Flood hazard mapping is an exercise to define those coastal areas which are at risk of flooding under extreme conditions. As such, its primary objective is to reduce the impact of coastal flooding. However, mapping of erosion risk areas may serve to achieve erosion risk reduction. It acts as an information system to enhance our understanding and awareness of coastal risk.
The description of this technology originates from Linham and Nicholls (2010).
Flood Hazard Mapping is a vital component for appropriate land use planning in flood-prone areas. It creates easily-read, rapidly-accessible charts and maps which facilitate the identification of areas at risk of flooding and also helps prioritise mitigation and response efforts (Bapulu & Sinha, 2005).
Flood hazard maps are designed to increase awareness of the likelihood of flooding among the public, local authorities and other organisations. They also encourage people living and working in flood-prone areas to find out more about the local flood risk and to take appropriate action (Environment Agency, 2010).
It is important to note here, that climate change must be carefully considered when implementing flood hazard mapping. Flood hazard mapping typically provides a ‘snapshot’ of flood risk at a given point in time. When considering the effects of climate change however, it is important to consider the dynamic nature of flood risks. For example, SLR and changes in storm intensity, occurring as a result of climate change, will causes changes in the areas susceptible to flooding. See, for example, Figure 1.
Due to climate change and changes in relative sea level, it is important to note that flood hazard maps will require periodic updates in order to reflect the changing risk of flooding. These updates should account for RSLR, erosion, changes in storm frequency and intensity, etc.
Flood hazard maps can be used by developers to determine if an area is at risk of flooding, and by insurers to determine flood insurance premiums in areas where flood insurance exists.
Due to sparse empirical records and the statistical rarity of extreme coastal events, coastal flood prediction often relies on complex numerical models that approximate the processes and phenomena that lead to coastal floods (Water Science and Technology Board, 2009). Coastal flood hazards are determined by the interaction of storm surges and waves with seabed bathymetry and coastal land cover. These factors determine the inland extent of flooding. Coastal flood models must therefore account for these features, as well as the processes associated with storm surges and waves (Water Science and Technology Board, 2009).
The creation of flood maps usually combines topographic data with historic or modeled information on extreme sea levels and wave heights. This allows determination of the water level at the coast under extreme conditions and shows how this water could flood inland. This is likely to involve the deployment of storm surge and wave models.
The level of protection offered by existing coastal defences should also be accounted for. This helps to determine when overtopping of defences will occur, causing flooding of defended areas.
Geographic Information Systems (GIS) are frequently used to produce flood hazard maps. They provide an effective way of assembling information from different maps and digital elevation models (Sanyal & Lu, 2003). Using GIS, the extent of flooding can be calculated by comparing local elevations with extreme water levels.
Identification of those areas at risk of flooding will help inform emergency responses. For example, areas that are likely to require evacuation can be identified, and evacuation routes can be planned and clearly signposted so local communities are made aware in advance of an emergency. The identification of flood risk areas will also help in the location of flood shelters for evacuees.
Identification of flood risk areas is likely to help in the planning of a more effective emergency response. It is essential that certain infrastructure, such as electricity supplies, sewage treatment, etc., and services, such as the emergency services, continue to function during a flood event. The creation of flood hazard maps will therefore allow planners to locate these elements in low risk areas so that they can continue to serve during an extreme event. Alternatively, flood hazard mapping may highlight a requirement to defend these elements from flooding.
Flood hazard mapping will allow quantification of what is at risk of being flooded such as the number of houses or businesses. This will help identify the scale of emergency and clean-up operations.
The creation of flood hazard maps should promote greater awareness of the risk of flooding. This can be beneficial in encouraging hazard zone residents to prepare for the occurrence of flooding. In order to achieve this however, local authorities must ensure that emergency procedures are established, and that information about what to do in the event of a flood is made available to the general public.
By identifying buildings at flood risk, awareness raising campaigns can also be targeted at high risk properties. This may include raising awareness of emergency flood procedures and may also promote the implementation of flood-proofing measures.
In the longer-term, flood hazard maps can support planning and development by identifying high risk locations and steering development away from these areas. This will help to keep future flood risk down and also encourages sustainable development. In order for this to occur, the consideration of flood hazard maps must be integrated into planning procedures.
In itself, flood hazard mapping does not cause a reduction in flood risk, It must be integrated into other procedures, such as emergency response planning and town planning, before the full benefits can be realised.
More advanced, accurate flood hazard maps are likely to rely on complex numerical models due to the lack of observed extreme event data. This requires a degree of expertise to implement. The collection of topographic and bathymetric data to complement extreme water level and wave height information could also be expensive to collect.
To realise the full benefits of flood hazard mapping, it is important to provide people in the hazard zone with information about emergency procedures and ways of reducing flood risk. If information on what to do in the event of an emergency is not provided, flood hazard maps may serve only to increase fear and anxiety as residents are more aware of the risk of flooding.
The costs of flood hazard mapping are not widely known. Therefore it is not possible to provide likely cost estimates here. However, we provide a number of factors which are likely to contribute toward the cost of flood hazard mapping:
- External expertise on numerical modelling of flood risk brought in from academic institutions or commercial organisations
- Topographic surveys (LiDAR or remote sensing) to provide information on land elevation which will feed back into the flood risk model
- Historic costs of collecting extreme event data such as water levels, wave heights, etc.
- Cost of employing a Geographic Information System (GIS)
Flood hazard mapping may be difficult to undertake at the community level due to the need for complex numerical modelling for the forecast of extreme water levels, storm surges and wave heights. The required expertise and modelling capacity is unlikely to be locally available, especially in developing countries. As such, it may be necessary to enlist the help of external organisations. Following developed country examples, this type of mapping has been accomplished via national programmes.
Flood hazard mapping relies on the availability of topographic, and long-term extreme event data and complex numerical modelling techniques. This requires specific modelling capabilities and expertise which may not be readily available.
A lack of public understanding about the benefits of flood hazard mapping may also provide a barrier to implementation. If the public is unaware of the benefits of flood hazard mapping, they may prefer to see public money spent on more tangible flood and erosion protection measures.
Flood hazard mapping complements and strengthens other adaptation options, such as flood-proofing measures, emergency planning, provision of flood shelters and evacuation planning. As such, this approach could be applied almost universally, irrespective of the other adaptation technologies that are used.
Bapulu, G.V. and Sinha, R. (2005) GIS in Flood Hazard Mapping: a case study of Kosi River Basin, India. Noida: GIS Development. Available from: http://home.iitk.ac.in/~rsinha/PDF's/2006_FloodGISdevelopment.pdf [Accessed: 21/07/10].
Environment Agency (2010) Flood Map - your questions answered. Rotherham: Environment Agency. Available from: www.environment-agency.gov.uk/homeandleisure/31662.aspx [Accessed: 21/07/10].
Linham, M. and Nicholls, R.J. (2010) Technologies for Climate Change Adaptation: Coastal erosion and flooding. TNA Guidebook Series. UNEP/GEF. Available from: http://tech-action.org/Guidebooks/TNAhandbook_CoastalErosionFlooding.pdf
Nicholls, R.J., Wong, P.P., Burkett, V.R., Codignotto, J.O., Hay, J.E., McLean, R.F., Ragoonaden, S. and Woodroffe, C.D. (2007a) Coastal systems and low-lying areas. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 315-356.
Sanyal, J. and Lu, X.X. (2003) Application of GIS in flood hazard mapping: a case study of Gangetic West Bengal, India. Map Asia 2003, Poster Session. Available from: http://tiny.cc/r7zci [Accessed: 21/07/10].
Water Science and Technology Board (2009) Mapping the Zone: Improving Flood Map Accuracy. Washington DC: National Academies Press.
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