Flood Risk Explained
The summer of 2012 was the wettest on record with over 7,000 properties flooded in the UK. 5 million properties are at risk from flooding from rivers or the sea with a further 2.4 million at risk from surface water flooding. Climate change is making flooding more frequent and today's extreme event may be the norm in the future. Flood risk is now a major concern for home owners, potential home buyers and developers. This paper will explore and explain
- the risk posed by flooding in England,
- national and local planning policy requirements,
- flood risk assessment process and requirements,
- how to manage and mitigate the risk.
2. Flood Risk in England
In England, the most common forms of floods are:
- River flooding that occurs when a watercourse cannot cope with the water draining into it from the surrounding land. This can happen, for example, when heavy rain falls on an already waterlogged catchment.
- Coastal flooding that results from a combination of high tides and stormy conditions. If low atmospheric pressure coincides with a high tide, a tidal surge may happen which can cause serious flooding.
- Surface water flooding which occurs when heavy rainfall overwhelms the drainage capacity of the local area. It is difficult to predict and pinpoint, much more so than river or coastal flooding.
- Sewer flooding that occurs when sewers are overwhelmed by heavy rainfall or when they become blocked. The likelihood of flooding depends on the capacity of the local sewerage system. Land and property can be flooded with water contaminated with raw sewage as a result. Rivers can also become polluted by sewer overflows.
- Groundwater flooding that occurs when water levels in the ground rise above surface levels. It is most likely to occur in areas underlain by permeable rocks, called aquifers. These can be extensive, regional aquifers, such as chalk or sandstone, or may be more local sand or river gravels in valley bottoms underlain by less permeable rocks.
- Reservoir flooding when a large raised reservoir fails resulting in flooding of the land downstream of the reservoir. The risk of flooding is a product of both the flood event itself and the vulnerability of the person, property or environment exposed to the event.
The Environment Agency’s National Flood Risk Assessment examines where flooding could occur in all 69 river catchments and the coastline around England using 39 weather patterns of varying severity and likelihood.
The assessment includes the extent to which flood defence structures reduce the chance of flooding and what might happen if they overtop or fail. The assessment identifies land at risk from flooding using three risk categories. These consider the chances of weather severe enough to cause a flood, and the likelihood this will overwhelm defence structures or lead to their failure.
The National Flood Risk Assessment is an assessment of the risk now, and therefore does not take account of increased likelihood of flooding due to the possible future effects of climate change.
Although there is still debate about the human influence over climate change, the observed trends for the UK climate are real:
- Average UK temperature has risen since the mid 20th century, as have average sea level and sea surface temperature around the UK coast. Over the same time period, trends in precipitation are harder to identify.
- All regions of the UK have experienced an increase over the past 45 years in the contribution to winter rainfall from heavy precipitation events; in summer all regions except north east England and north Scotland show decreases.
- Severe windstorms around the UK have become more frequent in the past few decades, though not above that seen in the 1920s.
- There has been considerable variability in the North Atlantic Oscillation, but with no significant trend over the past few decades.
- Sea level around the UK rose by about 1 mm/yr in the 20th century, corrected for land movement. The rate for the 1990s and 2000s has been higher than this.
3. Planning Policy
The National Planning Policy Framework (NPPF) is a key part of the UK Government’s planning reforms. It replaces most of the Planning Policy Guidance Notes (PPGs) and Planning Policy Statements (PPSs). The NPPF concentrates on high level national policy and avoids prescriptive guidance. However, some technical guidance has been published alongside the NPPF and the PPS25 Practice Guide relating to flood and coastal risk management still applies.
The NPPF aim is that inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere. This is achieved by applying sequential and exception tests at all levels of the planning process, from national and regional plans through to local plans and site plans.
For planning purposes, flood risk is defined on the Environment Agency’s flood map and categorised into three flood zones (see box).
Clear , Flood Zone 1, low probability. Flooding from rivers and the sea is very unlikely. There is less than a 0.1 per cent (1 in 1000) chance of flooding occurring each year. Light blue , Flood Zone 2, shows the additional extent of an extreme flood from rivers or the sea. These outlying areas are likely to be affected by a major flood, with up to a 0.1 per cent (1 in 1000) chance of occurring each year.
Dark blue , Flood Zone 3, shows the area that could be affected by flooding, either from rivers or the sea, if there were no flood defences. This area could be flooded:
- from the sea by a flood that has a 0.5 per cent (1 in 200) or greater chance of happening each year;
- or from a river by a flood that has a 1 per cent (1 in 100) or greater chance of happening each year.
3.1. Sequential Tests
The overall aim of the sequential test is to steer new development to Flood Zone 1.
3.2. The Exception Test
Having determined that there are no reasonably available sites in a lower flood risk area through the sequential test, then the site may be subject to the exception test (see table opposite) to establish whether:
1. There are wider sustainability benefits to the community resulting from this development that outweigh the flood risks, and;
2. A flood risk assessment has been undertaken that demonstrates that the development is safe and will not increase the risk of flooding
The sites wider sustainability benefits are defined by the opportunities that the development brings in terms of affordable housing, employment, regeneration, transport etc. and should contribute towards the achievement of the local planning authority’s objectives as defined in their sustainability appraisal.
A flood risk assessment will quantify the flood risk at the site from all sources of flooding (also taking account of climate change) and identify the flood risk management measures that need to be included within the development to ensure that it is safe for the residents/occupiers and does not increase the risk of flooding for others.
1. Steer new development to Flood Zone 1
•Preferable for all new development.
2. Consider reasonably available sites in Flood Zone 2
•Where no reasonably available sites in FZ1;
•take account of flood risk vulnerability and apply the Exception Test.
3. Consider suitability of sites in Flood Zone 3
•Where no reasonably available sites in FZ 1 or 2;
•take account of flood risk vulnerability and apply the Exception Test.
4. Flood Risk Assessments
Strategic Flood Risk Assessments (SFRAs) refine information on the probability of flooding, taking other sources of flooding and the impacts of climate change into account. SFRAs are used to inform Local Plans, particularly the identification and comparison of potential sites through the sequential test on the basis of the flood zones. SFRAs also include local policy and guidance for site specific flood risk assessments.
Site-specific flood risk assessments (FRAs) are carried out by, or on behalf of, a developer to assess the risk to a development site and demonstrate how flood risk from all sources of flooding to the development itself and flood risk to others will be managed now, and taking climate change into account. The technical guide expects there to be iteration between the different levels of flood risk assessment.
Environment Agency standing advice is that FRAs should be proportionate to the nature, scale and location of development, and undertaken under the supervision of an experienced flood risk management specialist (who would normally be expected to have achieved chartered status with a relevant professional body).
4.1. Sources of information
For flooding from rivers or the sea, the Flood Map combines detailed local data from modelling and mapping studies with information from a national model of England and Wales. The national model gives a consistent picture of flood risk for all rivers with a catchment size greater than 3 km2 and the sea. Where detail modelling exists, then the EA will supply the data necessary for FRAs (flood extents, levels, flows) to assess the flood risk from rivers and the sea through one of six standard data products. The detail provided with each product increases so that the degree of assessment and cost is proportionate to the nature, scale and location of developments: Product 1. Flood map Product 3. Basic FRA/FCA Map Product 4. Detailed FRA/FCA Map Product 5. Reports Product 6. Model Output Data Product 7. Calibrated and Verified Model Input Data (CaVMID). Where the flood map is defined by the national model or for catchments smaller than 3km2, then a specific modelling study is likely to be necessary to determine flood extents and levels. The scale and complexity of the modelling required should be proportionate to the nature, scale and location of development and preferably agreed in advance with the EA. In addition to the above products, the EA is able to supply surface water flood maps that indicate areas susceptible to surface water flooding, either greater than 0.1m deep or greater than 0.3m deep. The Areas Susceptible to Surface Water Flooding map was produced using a simple method that assumes that underground sewerage and drainage systems, and smaller over ground drainage systems are full to capacity. The main limitation is that the surface water map has used a national average drainage capacity and are therefore indicative at best. EA advice is that they should not be used as the sole evidence for assessing risk or for any specific planning decision without further supporting studies or evidence, for example historic surface water and sewer flooding records from water companies.
Reservoir flood maps have been produced primarily to assist in the production of emergency plans, and their availability and distribution is closely controlled in the interests of national security. However, the EA flood map includes a layer which indicates areas that are at risk from reservoir flooding and is a useful starting point for assessing risk at a specific site. Groundwater flooding is a particularly concern in chalk and limestone aquifers on the southern counties of England. Although the EAs flood map does not indicate areas susceptible to ground water flooding, there are data layers where the aquifers are and their type. The EA may hold historic data on ground water flooding however,and so where this is a risk then you should include a request for any such data.
4.2. Residual risk and other factors
Residual risks are those remaining after applying the sequential approach and taking mitigating actions. The Technical guide stresses: “… it is the responsibility of those planning development to fully assess flood risk, propose measures to mitigate it and demonstrate that any residual risks can be safely managed.”
Flood defences reduce the risk of flooding, but do not completely eliminate flood risk. The reduction in flood risk that the defence provides depends upon the standard of protection, performance and reliability of the defence. The residual risks associated with defences are those that remain with the defence in place.
Flooding may still occur in defended areas if the defence is overtopped or breached, or if flooding occurs because of other sources such as groundwater flooding or poor drainage.
It should be appreciated that even if the probability of flooding is low, the consequences can be high. Should the defences fail, the consequences could be severe. In particular, flow depths and velocities may be very high and therefore the danger to people could be high if suitable mitigation measures are not put in place.
FD2320 provides methods for assessing flood risk to people in defended areas. It uses three levels of complexity in its approach, simple, intermediate and complex. The FD2320 approach has been adopted because the most serious risk associated with development behind defences is the risk to people, including entering and leaving properties during a flood.
The safety of the public is the single most influential consideration for decision makers. The approach in FD2320 bases consequences of flooding on the danger to people by providing generic lookup tables relating to the level of danger to people to the distance for an overtopping scenario and a breach scenario (Table 12.1 and 12.2 of FD2320).
The tables provide a ‘danger to people’ classification for developments located behind defences. The following provides a very simplified guide as to the groups of people that should be considered as falling into these danger classifications:
Danger for some – including children, the elderly and the infirm
Danger for most – including the general public
Danger for all – including the emergency services
FD2320 advises that the classifications should be considered as fairly subjective and should not be used as a decision-making mechanism to refuse a planning application, especially as measures to mitigate residual risk could reduce risk to acceptable levels. In some cases, development will not be acceptable and the Emergency Planning Officers should seek to agree this criterion up front.
A further residual risk that should be considered is the potential for blockage of nearby culverts artificially raising water levels upstream. EA flood maps do not include this risk and local assessment of the potential for a blockage is required with detail modelling to establish increased flood extents and levels. Functional flood plain is defined as land where water has to flow or be stored in times of flood. Historically, land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or designed to flood in an extreme (0.1%) flood, or at another probability has been a starting point for defining this area. The scope for development in this area even with mitigating measures, is limited to water compatible uses.
5. Managing and Reducing Flood Risk
For new developments, NPPF describes a policy to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. However, where new development is by exception necessary in such areas, the policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall.
The approach to achieving this is one of appraising risk, managing risk and reducing risk. This approach can be summarised by the hierarchy of methods:
The sequential test, exception test and associated flood risk assessments deliver the Assess, Avoid and Substitute stages of the policy by relocating new developments out of flood risk areas and/or by substituting more vulnerable with less vulnerable uses.
The key aims of ‘control’ within PPS25 are surface water management and flood risk management by design. At the site-specific level the former aim means making best use of sustainable drainage systems (SUDS), designing the drainage scheme for exceedence and not increasing runoff rates leaving the site. For new developments, the latter aim can be achieved through:
- Avoidance by re-location
- Optimised site layout
- Raising floor levels
- Modification of ground levels
- Provision of upstream flood storage
• Building design
Having applied the Assess – Avoid – Substitute – Control approach to new development, mitigation may be introduced as a way to deal with the residual risk. However, the Technical Guidance makes it clear that:
“Flood resistance and resilience measures should not be used to justify development in inappropriate locations.”
However residual risk may be mitigated by selecting the most appropriate flood resistance and/or resilience measures available to limit damage to the building and its contents and flood planning to ensure the safety of people and ongoing effectiveness of the mitigation measures.
5.1. Safe access and exit
The NPPF and Flood Risk Assessment Guidance for New Development, Phase 2: FD2320 require that where important to overall safety, safe access and escape is available to and from new developments in flood risk areas. Access routes should allow occupants to safely access and exit their dwellings during the design flood event. Vehicular access to allow the emergency services to safely reach the development during design flood conditions will also normally be required.
FD2320 states that a route can only be completely safe in flood risk terms if it is dry at all times. However, a window of opportunity may provide a dry route to safely evacuate people from flood risk areas before it floods. In this situation, there will become a point when the message will need to be changed to ‘Go in, Stay in, Tune in’ to prevent people from being at greater risk of flooding whilst out on the road network and allowing them to be rescued safely by the emergency services.
FD2320 defines the requirements for safe access and exit from new developments in flood risk areas as follows:
- Safe dry route for people and vehicles
- Safe dry route for people
- If a dry route for people is not possible, a route for people where the flood hazard (depth and velocity of flooding) is low and should not cause a risk to people. The route should be clearly marked by appropriate signs to guide people away from floodwater safely.
- If a dry route for vehicles is not possible, a route for vehicles where the flood hazard (depth and velocity of flooding) is low to permit access for emergency vehicles. However, the public should not drive vehicles in floodwater.
Where it is not possible to provide safe access routes above the design flood levels, limited depths of flooding may be acceptable. The acceptable flood depth for safe access will vary depending on flood velocities and the risk of debris within the floodwater. Section 13.0 S3.3 of FD2320 provides useful guidance on the danger to people for different combinations of depths and velocities.
FD2320 indicates that flood depths below 0.25m and velocities below 0.5m/s are generally considered to be a low hazard. Table 13.1 of FD2320 should be used when designing safe access and exit routes for developments located in flood risk areas. As flood depth and/or velocity increase the hazard to people increases, therefore the combination of depth and velocity on the access and exit routes should correspond to the white boxes (very low hazard) in Table 13.1.
FD2320 also discusses safe flood depths for vehicles. The guidance indicates that cars will stop and/or float in water as shallow as 0.5m, whilst some emergency vehicles are able to operate safely in water up to 1m. Fire engines remain controllable in depths of 0.5m up to a flow velocity of 5m/s.
Underwater hazards should be taken into consideration by the emergency services when driving through floodwater. Hazards such as missing service covers and ditches and dykes at the side of roads could be difficult to see even in shallow water. Where possible, routes should be clearly marked in areas at risk of flooding.
5.2. Mitigation measures
Only after the sequential approach has been applied to development proposals should mitigation measures be considered. It is necessary to always try to locate development in areas of lowest flood risk first. Only when it has been established that there are no suitable alternative sites in lower flood risk areas should mitigation measures be considered to allow development to proceed in flood risk areas, by exception.
Where mitigation measures are considered appropriate, they need to meet the requirements of the NPPF (paragraph 103) that it must be safe without increasing flood risk elsewhere and where possible reduce flood risk overall. A range of mitigation measures can be used to manage flood risk at development sites.
The best way to avoid flood risk is to locate the development in areas considered to be at little or no risk of fluvial, coastal or tidal flooding i.e. Flood Zone 1. Where this cannot be accomplished, the sequential test should be applied within the development site to locate the most vulnerable elements of the development in the lowest flood risk areas.
Where it is not possible to avoid flood risk or minimise it through site layout, raising ground levels and/or raising floor levels above the flood level is a possible option to manage flood risk to new developments.
Wherever possible the construction of new defences to enable development to take place should be avoided, so that additional residual risks are not created. Where this solution is proposed, developers will need to show that other options such as upstream storage and attenuation have been considered and justify why they are not suitable. Developers will also need to demonstrate that the proposal is compatible with long-term plans for general flood risk management in the area, such as Catchment Flood Management Plans and Shoreline Management Plans.
In some cases, developer contributions to flood risk management may be necessary. It may be reasonable for developers to contribute to the upgrade or redesign and replacement of existing flood defences, or to flood alleviation schemes, which would provide benefits to the wider community. Local development documents should include general policies about the principles and use of planning obligations for flood risk management.
If a decision to build in a flood risk area has been justified and the above options are unsuitable, the next best option would be to avoid flooding of buildings through design. The NPPF technical guidance (paragraphs 16 - 19) identifies a hierarchy of building design including, flood avoidance, flood resistance, flood resilience and flood repairable. Non-structural measures must also be taken into consideration for example the availability of a flood plan.
5.3. Evacuation, rescue and safe refuge
Appropriate evacuation and flood response procedures must be put in place by developers to manage the residual risk associated with an extreme flood event. In locations where flood defences create a residual risk of flooding, judgements on whether a proposal can be regarded as safe will need to consider the feasibility of evacuation from an area should it be flooded.
It should be noted that the emergency services are unlikely to regard developments that increase the scale of any rescue that might be required, as being safe.
- The practicality of safe evacuation from an area will depend upon:
- The type of flood risk present, and the extent to which advance warning can be given of a flood event
- The number of people that would require evacuation from the area potentially at risk
- The number of vulnerable people who will require assistance and appropriate resources to evacuate
- The adequacy of both evacuation routes and identified places that people would be evacuated to
- The length of time that the evacuation may need to last
- Sufficiently detailed and up-to-date evacuation plans being in place for the locality that address these and related issues.
It is also important to consider the cumulative effect on the surrounding area especially in relation to the impact on the supporting road infrastructure. Heavy congestion points may have been identified in well-developed areas, consideration needs to be factored in to the evacuation process. It may be necessary for developers to provide public awareness and education on the risk of flooding that the area faces.
The NPPF technical guidance indicates that those proposing developments in flood risk area should take advice from the emergency services when producing an evacuation plan for the development as part of the flood risk assessment.
Evacuation is defined as,
‘Where flood warnings provided by the Environment Agency can enable timely evacuation of residents to take place unaided (i.e. without the deployment of trained personnel to help people from their homes, businesses and other premises).’
Rescue is defined as,
‘Rescue by the emergency services is likely to be required where flooding has occurred and prior evacuation has not been possible.’
Whilst provisions such as safe refuge and raised walkways play a role in reducing the overall risk posed by a flood, they do not themselves make a development safe, as they relate more to a rescue situation than to effective evacuation in advance of a flood occurring.
Single storey residential development is generally more vulnerable to flood damage and occupants do not have the opportunity to retreat to higher floor levels. Safe refuge above the appropriate flood level should be incorporated into the design of new developments within flood risk zones.
6. Surface Water Management
Surface water management provides an opportunity to reduce flood risk as a result of development and may contribute to the desirability of a proposal. The SUDS manual provides the industry standard guidance for design of Sustainable Drainage Solutions (SUDS). The detail regarding surface water disposal within a flood risk assessment is again proportionate to the nature, scale and location of the development. However, the Flood and Water Management Act is imposing new responsibilities on Lead Local Flood Authorities particularly in respect of Surface Water Management and SUDS. After significant consultation, Defra is set to release the new “National Standards for sustainable drainage systems” by the end of the year. From April 2014 Construction work that has drainage implications may not be commenced unless the drainage system has been approved by the approval body (SAB).
Flooding is a significant risk in England, the Cabinet Office national risk register assesses coastal flooding as the third highest risk to the UK. The NPPF and associated Terchnical Guidance describes a policy to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas at highest risk. However, where new development is by exception necessary in such areas, the policy aims to make it safe without increasing flood risk elsewhere and where possible, reducing flood risk overall. The approach to achieving this is one of appraising risk, managing risk and reducing risk. The sequential test seeks to avoid risk or substitute more vulnerable with less vulnerable development and where, by exception development is still necessary, the exception test identifies appropriate risk controls and mitigating measures.
Safe refuge is defined as,
‘Somewhere located above the design flood level, usually an upper floor in a development, which can be accessed by the emergency services in a situation where occupants will need to be rescued.’
A place of safety is defined as,
‘Somewhere located within flood zone 1 which will not require ongoing support by the emergency services.’
Flood risk assessments should ensure that the development is safe for the residents/ occupiers and does not increase the risk of flooding for others, be proportionate the nature, scale and location of the development and should be undertaken under the supervision of an experienced flood risk management specialist.
Russell Burton BEng CEng MICE MEPS
RAB Consultants Ltd
Lichfield Business Village
Lichfield WS13 6QG
Tel: 01543 308631 mobile:07753 730886