AEP (Annual Exceedance Probability)

A term used to describe the frequency or probability of floods occurring. Large floods occur rarely, whereas small floods occur more frequently. For example, a 1% AEP flood occurs (or is exceeded) on average once every 100 years. A so-called 100-year flood does not mean that there is only one flood of this size every 100 years. It means that there is a 1 in 100 chance in any given year that a flood of this size or bigger will happen; it is therefore more correctly called a 1% AEP flood. In any given year, there is about a 65 percent chance that there will be at least one 1% AEP flood in populated catchments of New Zealand (NIWA 2007).

Block slide:  

A translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a single unit (Wold, 1989).

EDITORIAL NOTE: Translational in this context means the movement of a mass in such a way that every point moves in the same direction from one place to another more or less over the same distance.

Debris avalanche  

A very rapid to extremely rapid landslide on a steep slope which is unconfined to a channel. Debris avalanches often initiate debris flows.

Debris flow

A form of rapid mass movement in which soils, rocks, and organic matter combine with entrained air and water to form a slurry that then flows down a slope in a confined channel. Debris flows are associated with steep confined gullies (Wold, 1989).

Earth flow

A bowl or depression forming at a head where unstable material collects and flows out.  The central area is narrow and usually becomes wider as it reaches the valley floor. Flows generally occur in fine-grained materials or clay-bearing rocks on moderate slopes and with saturated dry conditions. Dry flows of granular material are also possible. Earth flows have a characteristic 'hour glass' shape (Wold, 1989).


The landscape of all hazards in a particular place or the net result of natural and man-made hazards and the risks they pose cumulatively across a given area.

Lateral spreads

The result of the nearly horizontal movement of geologic materials, distinctive because they usually occur on very gentle slopes. The movement is caused by liquefaction triggered by rapid ground motion, such as that experienced during an earthquake (Wold, 1989).


A process that causes some soils to lose their strength and behave more like a liquid than a solid during an earthquake.

Mitigation measures

Mitigation involves taking steps to reduce the likelihood of a natural hazard occurring or the consequence of its impact. 
Mitigation aims to reduce the likelihood of a natural hazard occurring and/or reduce the consequences of a natural hazard event.
Mitigation measures will differ depending on the activity, location and nature of the particular hazard but may include:

  • structural measures, for example, constructing a stopbank and raising floor levels within areas subject to flood hazard
  • non-structural measures, for example, revegetating a hillside to reduce landslide hazard.

Natural Hazard  

RMA 1991 definition of natural hazard

’Natural hazard means any atmospheric or earth or water related occurrence (including earthquake, tsunami, erosion, volcanic and geothermal activity, landslip, subsidence, sedimentation, wind, drought, fire, or flooding) the action of which adversely affects or may adversely affect human life, property, or other aspects of the environment.'

CDEM Act 2002 definition of hazard

’Hazard means something that may cause, or contribute substantially to the cause of, an emergency.'

Building Act 2004 definition of natural hazard

’Natural hazard means any of the following:

  • erosion (including coastal erosion, bank erosion, and sheet erosion)
  • subsidence
  • inundation (including flooding, overland flow, storm surge, tidal effects, and ponding)
  • slippage.'

Net present value

The net present value (NPV) method of evaluating a major project allows the changing value of money over time to be considered. Essentially, it helps find the present value in ‘today's dollars' of the future net cash flow (or the value) of a project. It is then possible to compare that amount with the amount of money needed to implement the project.
If the NPV is greater than the cost, the project will be beneficial. If there is more than one project being evaluated, it is possible to compute the NPV of each, and choose the one with the greatest difference between NPV and cost.

Overland flow   path

The route taken by stormwater which becomes concentrated as it flows overland, making its way downhill following the path of least resistance towards the stormwater   network, streams or the coast. Overland flow paths vary in width depending on the shape of the ground over which they flow, but once the contributing catchment area exceeds 30,000 m2, they are referred to as major overland flow paths. Overland flow paths include secondary flow paths which result when the piped stormwater system gets blocked or when the capacity is exceeded. Secondary overland flow paths are the backup stormwater system.

Precautionary   Principle

Defined as "the lack of full scientific evidence shall not be used as reason for postponing cost-effective measures to prevent environmental degradation"(1992 Rio Declaration).

Qualitative   analysis

In the context of natural hazard management qualitative analysis means using words to describe the magnitude and likelihood of potential consequences arising out of a natural hazard event.

Quantitative   analysis

In the context of natural hazard management means using numerical values for both the magnitude and likelihood of natural hazard consequences that may occur.


Developing operational systems and capabilities before an emergency happens. These include self-help and response programmes for the general public, as well as specific programmes for emergency services, utilities, and other agencies.


Activities beginning after initial impact has been stabilised and extending until the community's capacity for self-help has been restored.


Identifying   and analysing long-term risks to human life and property from natural or   man-made hazards, taking steps to eliminate these risks where practicable   and, where not, reducing the likelihood and the magnitude of their impact.


Actions taken immediately before, during or directly after an emergency, to save lives and property, as well as to help communities recover.


One or more pieces of rock falling from a steep rocky slope whether one at a time or all at once.

Rotational   landslide

A landslide in which the surface of the rupture is curved concavely upward (spoon shaped) and the slide movement is more or less rotational about an axis parallel to the contour of the slope (Wold, 1989).

The SMUG system  

Acronym for Seriousness, Manageability, Urgency, Growth where:
Seriousness is the relative impact in terms of people and/or dollars
Manageability is the relative ability to reduce the risk (through managing the hazard or the community or both)
Urgency is the measure of how imperative or critical it is to address the risk (associated with the probability/likelihood of the risk from the hazard, including return period considerations)
Growth is the rate at which the risk will increase (through an increase in the probability of the extreme event occurring, an increase in the exposure of the community, or a combination of the two).
The SMUG system, advocated by the Ministry in 2002, has been subsequently modified by many CDEM groups. Many groups removed the 'urgency' component (as it may be adequately covered under 'seriousness') and expanded the 'manageability' table to include the subcomponents of 'difficulty' (how difficult the hazard is to manage) and 'effort' (how much effort is currently being put into managing the hazard). Manageability ratings were then given to each subcomponent for each of the 4 ‘Rs' (reduction, readiness, response and recovery), thereby a manageability rating derived from eight manageability values.

The 4 'Rs' of community resilience

Reduction, Readiness, Response, Recovery


A block of rock that tilts or rotates forward, eventually to fall, bounce, or roll down the slope as a rockfall (Spiker & Gori, 2003). Often also used for the whole event, including the rockfall deposit.

Transitional slide

A landslide in which the mass of soil and rock moves out or down and outward with little rotational movement or backward tilting (Spiker & Gori, 2003).

Risk management definitions: as per AS/NZS standard 4360


Means the outcome or impact of an event.

NOTE 1: There can be more than one consequence from one event.
NOTE 2: Consequences can range from positive to negative.
NOTE 3: Consequences can be expressed qualitatively or quantitatively.
NOTE 4: Consequences are considered in relation to the achievement of objectives.

Event occurrence

Means: of a particular set of circumstances.
NOTE 1: The event can be certain or uncertain.
NOTE 2: The event can be a single occurrence or a series of occurrences.


A measure of the number of occurrences per unit of time.


A source of potential harm.


Used as a general description of probability or frequency.

NOTE: Can be expressed qualitatively or quantitatively.


To check, supervise, observe critically or measure the progress of an activity, action or system on a regular basis in order to identify change from the performance level required or expected.


A measure of the chance of occurrence expressed as a number between 0 and 1.

NOTE 1: ISO/IEC Guide 73 defines probability as the 'extent to which an event is likely to occur'.
NOTE 2: ISO 3534-1:1993, definition 1.1, gives the mathematical definition of probability as 'a real number in the scale 0 to 1 attached to a random event'. It goes on to note that probability 'can be related to a long-run relative frequency of occurrence or to a degree of belief that an event will occur. For a high degree of belief, the probability is near 1'
NOTE 3: 'Frequency' or 'likelihood' rather than 'probability' may be used in describing risk.

Residual risk:

The risk remaining after implementation of risk treatment


The chance of something happening that will have an impact on objectives

NOTE 1: A risk is often specified in terms of an event or circumstance and the consequences that may flow from it.
NOTE 2: Risk is measured in terms of a combination of the consequences of an event and their likelihood.
NOTE 3: Risk may have a positive or negative impact.
NOTE 4: See ISO/IEC Guide 51, for issues related to safety.

Risk analysis:

A systematic process to understand the nature of and to deduce the level of risk.

NOTE 1: Provides the basis for risk evaluation and decisions about risk treatment.
NOTE 2: See ISO/IEC Guide 51 for risk analysis in the context of safety.

Risk assessment:

The overall process of risk identification, risk analysis and risk evaluation.

Risk avoidance

A decision not to become involved in, or to withdraw from, a risk situation.

Risk evaluation:

A process of comparing the level of risk against risk criteria.

NOTE 1: Risk evaluation assists in decisions about risk treatment.
NOTE 2: See ISO/IEC Guide 51 for risk evaluation in the context of safety.

Risk identification:

The process of determining what, where, when, why and how something could happen.

Risk management:

The culture, processes and structures that are directed towards realising potential opportunities whilst managing adverse effects.

Risk management process:

The systematic application of management policies, procedures and practices to the tasks of communicating, establishing the context, identifying, analysing, evaluating,   treating, monitoring and reviewing risk.

Risk management framework:

A set of elements of an organisation's management system concerned with managing risk.

NOTE 1: Management system elements can include strategic planning, decision-making, and other strategies, processes and practices for dealing with risk.
NOTE 2: The culture of an organisation is reflected in its risk management system.

Risk reduction:

Actions taken to lessen the likelihood, negative consequences, or both, associated with a risk.

Risk treatment:

Process of selection and implementation of measures to modify risk

NOTE 1: The term 'risk treatment' is sometimes used for the measures themselves.
NOTE 2: Risk treatment measures can include avoiding, modifying, sharing or retaining risk.