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Guidance for regional authorities, contractors and owners of small dams

Small dams and the Building Code

On this page:

  1. Purpose of this article
  2. Introduction
  3. What happens if building work does not comply with the Building Code?
  4. The construction of a dam
  5. Small dam inspection and tell tale signs of distress
  6. Upstream slope
  7. Crest
  8. Downstream slope
  9. Further information

Purpose of this article

The purpose of this document is to assist owners of small dams, contractors, and regional authorities understand how the construction of small dams relates to the Building Code. This document is not a guide prescribing how you should build a small dam but rather an explanatory note on how you should consider the Building Code when building a small dam.

Introduction

Small dams play an important role in the lives of many rural New Zealanders and are often used to store or divert water for rural activities.&nSmall dams (that are not large dams as defined by section 7 of the Building Act 2004 [the Act]) do not require a building consent1. This is because a small dam would pose generally less risks to people or property as a consequence of a failure.

A dam is defined as:

(a) means an artificial barrier, and its appurtenant structures, that –

(i) is constructed to hold back water or other fluid under constant pressure so as to form a reservoir; and

(ii) is used for the storage, control, or diversion of water or other fluid; and

(b) includes –

(i) a flood control dam; and

(ii) a natural feature that has been significantly modified to function as a dam; and

(iii) a canal; but

(c) does not include a stopbank designed to control floodwaters.

A small dam can be considered any dam that meets any of the following criteria:

(a) retains less than three metres depth; or

(b) holds less than 20,000 cubic metres volume; or

(c) retains less than three metres depth and holds less than 20,000 cubic metres volume.

However, the Act states that all building work carried out must comply with the Building Regulations 1992 (also known as the Building Code). Building work includes the construction and alteration of small dams even though they are exempt from requiring a building consent for this work.

The Building Code sets out performance standards that building work must meet, and covers aspects such as structural stability and durability. The Building Code does not prescribe how building work should be done (ie, no detailed requirements for design and construction), but states how completed building work, and its components, must perform. This is important when considering constructing a small dam, as each dam is unique to its location and environment. A dam, however, should be designed and constructed and maintained in a manner that safeguards people and property from structural failure and throughout its life continues to comply with the Building Code and have a low probability of failure. 

What happens if building work does not comply with the Building Code?

If a regional authority (council) considers on reasonable grounds that a small dam does not comply with the Building Code they may issue a notice to fix to the owner. A notice to fix is a statutory notice requiring a person to remedy a breach of the Act. i.e. a small dam constructed that does not comply with the Building Code. In extreme cases where the dam poses an immediate danger2 to persons, property or the environment, a council may order any action necessary to remove the danger.

The construction of a dam

The construction of a small dam, although not requiring building consent, will nevertheless require careful consideration concerning design and construction methodology. In the first instance, one should employ a technical expert to provide advice on designing and constructing a small dam to ensure it is fit for the purpose and it complies with the Building Code.

A small dam would be expected to the have the following features:

Foundation

The areas of ground on which the dam is located including the areas of ground adjacent, form part of the total water barrier. If the foundations do not adequately support the basic small dam structure, or are themselves weak or prone to high seepage flows, then they can cause the small dam to be rendered useless or to fail. The foundation of a small dam is often the natural materials on which it stands. A clean stable foundation of adequate strength and low permeability is vital for a small dam’s durability and performance. An adequate seal at the dam foundations and abutments must be formed to reduce leakage from the reservoir otherwise it may not fill or excessive seepage may cause failure of the dam.

Spillway

All dams require at least one working spillway. A flood spillway prevents high stream flows generated by heavy or prolonged downpours overtopping the dam crest causing subsequent erosion of the dam materials that may lead to a breach of the dam. The flood spillway is normally formed around the end of the small dam and extends downstream clear of the dam toe. The flood spillway was must be of a size adequate for flood flows expected for the rainfall and catchment size/topography.  A smaller service spillway for a small dam may also exist and will normally be a culvert/pipe which takes normal flows.

Storage capacity

The purpose of most dams is to store water for use when required. The volume of storage needs to be assessed to ensure it is sufficient for its intended purpose. Sufficient freeboard3 must be provided to prevent overtopping of the dam.

Embankment

The embankment must have a crest of sufficient width and may require protection if it is intended that vehicles or heavy stock will have access. The upstream and downstream slope angles need to be chosen carefully to ensure the embankment slopes are stable. The fill material needs to be carefully selected, sufficiently impervious, placed at the correct moisture level and thickness of each layer to ensure a high standard of compaction is achieved. Riprap4 may be required on the upstream face to protect the dam against wave lap erosion.

Pipes and Conduits

Pipes are often put through the bottom of the dam for the drawing of water; however it is important to note that these can also be weak point for seepage, causing erosion of the dam fill. Technical advice is recommended for correct design details where pipes pass through a dam. 

Small dam inspection and tell tale signs of distress

Once a small dam has been constructed it is an asset that requires monitoring and maintenance and it is important to ensure it maintains compliance with the Building Code to minimise the risk of it failing. The most common failure mechanisms for a typical small dam (earth dam) are surface erosion from overtopping, internal erosion (i.e. piping), and embankment slumping. These failures arise from defects such as spillway inadequacy, uncontrolled seepage, design and construction deficiencies, and a lack of maintenance. It is recommended that regular inspections are carried out in order to identify any tell tale signs of distress at an early stage when it is possible that maintenance and repair could prevent a potential failure.

Upstream slope

The upstream slope of an earth dam should be examined for any sign of erosion, beaching, or slumping. These may be caused by wave action, flooding, or a rapid drop in the reservoir capacity.

A damaged upstream face reduces the stability of the dam by limiting its ability to resist wave action and high reservoir levels.

Failure of the upstream slope can result from undercutting, erosion, depressions, and other evidence of the initiation of a possible slip or landslide.

Crest

The crest of the dam should be examined for shape and cracks. A variation in level across the top of the dam may indicate abnormal settlement (the vertical downward movement) or possibly an underlying empty space. Undetected this may lead to the eventual failure of the dam as result of piping (the progressive development of internal erosion).

Downstream slope

Ideally, an inspection for seepage should be made when the reservoir is at or near its highest level. Areas to be examined include the downstream slope, downstream toe, abutments, areas near spillways, and around and adjacent to outlets.  Seepage areas can be identified by wet spots or muddy areas, usually accompanied by the lush growth of tussock and other grasses. Small amounts of steady seepage (not concentrated flows) do not represent a serious condition, as long as controlled drainage is provided and ponding is not allowed to occur.

These are only a few of the more common signs of distress possible in a dam. If any of these signs of distress or other unusual characteristics develop a technical expert should be contacted to investigate the dam to ensure it is safe and compliant with the Building Code. 

Further information

This document has been based on the following articles of note. For further general information on inspecting small dams, constructing small dams or the Building Code please refer to the following:


  1. Although small dams are exempt from requiring a building consent, they may be required to have land use/ resource consent from a territorial authority or regional authority.
  2. Section 157 of the Act (Measures to avoid immediate danger) outlines what a regional authority can do to remove the danger to the safety of the people, property and environment. The approach each regional authority will take in respect to immediate danger is detailed within each regional authority’s Dangerous dams policy (refer to the Dam Safety Scheme). 
  3. Freeboard. The distance between normal reservoir level and the top of dam. 
  4. Riprap. A layer of large quarried stone, precast blocks, bags of cement, or other suitable material, generally placed on the slope of an embankment or along a watercourse as protection against wave action, erosion, or scour. Riprap is usually placed by dumping or other mechanical methods, and in some cases is hand placed. It consists of rock pieces of relatively large size, as distinguished from a gravel blanket