Your Dam Your Responsibility (PDF~1.2MB)

4 Planning to Build a Dam
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2. Turkey’s Nest
This is a similar construction to the ring tank but the material is excavated from outside
of the embankment. All water is therefore stored above natural surface level allowing
gravity outflow for most of the contents.
3. Excavated Tanks and dragline holes
As the name implies, these are below ground level storages. The surplus material from
the excavation is not used to contain water and therefore the construction of this type
of dam does not normally come under the jurisdiction of the Water Act or this booklet
concerning safety aspects. If the purpose of the excavation is to intersect groundwater,
the excavation may be considered to be a bore and a construction licence may be
required. However a Take and Use Licence is required for irrigation or commercial use of
the water irrespective of whether it is surface water or groundwater.
Note: For structures Types 1 and 2 above, it is frequently considered not to require
a spillway, because they are filled by pumping and not by incoming streamflow or
overland runoff. However, each situation needs to be looked at on its merits: if there is
any risk that ‘overpumping’ may occur (eg. due to failure of the cut-out switch at the
pump) water may overflow the embankment, and if so may cause failure. A spillway
may be required in these circumstances, or some other means of securing against this
risk. Similarly, the design needs to provide security against overland runoff eroding the
outside toe of the embankment, thereby risking failure.
4.4 Site Investigations
4.4.1 Selecting the Dam Site
Locating a dam depends on a number of factors including:
• suitable valley or hillside that will catch enough water;
• location that will maximise the available catchment area and minimise the cost of the
dam;
• suitability of the foundations and materials available at the site; and
• location of the dam relative to where the water is to be used.
Each of the above points will require some degree of compromise to get the most costeffective
result.
There may also be a need to give thought to the visual impact of a proposed dam to
your neighbour, as they have a right to object to the structure if it affects their residential
amenity.
From an economic viewpoint, a dam should be located where the required storage
volume can be captured with the least amount of earthworks. This is generally possible if
the valley is narrow with a steep side-slope and the slope of the valley floor will permit a
deep basin. These sites also tend to minimise evaporation losses from the dam.
You should avoid locating the dam where run-off from houses, piggeries, dairies or septic
systems can pollute the water.
When choosing the location and size, you should also take into account what would
happen if the dam failed suddenly and whether it would result in loss of life, injury to
persons or livestock, damage to houses, buildings, roads, highways or railtracks.
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