A flood warning system is a way of detecting threatening events in advance. This enables the public to be warned en masse so that actions can be taken to reduce the adverse effects of the event. As such, the primary objective of a flood warning system is to reduce exposure to coastal flooding.
The description of this technology originates from Linham and Nicholls (2010).
The purpose of a flood warning service is to detect and forecast threatening flood events so that the public can be alerted in advance and can undertake appropriate responses to minimise the impact of the event. This is a particularly important technology in developing countries, where flooding results in massive loss of life and property.
Flood warnings are a highly important adaptive measure where protection through large scale, hard defences, is not desirable or possible. This may be the case if defences would cause adverse environmental or social problems, or where the cost of defence construction would be prohibitive.
A flood warning process has two distinct stages: (1) flood warning and (2) response. These stages are composed of a number of sub-stages and are linked through the dissemination of warnings as shown in Figure 1.
The flood warning stage requires constant monitoring of meteorological conditions. This allows detection and assessment of threatening events to take place before it hits a community. Forecasts may also be made to help decision-makers model how an event is likely to develop, how significant it will be upon arrival, and what sections of the population are likely to be at risk. This is necessary because simple detection of an event will not provide enough time to undertake appropriate responses. To achieve monitoring and forecasting, it is likely that a flood warning system will include meteorological and tidal detection systems and river and coastal flood forecasting models.
Once an event exceeds a given threshold, a warning will be issued. This message is likely to be disseminated to the ‘at risk’ population via a number of channels. The media, services such as the police and fire departments and basic signals such as sirens and flags all have important roles to play.
After the at risk population have been warned, the second stage of the flood warning service is initiated; the response. Communities in the hazard zone are required to take action to minimise their exposure to the hazard and to reduce the consequences of flooding. It is important that appropriate actions are communicated to the public through awareness raising campaigns, prior to an emergency. Doing so, will mean actions can be quickly taken, helping to mitigate the consequence of flooding to the greatest degree.
An effective flood warning service requires cooperation between different agencies, such as the government, relief agencies and local communities. As such, this approach not only provides technical challenges but also, organisational ones.
At its simplest, the task of flood warning consists of answering the following five questions (EMA, 1999):
1) How high will the flood reach and when?
2) Where will the water go at that predicted height?
3) Who will be affected by flooding?
4) What information and advice do the people affected by flooding need to respond effectively?
5) What is the best way of giving the people affected by flooding the appropriate information?
Some of the essential components required in an effective coastal flood warning system are shown in Table 1.
Table 1: Typical components of a flood warning, forecasting and emergency response process
- Monitoring meteorological, river & tidal conditions
- Meteorological forecasting (e.g. weather prediction)
- The meteorological, river & coastal conditions under which decisions are taken to issue flood warnings
- Procedures and techniques for issuing warnings to the public, local authorities, emergency services, etc.
- Models for forecasting future river and coastal conditions
- Emergency works. E.g. temporary barriers, flow control, evacuation, recue, incident management, decision support
- Repair, debris removal, reuniting families, emergency funding arrangements
- Providing shelter, food, water, medical care, counselling
- Review performance of all components of the system
- Recommendations for improvements
- Emergency planning, public awareness campaigns, training, systems improvements, flood risk mitigation
It is important to note that a flood warning system is not a standalone response to minimisation of the impacts of coastal flooding. An early warning system should be coupled with emergency planning measures, such as the provision of evacuation routes and flood shelters, and should also contain an awareness raising element. These systems are only useful when everybody knows what the system of warning means, what the stages of warning are and what to do when the warnings are given (Tompkins et al., 2005).
Coupling this measure with technologies, such as flood hazard mapping (see Section 4.2.4), will improve the effectiveness of flood warnings and will help to further raise awareness of the local risk of flooding.
HR Wallingford (2006) state that flood warning systems provide advance warning of flood events which can potentially allow:
- The risk to life to be minimised
- Evacuation of vulnerable groups
- Residents to move assets (e.g. food, livestock, personal effects) to safer locations
- Timely operation of flood control structures (e.g. storm surge barriers, temporary flood defences, etc.) to prevent inundation of property and land
- Installation of flood resilience measures (e.g. sandbags, property flood barriers)
- Pre-event maintenance operations to ensure free channel conveyance
If warnings can be disseminated to the public, it will also be possible to give communities advice on what to do in the event of a flood, as well as providing further information to limit losses. This may include areas to be evacuated, evacuation routes and the location of refuges for evacuees. It is likely that advice and guidance can be issued through the same channels used to notify communities of the flood risk as well as being made available prior to flood events.
Flood warning technologies are relatively low-cost and have been successfully employed in a diverse range of countries from developed countries, such as the USA, to developing ones, like Bangladesh (IOC, 2009).
As stressed above, a flood warning system is not sufficient on its own to reduce risk; people’s reactions to warnings – their attitude and the nature of their response – has an important bearing upon the effectiveness of a warning system (Haque, 1995). Flood warnings must be disseminated to local communities and responses must be made to minimise risks. Without these elements, the effectiveness of flood warning systems is compromised. It is therefore highly important that warnings can be communicated effectively to the public and that emergency responses are implemented. It is thus essential that the public are educated about appropriate responses to flood warnings, in advance of a flood emergency.
It is also essential that the flood warning system is accurate – system inaccuracies may lead to complacency if previous warnings were unfounded, or fear by causing unnecessary anxiety (UNFCCC, 1999). In order for a flood warning system to be successful, it is essential that communities heed the warnings issued – this requires the public to trust the agency providing the warning.
The costs of implementing flood warning systems are expected to differ widely, depending on the level of sophistication of monitoring and forecasting technologies.
In developing countries, meteorological observations are frequently made using basic methods, which may include ground-based methods and weather balloon observations, coupled with limited computing. In these cases, annual running costs are expected to be in the hundreds of thousands of pounds. It is also not unusual for flood warning schemes in developing countries to be heavily funded by international civil society organisations (UK POST, 2005).
In more developed countries, where more sophisticated meteorological observations are made, and where computing power is more advanced, annual running costs are expected to be in the hundreds of millions of pounds (UK POST, 2005).
It is not necessarily the case that lower technology systems offer less effective protection against flooding. Community-based, early warning systems such as those frequently applied in developing countries can sometimes be more effective than top-down, centralised systems. This is attributed to the fact that they can be more directly integrated into local response and risk reduction strategies (DFID, 2004).
The effectiveness of flood warnings can even be improved by involving local communities, for example, in the creation of flood hazard maps, scientific monitoring and contingency planning, because these activities help to increase awareness and understanding of the impacts of natural hazards (UKPOST, 2005). People-centred strategies which increase access to, and understanding of, information can even help to provide a more robust defence against a number of stresses, not just those related to climate change (Hay, 2009).
Because of their ability to drastically reduce property losses and loss of life, flood warning services may be seen as a cost-effective means of mitigating flood hazards. This is especially the case when compared against hard technologies, such as seawalls and dikes, which are often prohibitively expensive to construct.
Some of the key factors which contribute to variations in the cost of flood warning systems are provided below:
- Extent of meteorological monitoring network
- Cost of sourcing meteorological data
- Set up costs of warning dissemination system and its degree of sophistication
- Training and employment costs of meteorological data analysts
- Cost of associated measures:
- Provision of flood shelters
- Creation of evacuation routes
- Awareness raising
- Training of emergency services
The organisation of a flood warning service varies widely between countries and depending on the scale of the overall system. Sene (2008) indicates that it may include some, or all, of the following activities:
- Detection: design, installation and operation of rainfall, river level, tidal level, wind, wave and other monitoring equipment
- Design: design of flood warning schemes, including contributing to decisions on who should receive warnings, setting flood warning thresholds, deciding how flood warnings should be disseminated and under what circumstances
- Dissemination: monitoring measurements and forecasts against thresholds and issuing warnings following agreed procedures and public awareness activities
- Operation: suggesting actions which should be taken to mitigate flooding risks/losses
- Management: general management activities, including defining staff rotas, procurement, performance monitoring and reporting, research and development, etc.
- Forecasting: development and operation of flood forecasting models to provide estimates of river levels, river flows, tide levels, wave overtopping, etc.
Some of these tasks may be unnecessary for a small-scale, community-based warning system where the primary needs are for detection and dissemination of warnings. However, for a regional or national programme, most of the tasks will be necessary, although some may be shared with other organisations (Sene, 2008).
It is possible to employ low technology methods in warning systems. For example, in Bangladesh, warnings are disseminated by local trained volunteers or alternatively, through channels such as newspapers, television and radio. The use of volunteer messengers has been very successful in Bangladesh, since warnings may even be viewed as more relevant and person-specific when delivered by other members of the community. This demonstrates real potential for flood warnings in developing countries.
Responses to flood warnings can also be conditioned at the community level. This may include the provision of sandbags, designing and implementing evacuation procedures, or distributing relief goods, amongst other activities. In Bangladesh, this is undertaken by local volunteers. Education may also be offered to communities at risk in advance of a significant event. This is likely to make people more aware of the severity of hazards and of the precautionary options available (Haque, 1995).
It may also prove beneficial to teach coping strategies at a community level. Strategies may include swimming lessons or providing information on evacuation. Haque (1995) found that the majority of communities in Bangladesh had not received information from government departments regarding coping strategies for cyclones.
It can be seen that to be effective, warning systems require the development, implementation and coordination of quite diverse flood responding technologies (IOC, 2009). This may prove challenging for local communities to achieve, especially given the involvement of multiple organisations in flood warning.
Despite the fact that these actions can take place on a local level, involving larger organisations, with superior resources, knowledge and know-how may still prove beneficial in improving the quality of warning messages from the warning systems. Better still, by working together with neighbouring countries that may also operate flood warning systems, it may be possible to obtain more complete and timely meteorological data, better dissemination of warnings and improved responses.
One of the main barriers to implementation of flood warning systems in developing countries is the availability of communication channels, through which warnings can be disseminated to the public. In developed countries this can be achieved through radio and television channels and the internet. These resources are less widely available in developing countries therefore sending out the warning messages in a timely manner to the targeted audience can be problematic.
The approach also requires significant volumes of detailed information to be collected and analysed in order to detect flood threats. It needs significant investment in equipment and training. This has, however, been achieved in developing countries such as Bangladesh (Haque, 1995; Mirza et al., 2005) and Vietnam (Pilarczyk & Nuoi, 2010) with the help of foreign organisations who can supply information and real-time data on weather patterns (Haque, 1995). Locally recognised indicators may also be important when developed by coastal communities with a close relationship with the land and sea.
Even once a warning system has been implemented, significant barriers to the effectiveness of this approach may still exist. In a field study following the April 1991 cyclone in Bangladesh, Haque (1995) found that despite receiving flood warnings, a large proportion of the population took no deliberate emergency action. Therefore, a large segment of the population remained vulnerable. Three main factors were cited as reasons for failing to take action:
1) Fear of losing household assets through looting if the house is abandoned
3) Disbelief of flood warnings
Fear of looting may be addressed by providing a denser network of smaller shelters to reduce the distance between homes and shelters and to allow better protection of property (Haque, 1995). Improved law enforcement is also needed for better protection of private property during disaster events.
Fatalism typically stems from a sense of powerlessness to influence events. It has been suggested that some individuals believe flooding is God’s will and that individuals must instead just learn to live with the consequences (Haque, 1995).
Disbelief of flood warnings may be due to past false warnings. It can be hard to forecast significant flood events due to their unpredictable nature. Therefore, it may be wise to implement a trade-off between the gains of advance warning when the hazard probability is low, and gains resulting from enhanced responses when the incidence of false alarms is reduced (Haque, 1995).
Additional reasons noted by Haque (1995) for failing to take action include disbelief that floods would occur in that area due to a lack of experience within living memory, over-filled shelters, the fact that shelters were crowded by men, which discouraged females users and finally, a lack of awareness of the limited amount of protection that homes would provide.
It is possible to implement flood warning systems together with other adaptation measures , as part of an integrated flood risk management plan. Complementary actions could be part of a protect, accommodate or retreat approach. In London, flood warnings inform operation of a storm surge barrier and embankments have also been constructed along the majority of the riverside.
The costs involved in implementation of a flood warning system could be offset through the construction of multi-purpose shelters which could also serve as schools, health facilities and agricultural extension centres (Haque, 1995). This has already proven successful in Indian communities (Mishra & Prakash, 1982).
Technology used for detecting flood risk may also be used for forecasting rainfall when flood risk is low. This could benefit agricultural practices in these regions.
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Matthew M. Linham, School of Civil Engineering and the Environment, University of Southampton, UK
Robert J. Nicholls, School of Civil Engineering and the Environment and Tyndall Centre for Climate Change Research, University of Southampton, UK