1. Understanding flood risk
Flood risk is a measure of the likelihood that a flood event will happen and of the potential consequences of that event. The long-term aim of the Scottish Government is to reduce these risks.
Robust and reliable information on the causes and consequences of flooding are needed to promote well informed decisions on how to tackle flood risk.
Sources of flooding
This guidance does not deal with floods caused solely by a failure in or blockage of a sewerage system although many of the same principles will apply. Scottish Water has statutory responsibilities for maintaining the sewerage network. Floods caused solely by a failure in or blockage of a sewerage system should be dealt with through this existing channel.
This guidance covers all other potential sources of flooding. In fulfilling their flood risk management responsibilities, SEPA and the responsible authorities should focus on the sources of greatest risk, which should include the following primary sources of flooding.
- River (Fluvial) flooding - this occurs when the water draining from the surrounding land exceeds the capacity of the watercourse.
- Coastal flooding - is caused by a combination of high tides and stormy conditions where wave overtopping can occur.
- Surface water (pluvial) flooding - is caused when rainfall water ponds or flows over the ground before it enters a natural or man-made drainage system or watercourse, or when it cannot enter the drainage system because the system is already full to capacity.
- Sewer flooding - this occurs when combined sewers are overwhelmed by heavy rainfall. Sewer flooding is often closely linked to surface water flooding, and may contain untreated foul water.
- Groundwater flooding - this occurs when water levels in the ground rise above surface levels.
- Reservoir flooding and flooding from other infrastructure - Although unlikely, failure of infrastructure such as dams, could result in a large volume of water being released very quickly.
Characteristics of a flood
The causes and consequences of flooding can only be fully understood when the characteristics of a flood are examined. The Act specifies particular flood hazard characteristics that must be assessed and mapped, which are described in more detail in Table 1. Flood hazard maps will show where flooding has the potential to do harm. Where necessary, additional factors should be considered to give a full picture of the likely impacts resulting from a flood.
Table 1 Flood Hazard characteristics
Helps show where flood waters will penetrate and what may be affected.
Helps understand the potential impacts of a flood. For instance, extensive shallow water flooding is likely to be less damaging than more localised areas of deeper water
The duration of a flood can have an impact on the damage caused, for long duration flooding can increase impacts to crops and services.
High velocity flood waters can increase risk to health and safety and cause greater damage.
Flood that carry pollutants, for instance where sewer flooding occurs often carry a greater risk to health and safety as well as the potential to cause greater economic damage.
Flood waters with a high sediment or debris content can create additional risks to health and safety, and may increase the risk of damage to infrastructure (e.g. bridges).
Likelihood of flooding
For flood mapping purposes, the Act requires three flood scenarios to be assessed: high, medium and low probability floods. The Scottish Government will issue regulations defining the flood probabilities that should be applied to each scenario.
A variety of methods can be used to estimate the probability of flooding. After considering existing methodology and work being undertaken in other Member States implementing the Floods Directive, SEPA should take a lead role in developing and disseminating guidance on the analysis of flood probabilities, including techniques to examine multiple or combined sources of flooding.
In many instances, different sources of flooding can combine to intensify a flood. For example, high tides in estuaries can occur simultaneously with high river levels. Understanding these interactions (including their likelihood) will be an important part of understanding and managing flood risk.
Assessing the impacts of flooding
A wide range of impacts to society, the economy, the environment and cultural heritage should be assessed where appropriate, including those set out in Table 2.
To gain a fuller appreciation of the impacts of flooding, SEPA and the responsible authorities should also consider the following factors:
- Exposure - what will be exposed to the flood;
- Vulnerability - the vulnerability of those things that are exposed to the hazard; including the ability to recover, which may include the availability of insurance;
- Value - the value of things exposed to the hazard, which could include costs or how critical the item is.
Wherever possible, both aspects of vulnerability should be considered - susceptibility and resilience. Susceptibility is a measure of how prone to impacts particular elements will be during a flood event. For example, the elderly, frail or sick can be more susceptible to injuries or loss of life. Resilience is a measure of the ability of something to recover from a flood. For instance, businesses can be more resilient to flooding through the use of insurance. Buildings can be made more resilient through the use of water resilient materials in construction.
In particular, measuring the impacts on the environment poses a significant challenge, and concepts like ecosystem services set out later in the guidance should be explored to help assess these impacts.
The indirect impacts of flooding can also cause problems. For example, the costs of disruption to transport and power supplies or the costs to emergency services. It is important that these indirect impacts are included wherever practical to do so.
Table 2 Measuring the impacts of flooding
Categories and descriptions
Human Health (Social)
Human Health: includes immediate or consequential impacts
Community: impacts to emergency response, education, health and social work facilities
Water body Status: permanent or long-term impacts to ecological or chemical status of surface water bodies including those caused by hydro morphological impacts of flooding.
Protected Areas : adverse permanent or long-term impacts to protected areas or water bodies.
Pollution Sources: sources of potential pollution in the event of a flood, such as IPPC and Seveso installations, or point or diffuse sources.
Wider environment : Other potential permanent or long-term environmental impacts, such as those on soil, biodiversity, flora and fauna, etc.
Cultural Assets: permanent or long-term impacts to cultural heritage, which could include archaeological sites / monuments, and architectural sites.
Property: impacts to property, which could include homes, insurance availability
Infrastructure: impacts to infrastructural assets such as utilities, energy generation and transmission, transport, storage and communication.
Rural Land Use : impacts to uses of the land, such as agricultural activity (livestock, arable and horticulture), forestry, mineral extraction and fishing.
Economic Activity: impacts to other sectors of economic activity, such as manufacturing, construction, retail, services and other sources of employment.
Analysing flood risk
An integrated approach to assessing risk
Many of the assessments undertaken by SEPA will be strategic level assessments that will support the preparation of flood management plans. These assessments will also identify where more targeted or detailed assessment should be carried out, potentially by local authorities, Scottish Water or any other designated responsible authorities.
Ultimately this should form a cyclic process where information and knowledge is built up over planning cycles (Figure 2).
Figure 2 Cycle of information and knowledge growth
To support the delivery of this integrated approach to assessing and managing flood risk, SEPA should:
- Make use of available historic flood and gauging data;
- publish and maintain advice on assessing, modelling, mapping and sharing data. In doing so, SEPA should work alongside the responsible authorities and other organisations to ensure that the information meets the needs of all relevant organisations;
- take all practicable steps to ensure that national assessments, including the national flood risk assessment and national flood maps, thoroughly consider and reflect local risks and issues;
- ensure that information generated on flood risk can be used in a consistent way at all stages of the flood risk management planning process, and in particular in the appraisal of options to manage flood risk;
- create and manage a repository of information on flooding and its impacts that can be shared with the responsible authorities and other interested parties. If possible this should include data on insurance claims.
The responsible authorities should provide active support and information to help SEPA in this work. Where necessary, the Act provides SEPA and local authorities with powers to request information and to seek assistance.
Residual risk and the effectiveness of actions
Residual risk is the risk that remains after management actions have been taken. Residual risks often have a low probability of occurrence, although the impacts can be severe. Residual risk should be considered in flood management decisions. Home and business owners should be encouraged to insure themselves against the residual risk where possible and all actions to manage flooding should include arrangements to deal with residual risks.
Understanding the effectiveness of existing actions and residual risk is an important step in identifying management actions. For example:
- How well are flood defence structures performing against their initial design standard?
- What difference does flood warning make to public safety?
- What are the consequences of an event exceeding the design standard?
- Is insurance available to cover the residual risk?
It is important that existing actions to manage flooding and their effectiveness are taken into account wherever it is practical to do so, particularly when undertaking work that will influence investment decisions, e.g. assessing areas vulnerable to flooding or preparing flood risk management plans.
Data on historical flood events and gauge data are essential to build up a picture of what contributes to flood events in a particular catchment. Additional information can be used to build a good understanding of the links between the sources and impacts of flooding. This will help identify the right combination of measures to tackle particular flooding problems. For instance, where high rates of run-off in upland areas is contributing to flooding problems, measures to store or slow run-off could be considered, including re-vegetating a hill slope to increase the interception of rainfall and increase the roughness of the land surface, thereby slowing runoff.
The same principles apply in urban areas, where an understanding the sources and pathways of flood waters can help identify where features to store or divert flood waters, including detention ponds and other Sustainable Urban Drainage System should be located. Drainage is discussed in more detail in section 4.
To help understand the interaction of different actions across catchments and coastlines, SEPA and the responsible authorities should adopt what is commonly referred to as the source -pathway- receptor -impact approach.
The approach is a well-established framework in flood risk management. It provides a basis for understanding the causal links between the source of flooding, the route by which it is transmitted and the receptor, which suffers some impact:
- Sources are the weather events or conditions that result in flooding (e.g. heavy rainfall, rising sea level, waves, dam break, river flows etc);
- Pathways are routes between the source of flood waters and the receptor. These include surface and subsurface flow across the landscape, urban drainage systems. The hydrological cycle can provide valuable insights into the pathways of flood waters;
- Receptors are the people, industries and built and natural environments that can be impacted upon by flooding;
- Impacts are the effects on receptors. The severity of any impact will vary depending on the vulnerability of the receptor.
Quantifying flood risk
Risks are evaluated by combining the likelihood and consequence of flooding. This can be achieved in a variety of ways, and the approach adopted should reflect the purpose of the assessment, the scale of the assessment and the data available.
There will be times where risk thresholds will need to be set, for instance, when undertaking the national flood risk assessment. Risk thresholds are highly subjective and can be influenced by societal preferences, values and opinions of acceptability. SEPA and the responsible authorities should present a consistent opinion of flood risk and its significance or acceptability. This must be done within the context of Government guidance and policy on these matters.
Climate change and other long term trends
Testing proposed flood management actions against long term trends is essential to selecting sustainable actions that will stand the test of time. SEPA and the responsible authorities should work to establish approaches to examining future scenarios that can be applied consistently across flood risk assessments and management decisions. Wherever possible, a range of future scenarios should be examined, including a 'worst case' scenario.
Climate change is likely to have a substantial impact on flooding. SEPA in collaboration with the responsible authorities should work to improve information on the affects of climate change on flood risk. This should include using information gathered over implementation cycles to detect changes in flood patterns, and developing new methods to detect and assess trends.
Other long term trends that could have a measurable impact on flood risk should also be considered, including urban creep, changes in land-use and societal changes.
Dealing with uncertainty
Floods are infrequent phenomena for which it is difficult to predict future events. Uncertainties can be divided into four main areas:
- natural variability, which can be subdivided into natural variability in time and natural variability in space;
- knowledge uncertainties that come from a lack of knowledge, for example about the behaviour of defences or climate change;
- modelling and data uncertainties in the quality of models or data that supports assessments, design and appraisal.
- fundamental uncertainties about things we cannot know, for example the distant future.
Uncertainty should be clearly presented in flood risk assessments showing what approaches have been used to quantify them and how decisions have been influenced by uncertainties. Any assumptions made should be clearly set out.
Communicating flood risk
It is important that the public understand the flood risk that they face. These can be complex concepts to explain. This means that special attention must be given to how information on flooding is conveyed to the public. Experience suggests that simply stating 'return periods' or probabilities for particular events can be very confusing, particularly to communities who have recently experienced flooding.
SEPA and the responsible authorities must investigate a range of options for expressing flood probabilities and risk to the public. Visual tools in particular can help with understanding such as figures, maps and diagrams to communicate the messages.
Options may include providing information on the chance that an individual or community could be affected by a flood, rather than information on the likelihood of particular flood occurring. Comparisons to other risks people face in daily life could also be used to help explain flooding issues. As no comparison is perfect, this approach should not be relied upon in isolation.
Where risk thresholds have be used, for instance when identifying areas potentially vulnerable to flooding, they must be accompanied by clear explanations of the criteria used, how risks were calculated or estimated, and how thresholds have been set.