Sections

Understanding and Managing Flood Risk

Please note that this paper is held here for archival purposes, the most up to date papers are currently held on the Engineers Australia website.

Introduction

This paper is concerned with floods in Australia, and the means by which the adverse impacts of flooding can be minimised.  Its purpose is to provide a general understanding of the causes of floods, assessment of flood risk, and the benefits of appropriate floodplain management.

It needs to be stated that Australia has a highly variable climate, and floods are a natural consequence of this.  However, while natural floods confer great benefits to the environment, many Australians may be at risk from flooding in their homes, at work, or while travelling.  Such floods can be widespread, causing major damage and disruption to commercial and agricultural activities: alternatively they can be quite localised in extent.

While it is not possible (nor desirable) to eliminate floods, the economic and social impacts of damaging events can be mitigated in a number of ways.  These include controls on land use and development in flood-prone areas, public education and warning systems, and flood-mitigation structures.  More details on each of these are given later in this paper.

Understanding Floods

Most floods are caused by heavy rainfall, but there can be other causes, including dam failure, blockage of streams and/or drainage systems, and (in coastal areas) storm surges and tsunami.  Because they dominate the flood issue in Australia, this paper deals primarily with rainfall-generated floods.

For a particular storm magnitude, there are a number of factors which determine the resulting flood flow at a given site.  These include:

It is important to note that changes in these factors will change flood behaviour.  Urbanisation, in particular, has a major impact on flood response to rainfall; the average annual peak flow can easily be increased by a factor of 5-10, and response times are much faster (perhaps only 10-20% of those of the natural catchment).

While the magnitude of a flood event is generally measured by the peak flow, the severity of the flood hazard also depends on a number of other flood characteristics:

These characteristics would normally be determined for a location by specialists using computer models to simulate flood behaviour for a range of event severities.

Risk of Flooding

The degree of flood risk for a specific location depends on both the frequency of damaging flood events and the consequences associated with these events.  Frequency of flooding is expressed by the probability that a given flood height (or river flow) will be equalled or exceeded in any year, termed the Annual Exceedance Probability (AEP).  [A related term (the reciprocal of AEP) is the Average Recurrence Interval (ARI), which is the (long-term) average number of years between events of given magnitude.]

A flood event criterion commonly used in flood risk assessments is the 100-year ARI or the 1% AEP.  This criterion is based upon the acceptance of a 1% chance of that event being exceeded within any one year. Statistics can be used to estimate the risk associated with occurrence of floods for any given period. For example, the odds of experiencing at least one 100-year ARI flood in a 25 year interval – perhaps a typical period of home ownership – are about 2 in 9 (or 22%).

Implicit in most analyses of flood risk is the assumption that the general climatic conditions do not vary and that past flood information is a reliable indicator of potential future events.  Recent research on the Greenhouse Effect indicates that this assumption may not be valid, and that potential climatic changes may need to be incorporated into future flood risk assessment. However, at present it is not possible to quantify the magnitude of potential climatic and streamflow changes, nor is there sufficient evidence yet to justify the inclusion of potential changes in assessments of flood risk. This situation may change in the future.

The risk of flooding can also change over time due to changing catchment conditions upstream.  Urbanisation, for instance, will almost always increase the volume and peak flows of flood runoff.  On the other hand, measures such as farm dam construction and soil conservation measures may reduce the flows for small to medium events.

The flood risk at a location, therefore, is not constant, and needs to be re-assessed at appropriate intervals.

[The basis of all flood risk assessment is data, either streamflow data collected primarily by Statutory Authorities or meteorological data collected by the Bureau of Meteorology. These data benefit many parts of society and, in the area of flood risk assessment, are essential for the determination of the flood risk.  It is important that these data collection programs continue.]

Mitigation of Flood Impacts

Engineers (and others) play a key role in the development of appropriate flood awareness in the public and in the mitigation of flood impact upon communities. There are three general approaches.

  1. The best outcome will be had by not having residential and commercial development in flood-prone areas in the first place.  Areas at risk can be shown on flood maps (or planning overlays) to indicate land subject to flood inundation (eg at the 100 year ARI level), and planning and land zoning legislation applied to them. These controls should be based upon assessment of the flood risk determined through a thorough hydrological and hydraulic analysis of flooding in the catchment. The responsibility for implementation of these controls currently rests with State and Local Government Authorities. 

    It is noted that there may be differences in standards applied in different regions of Australia, and even in different regions of the same state, based on the nature of the flood risk.
  2. A cost-effective way to reduce flood impacts in developed regions is to provide warnings in time for action to be taken (eg removal of people and property from the affected areas).  The magnitude of economic and social losses associated with a flood event can be significantly reduced with enough lead-time. In Australia, flood forecasting and flood warning services are provided primarily by the Bureau of Meteorology, with local authority involvement to disseminate the warning, provide flash flood warning services and coordinate the response action.
  3. The third approach for flood mitigation involves structural measures such as providing ‘flood-space’ in storage reservoirs, and/or constructing retarding basins, levees, and flood channels.  It is applied mainly when existing development is exposed to unacceptably high levels of flood risk.  Generally, structural measures are expensive (especially in retro-fit situations), highly visible, and require regular maintenance. Further, they may generate the perception that the area protected for the design level used is free from risk for even larger events; community education is necessary to negate this. 

An integrated approach to flood mitigation will generally involve a combination of measures and, if they are needed, multi-purpose structural measures.  Hence, retarding basins can be public open space with high amenity values, roads and paths can ‘double’ as floodways, artificial lakes can have flood ‘air-space’ included.  The infrequency of flood events makes such multiple uses highly desirable.

The perception by the local community of the flood risk is also important with those communities in districts where flood events have not occurred in recent times generally having a low perception of their risk. Public information and education programmes are essential to ensure that local communities are aware of their flood risk.

What Engineers Australia is doing

Engineers play an important role in assessing the risk of communities to flood events and the development and implementation of appropriate mitigation measures to reduce the severity and frequency of flooding and its damaging impacts. Through its National Committee on Water Engineering, Engineers Australia:

What Communities can do

For further information please contact

National Committee on Water Engineering,
The Institution of Engineers, Australia
11 National Circuit
BARTON ACT 2600

Telephone. (02) 6270 6555
Facsimile: (02) 6273 148

A position paper prepared by the National Committee on Water Engineering, Engineers Australia.