This page defines the types of structures that Dams Safety NSW will consider to be dams for the purposes of declaration under the Dams Safety Act 2015 (the Act).
2 The issue
Dams Safety NSW declares a dam, in accordance with Section 5 of the Act, when it meets the criteria prescribed in the Dams Safety Regulation 2019 (the regulation).
The criteria in the regulation are based on the consequence of dam failure (whether a dam failure endangers life, the environment, or the community).
The Act and regulation do not define what structures are taken to be a dam for the purposes of declaration.
This has led to questions from (potential) dam owners about whether their structure would be considered by Dams Safety NSW to be a dam for the purposes of the Act.
Types of structures that have been proposed as dams for the purposes of the Act, and which are evaluated here, include:
- off-river storages
- retarding basins
- road/rail embankments
- concrete or steel service tanks/reservoirs.
3 Definitions of dams
Because there is no definition of a ‘dam’ in the Act, the criteria for what constitutes a dam follows the generally accepted usage of the word ‘dam’.
3.1 Macquarie Dictionary definition of a dam
‘A dam is a barrier to obstruct the flow of water, especially one of earth, masonry, etc., built across a stream.’
Important aspects of the definition:
- because a dam is a ‘barrier to obstruct the flow of water’ it follows that the structure’s intention is to obstruct the flow
- the barrier is built across a stream, and
- because the barrier obstructs the flow across a stream, it follows that the barrier must be able to pass or release at least some of its contents. Otherwise the dam contents may continue to fill from the stream inflow until the dam overtops (or fails).
3.2 ANCOLD definition of a dam
The Australian National Committee on Large Dams (ANCOLD) general description of a dam is:
‘A dam is built to control and store water. Dams are made from earth, rocks or concrete and are usually constructed on rivers to store the water in a reservoir.’
The more precise ANCOLD (1) definition is:
An artificial barrier, together with appurtenant works constructed for storage, control, or diversion of water, other liquids, silt, debris, or other liquid-borne material.
3.3 The Act
The Act (section 19(2)) includes the wording: ‘…including the water or other material impounded by the dam…’.
This can also be used to determine whether a structure is a dam. That is, if its contents are ‘impounded’ by the structure.
4 Elements that need to be considered for a structure to be a dam for the purposes of the Act
For the purposes of the Act, a dam is a structure that is:
- intended to obstruct a flow and/or impound(2) its contents
- built(3)across a stream (or source)
- able to pass or release(4) at least some of the impounded contents.
The above three elements are used to determine what structures are taken to be a dam for the purposes of declaration.
Generally, all three of the above elements should be satisfied for a structure to be a dam for declaration purposes. However, there may be additional reasons for including or excluding a dam for consideration for declaration – see the table for details.
The following table summarises whether a structure is taken to be a dam for declaration purposes, based on the three elements above, with additional notes to provide clarification or justification for the determination.
5 Determination - structures to be considered for declaration
|Structure||Intended to impound its contents?||Across a source to obstruct a flow?||Able to pass or release its contents?||Considered to be a dam for the purposes of the Act?|
|Above ground concrete or steel tank||No(14)||No||Yes||No|
1. A structure that is currently declared will continue to be a declared dam.
2. There may be some structures that are on the boundaries of the rationale described by the element descriptors and table footnotes in this paper. Dams Safety NSW will consider each of these 'grey area' structures on their merits, with prime consideration given to whether the community would expect Dams Safety NSW to include the structure as a dam for declaration purposes, which is consistent with the objects of the Act.
(2) 'impound' is the wording used in the Act
(3) 'Built' is consistent with the ANCOLD definition of 'constructed' and 'an artificial barrier'
(4) Able to 'pass or release' is consistent with the ANCOLD definition 'control'
(5) This includes water supply dams and hydroelectric dams that are built across a source and consist of a constructed dam wall and discharge control – the classic ‘dam’ in accepted use
(6) Noting that weirs are constructed within the river channel and therefore the failure effects are usually confined mostly within the river channel
(7) Most off-river storages have the characteristics of a water supply dam, with the difference usually being their means of filling – intermittently by run-off, or via a channel or pipeline
(8) ‘yes’ applies to those storages with constructed dam walls and failure characteristics typical of water supply dams
(9) In-pit tailings with an in-pit embankment wall may also be considered
(10) Included as a dam for declaration as these dams were specifically mentioned in the Dams Safety Act 2015 second reading speech as ‘dams that play a vital role in containing contaminated run-off from mining activities’
(11) If the embankment is intended to impound water, is built across the source and is able to pass or release the contents, then ‘yes’
(12) A levee is not intended to impound its contents. A levee is intended to protect assets from overflow of a water source, its intent is to ‘augment the riverbank’ to allow a large volume of water to flow downstream.
(13) A levee will usually only pass or release the water when it fails. A levee may (very rarely) incorporate a spillway.
(14) They are intended to store water (as in the sense of a container) rather than intended to impound water (as in the sense of a reservoir). They generally do not have failure characteristics typical of constructed wall dams.