How do we manage flood risks?
Flood risk includes both the chance of an event taking place and its potential impact. Land use planning informed by floodplain management plans can reduce risk for new development areas. Flood risk is harder to manage in existing developed areas; however modification measures such as dams or levees can change the behaviour of floodwaters. Similarly, property modification measures can protect against harm caused by floods to individual buildings, and response modification measures help communities deal with floods.
Flood risk includes both the chance of an event and its potential impact
Flood risk is a combination of the chance of a flood occurring and the consequences of the flood for people, property and infrastructure (Figure 12). The consequences of a flood depend upon how exposed the community is to flooding and how vulnerable its people, property and infrastructure are to the flood’s impacts.
Managing risks from floods may involve altering the chance of flooding affecting a community; and/or reducing the impacts of flooding by reducing the community’s vulnerability and exposure to flooding. The methods that are effective in reducing flood risk are very location specific. There is no one-size-fits-all solution and a variety of measures are generally necessary to reduce risk.
Flood risk management is a partnership between government and the community using a range of measures to reduce the risks to people, property and infrastructure. Decisions on managing flood risk should be made in consultation with the community that may be impacted by floods.
Preparing a floodplain management plan that outlines how flood risk to existing and future development can be managed for a particular location can inform these decisions. This process involves more than simply ensuring that building floors are above a particular flood level. It also considers how flood behaviour and hazard may vary in different parts of the floodplain and how different sized flood events might have an impact on people, property and infrastructure.
Floodplain management plans can reduce risk for new development areas
Managing flood risk is generally simpler in new development areas. Preparing a floodplain management plan enables strategic decisions about where, what and how to develop the floodplain while reducing residual flood risk (i.e. the risk left after management measures are put in place) to an acceptable level.
Local councils can use local planning instruments to influence the long-term development of an area in consideration of flooding, by restricting the location of development (zonings) and placing conditions (controls) on development.
Zonings can limit the impacts of new development on flood behaviour in other areas and the exposure of people and property to risk by locating new development away from areas where:
- The main floodwaters flow. Development in these areas may alter flood behaviour affecting other properties. Maintaining these flowpaths can also provide green corridors through cities.
- The speed and depth of water make it hazardous to people, property and infrastructure.
- It is not possible to evacuate people to flood free areas and there is no practical alternative.
Zonings can also limit the development of the remaining available land by considering:
- The use of community facilities during a flood. Critical facilities, such as emergency hospitals, should ideally be located in areas where they will not flood and can operate during a flood event (see Figure 13).
- The vulnerability of occupants to flooding. Aged care and disabled facilities should generally be located in areas where they can be readily evacuated to dry land.
- The vulnerability of buildings and contents. Homes and their contents are generally more vulnerable to flooding than industrial and commercial buildings and therefore should be located in less vulnerable areas.
Conditions on development can include: minimum fill levels for land and minimum floor levels for buildings (to reduce how often people and property are exposed to flooding); building regulations (that reduce the potential for structural building damage); and the ability to evacuate people to flood free areas (which may affect the way land is developed).
Flooding can also have significant impacts on infrastructure, which needs to be considered when designing infrastructure. Appropriate design standards for infrastructure exposed to flood risk can reduce its vulnerability to flooding.
Flood risk is harder to manage in existing developed areas
Flood risk is harder to manage where development, or the right to develop, already exists. Flood risk to existing infrastructure is usually altered through improvements to protection as part of any upgrade. However, for people and property there are basically three ways of managing flood risk to reduce the consequences of flooding: by modifying flood behaviour, property, or community response.
None of these measures is a stand-alone solution for addressing flooding issues. The preferred option is often a combination of flood-, response- and property-modification measures to reduce risk to an acceptable level and to manage this residual risk appropriately.
Flood modification measures change the behaviour of floodwaters
Flood modification measures aim to reduce flood levels, velocities or flows, or exclude floodwaters from areas under threat for events up to their designed capacity. They are a common and proven means of reducing damage to existing properties under threat from flooding. They tend to be more expensive than response or property modification measures but will often protect a larger number of properties.
Flood mitigation dams can reduce downstream flood levels by temporarily storing and later releasing floodwaters. Most dams are used to supply water to the community, but they can, when purpose built, also provide some flood mitigation for events up to their flood storage capacity. In larger floods, this mitigation capacity can be exceeded and floods pass through with little, if any, reduction. On the negative side, dams can cause disruption to existing communities, loss of valuable land and negative environmental impacts, and good sites for dams are difficult to locate. Detention basins act like dams but at a much smaller scale and are most suitable for ‘green field’ developments, where sizing constraints tend not to exist.
Levees are generally raised embankments built to eliminate inundation of the areas protected by the levee up to a certain size event. In larger floods, levees can be overtopped with water flooding into and inundating areas protected in the smaller events. Levees can trap local stormwater, causing damage unless flood gates and pumps are provided. However, levees, whether temporary or permanent, can increase flood levels in areas not protected by the levee (as noted in Question 2).
Waterway or floodplain modifications such as widening, deepening, realigning or cleaning rivers and flowpaths can improve the transport of floodwaters downstream and reduce the likelihood of blockage, but can increase velocities and erosion and cause negative environmental impacts. The benefits of cleaning and clearing are only temporary unless these continue to be maintained.
Other structures such as roads, railways and embankments also have an impact on flood risk management because they can alter flood flows and behaviour. Floodgates can also be used to prevent backflow from river systems into drains.
Property modification measures can protect against harm caused by floods
In addition to the zoning and development controls for new and re-developments mentioned above, modifications to existing property are also essential if the growth in future flood damage is to be contained.
Land filling involves building up low-lying areas and can improve the flood immunity of structures constructed on that land, but can adversely affect flood behaviour elsewhere and therefore is generally limited to the fringes of the floodplain.
Flood proofing involves the sealing of entrances, windows, vents, etc. to prevent or limit the ingress of floodwater. Generally it is only suitable for brick commercial buildings with concrete floors and it can prevent ingress for outside water depths up to approximately one metre. Ideally, new developments would use flood resilient designs and materials, as addressed in Question 8.
House raising is widely used to reduce the frequency of inundation of habitable floors, thereby reducing flood damage. This approach provides more flexibility in planning, funding and implementation than removal of development. However, its application is limited as it is not suitable for all building types and only becomes economically viable when above-floor inundation occurs frequently (for example, on average at least once in every 10 years). It also does not remove the risk to people who occupy the house, particularly in larger flood events.
Removal of development is generally only considered where there is significant potential for fatalities to residents and/or potential rescuers due to flooding, and where other measures are not able to reduce these risks. There are instances where a large proportion of, or an entire town has been relocated due to flooding. For example Clermont, Queensland, was relocated to higher ground after the flood of 1916. This approach generally involves voluntary purchase and demolition of the residence to remove it from the floodplain. Voluntary purchase has no environmental impacts, although the economic cost and social impacts can be high. Communities often oppose such schemes due to the impact on their community, surrounding property values and way of life.
Figure 14. Property modification measures to manage flood risk in new and existing development areas. This schematic was developed utilising the Integration and Application Network (IAN) tool.
Response modification measures help communities deal with floods
Measures to modify the response of the community to a flood are essential to deal with residual flood risk, because development controls and flood mitigation works generally cannot deal with all possible floods. Response modification measures can include: flood warnings, upgrading flood evacuation routes, flood evacuation planning, flood emergency response and flood education programs.
Implementing effective flood response within the community can reduce the danger and damage associated with floods. Flood warning and evacuation plans can be very cost effective and may, in some cases, be the only economically justifiable risk management measures. However, like all mitigation measures, they require ongoing maintenance and support. This is discussed further in Question 5.
References and further reading
McLuckie, D, Kandasamy, J, Low, A & Avery, D 2010, ‘Chapter 5 – Managing risk to existing development’, in D McLuckie & J Kandasamy (eds), Floodplain Risk Management, University of Technology Sydney Course Notes, Sydney.
McLuckie, D, Kandasamy, J, Low, A & Avery, D 2010, ‘Chapter 10 – Managing risk to future development’, in D McLuckie & J Kandasamy (eds), Floodplain Risk Management, University of Technology Sydney Course Notes, Sydney.