The primary objective of flood-proofing is to reduce or avoid the impacts of coastal flooding upon structures. This may include elevating structures above the floodplain, employing designs and building materials which make structures more resilient to flood damage and preventing floodwaters from entering structures in the flood zone, amongst other measures.
The description of this technology originates from Linham and Nicholls (2010).
Flood-proofing measures are widely applied in the USA where two types of flood-proofing are widely recognised: wet and dry. Wet flood-proofing reduces damage from flooding in three ways; (1) allowing flood waters to easily enter and exit a structure in order to minimise structural damage; (2) use of flood damage resistant materials; and (3) elevating important utilities. On the other hand, dry flood-proofing is the practice of making a building watertight or substantially impermeable to floodwaters up to the expected flood height (FEMA, 2008).
Wet flood-proofing measures typically include structural measures, such as properly anchoring structures against flood flows, using flood resistant materials below the expected flood depth, protection of mechanical and utility equipment and use of openings or breakaway walls to allow passage of flood waters without causing major structural damage (FEMA, 2010). A typical example of wet flood-proofing is shown in Figure 1.
A dry flood-proofed structure is made watertight below the expected flood level in order to prevent floodwaters from entering in the first place. Making the structure watertight requires sealing the walls with waterproof coatings, impermeable membranes, or a supplemental layer of masonry or concrete, installing watertight shields on openings and fitting measures to prevent sewer backup (FEMA, 2007). A typical example of dry flood-proofing is shown in Figure 2.
Flood-proofing can be applied in residential and non-residential buildings and the principles of flood-proof design can also be applied to other important infrastructure such as electricity substations and sewage treatment works. Obviously, the decision to choose wet or dry flood-proofing should be influenced by the use of the structure being protected and the compatibility with flood waters.
One of the main advantages of flood-proofing is that it avoids the need to elevate, demolish or relocate structures and as a result, is often a much more cost effective approach to reducing flood risk (Powell & Ringler, 2009). Flood-proofing measures are also much more affordable than the construction of elaborate flood protection works such as seawalls and dike systems (FEMA, 2007).
Flood-proofing is also advantageous because it does not require the additional land that would be needed to offer the same degree of flood protection through seawalls or dikes.
Wet flood-proofing measures are beneficial because they allow internal and external hydrostatic pressures (relating to fluids which are not in motion (for example, the maximum still water level caused by extreme events) to equalise during a flood therefore lessening the loads on walls and floors (FEMA, 2007). This means structures are less likely to fail during floods.
Although flood-proofing will not allow residents to continue living in their house during flooding, flood-proofing measures will make it much quicker and easier to clean up and repair flood damage (FEMA, 1992).
Flood-proofing can also be undertaken by individuals, rather than requiring funding from central or local government bodies. Even small, inexpensive flood-proofing efforts are likely to result in worthwhile reductions in flood damage. Availability of funds to undertake more expensive flood-proofing measures will no doubt encourage the uptake of flood-proofing however.
Flood-proofing measures require the current risk of flooding to be known and communicated to the public through flood hazard mapping studies and flood warning systems. This will allow flood-proofing measures to be appropriately applied and will allow time for residents to vacate flood-proofed buildings in the event of an emergency. In the case of dry flood-proofing, it will also allow residents to close barriers in a timely fashion. Although the provision of flood hazard maps and flood warnings bring benefits themselves, it is an additional cost that must be borne when implementing flood-proofing measures.
Since residents are not able to continue living in flood-proofed houses during flooding, amenities for accommodating evacuated people must also be provided. These facilities may be required for some period after a flood event, as wet flood-proofing may leave the structure uninhabitable for a small period following flooding.
Flood-proofing measures are most effective when applied in areas where flood depth is low. The application of flood-proofing measures does little to minimise damage caused by high velocity flood flow and wave action (FEMA, 2007). If a flood larger than the design specification occurs, the effect will be as if there was no protection at all (FEMA, 2001).
Another disadvantage is that in the case of dry flood-proofing, flood shields are not aesthetically pleasing (FEMA, 2007). Shields for doors and windows are left in place in most circumstances, so that they can be quickly closed when required. However, this means that these measures are permanently on display. Ongoing maintenance of flood-proofing measures is also required to ensure they continue to provide appropriate protection (FEMA, 2007).
When wet flood-proofing measures are applied, flood waters still enter the structure. Therefore significant clean up may be required following floods to remove water borne materials such as sediments, sewage or chemicals (FEMA, 2007). The choices of materials used in these structures will still enable clean up to progress much more quickly than in non-flood-proofed structures.
In the case of dry flood-proofing, if design loads are exceeded, walls may collapse, floors buckle and homes may even float. This has the potential to cause more damage than if the home were just allowed to flood (FEMA, 2009).
In the absence of cost information from developing countries, cost estimates for a number of flood-proofing measures in the US are provided. The US is one country which widely applies flood-proofing measures.
In the US, the cost of elevating a structure above flood depth is likely to be between US$29 and US$96 per square foot of house footprint (FEMA, 2009). The range in cost is due to the construction and foundation type and the required elevation.
Wet flood-proofing measures are likely to include the addition of wall openings for the entry and exit of floodwaters, installing pumps, rearranging or relocating utility systems, moving large appliances and coating surfaces in coverings which make it easier to clean up after flood waters recede. According to FEMA (2009), the cost of wet flood-proofing in the US is likely to be between US$2.20 and US$17.00 per square foot of house footprint when considering basement flood-proofing up to a depth of approximately 2.4 m.
Dry flood-proofing measures in the US include sealing walls with waterproof coatings, impermeable membranes or supplemental layers of masonry or concrete and equipping doors, windows and other openings below the flood elevation with permanent or removable shields. Installation of backflow valves on sewer lines and drains is also likely to be required (FEMA, 2009). US cost estimates for these measures are given in Table 1.
Table 1: Approximate costs of dry flood-proofing measures in the USA
|Sprayed on cement||$55.10||Linear metre of wall covered|
|Waterproof membrane||$18.70||Linear metre of wall covered|
|Asphalt||$39.36||Linear metre of wall covered|
|Drainage line around perimeter of the house||$101.68||Linear metre|
|Plumbing check valve||$1060||Each|
|Sump and sump pump||$1710||Lump sum|
|Metal flood shield||$1230||Linear metre of shield surface|
|Wood flood shield||$383.76||Linear metre of shield surface|
Costs are relevant for flood proofing of approx. 0.9m. Costs are presented in 2009 $US
Source: FEMA, 2009
Wet flood-proofing is generally less expensive than dry flood-proofing since any action to reduce the number of items that are exposed to flood damage is considered a wet flood-proofing measure (FEMA, 2007). For example, moving valuable items to an upper story is a wet flood-proofing measure that can be undertaken at negligible cost.
The costs of dry flood-proofing a structure will depend on the following factors (FEMA, 2007):
- The size of the structure
- The height of the flood protection elevation
- Types of sealant and shield materials used
- Number of openings that have to be covered by shields
- Plumbing measures required to prevent water back-up
At the community level, flood-proofing costs will depend largely on the number of properties in the flood hazard zone and associated costs such as flood hazard mapping and modelling exercises to determine properties at risk.
Flood-proofing measures are very much possible at the community level. At its simplest, wet flood-proofing involves moving valuable objects to higher ground in order to avoid the effects of flooding. Since this can be undertaken at negligible cost, wet flood-proofing is highly achievable on a local level provided sufficient warning time is provided.
More advanced flood-proofing measures are not as capital intensive as the construction or realignment of coastal defences and therefore should also be achievable at the community scale. Implementation of this technology will however, require a proactive planning approach.
It may even be possible for individual households to finance basic flood-proofing measures themselves. This may include elevating valuable items and utilities above the expected level of flooding. This will be possible if households are given adequate information on the likely level of flooding. However, more advanced flood-proofing measures are likely to require the assistance of specialists. For example, the construction of houses within the flood zone will require experienced, professional engineers or architects to develop and/or review structure designs to ensure that structures are capable of functioning as designed.
Although flood-proofing is achievable at the community level, its effectiveness depends on community uptake and the standard to which measures are implemented. Few benefits will be gained from flood-proofing if the uptake is low or if measures are completed to a low standard. Potential unwillingness to undertake flood-proofing such measures has been highlighted by Mathis and Nicholson (2006) who found that only 63% of new buildings are in compliance with flood regulations in the US. Due to reluctance to undertake flood-proofing measures on an individual basis, it may be necessary to inspect properties in the hazard zone to ensure that flood-proofing measures have been employed and to an acceptable standard.
Funding may be provided to local communities in order to increase uptake of flood-proofing projects. This may increase uptake in poorer communities and may help to protect those at risk rather than just those who can afford such measures. A similar outcome may be achieved if flood insurance is regionally important. Reduced premiums for flood-proofed properties will encourage the uptake of flood-proofing.
Before communities can go ahead with flood-proofing measures, it will be necessary to undertake some form of flood hazard mapping. This will inform decision-makers on which buildings require flood-proofing and to what depth. It can also support the appropriate design of flood-proofing measures.
Although basic flood-proofing measures can be undertaken at negligible cost, the cost of implementing more advanced flood-proofing may be prohibitive in poorer communities. This may prevent implementation but could be addressed by providing funding opportunities.
For more advanced flood-proofing measures, such as anchoring structures and installing breakaway walls, specialist knowledge is likely to be required. This may require the input of experienced architects or engineers.
In areas where flood hazard maps do not currently exist, the uptake of flood-proofing measures may be problematic. Non-availability of flood hazard maps will make identification of properties at risk and the minimum specification of flood-proofing measures difficult to define.
The main opportunity for the implementation of flood-proofing lies in the capacity to allow development in the flood hazard zone to go ahead albeit, with explicit limitations. Where there is high demand for coastal land, flood-proofing measures present an opportunity to utilise this land. This is in contrast to policies such as building setbacks, which prevent coastal development.
FEMA (Federal Emergency Management Agency) (1992) Repairing your flooded home. Washington DC: US Dept. of Homeland Security. Available from: http://tiny.cc/mrzv8  [Accessed: 21/07/10].
FEMA (Federal Emergency Management Agency) (1998) Managing Floodplain Development Through the National Flood Insurance Programme. Washington DC: US Dept. of Homeland Security. Available from: www.fema.gov/pdf/floodplain/is_9_complete.pdf#nameddest=wet-flood  [Accessed: 27/07/10].
FEMA (Federal Emergency Management Agency) (2001) Engineering Principles and Practices for Retrofitting Flood Prone Residential Structures. Washington DC: Dept. of Homeland Security.
FEMA (Federal Emergency Management Agency) (2007) Selecting Appropriate Mitigation Measures for Floodprone Structures. Washington DC: US Dept. of Homeland Security. Available from: www.fema.gov/library/viewRecord.do?id=2737  [Accessed: 21/07/10].
FEMA (Federal Emergency Management Agency) (2008) Floodplain Management Bulletin: Historic Structures. Washington DC: US Dept. of Homeland Security.
FEMA (Federal Emergency Management Agency) (2009) Homeowner’s Guide to Retrofitting. Washington DC: Dept. of Homeland Security. Available from: http://tiny.cc/kfnxq  http://www.fema.gov/hazard/map/firm.shtm - 1 [Accessed: 10/08/10].
FEMA (Federal Emergency Management Agency) (2010) Wet Floodproofing. Washington DC: US Dept. of Homeland Security. Available from: www.fema.gov/plan/prevent/floodplain/nfipkeywords/wet_floodproofing.shtm  [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 
Mathis, M.L. and Nicholson, S. (2006) An Evaluation of Compliance with the National Flood Insurance Program Part B: Are Minimum Building Requirements Being Met? Washington DC: American Institutes for research. Available from: www.fema.gov/library/viewRecord.do?id=2590  [Accessed: 28/08/10].
Powell, M. and Ringler, R. (2009) Yorklin, DE, and other cities adopt plans to protect buildings in floodplains from water in Kemp, R.L. (ed.). Cities and Water: A Handbook for Planning. Jefferson: McFarland and Company, 180-184.
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