Flood prevention schemes: guidance for local authorities

ANNEX A: IDENTIFICATION OF RISKS

A.1 Risk categories and areas

Fundamental to both risk assessment and risk management is the identification of existing risks. This need not be an onerous task but it does require a careful and considered approach. It should be carried out for any project regardless of size or simplicity. It is not appropriate to adopt the risk identification from a previous project since each will have its own unique risks. This section provides some examples of the main risk areas to be considered at the project appraisal stage. The extent to which each area needs to be considered will vary from stage to stage and project to project.

For convenience in this Annex, risks are considered in the following way:

• Risks are divided between a number of categories as listed below.
• Within each category there are a number of areas for consideration. These are intended to provide a general indication of areas where risk issues may need to be addressed, though the lists should not be regarded as comprehensive, and several areas will be irrelevant to many projects.
• Specific risks within each area should be determined as part of the assessment procedure. They are not listed in this document as there are potentially a huge number of risks, many of which are project specific.

	Category
Non-project specific (A2)	1. Funding
	2. Statutory framework
	3. Socio-economic framework
Project development (A3)	1. Management of project development
	2. Strategic risks
	3. Impact of natural processes
	4. Performance of existing works
	5. Ecology, heritage and amenity
	6. Human intervention
	7. Design parameters
	8. Knowledge of principles/methods
	9. Scheme performance and response
Project implementation (A4)	1. Procurement and construction
	2. Operation and maintenance

Risks associated with each category are discussed in the sections below.

A.2 Non-project specific risks

Awareness of a range of non-specific project risks is important in the project appraisal process, though it will rarely be appropriate to consider many of these in great detail. Some will, however, be particularly relevant to large-scale planning or the development of major strategic plans.

A.2.1 Funding

The availability of funding for a project presents a risk which can be non-project specific. This can arise through changes in funding policy, particularly rules for grant aiding of schemes, and can apply at local or national level.

Areas which may be considered include:

• availability of funds;
• changes in funding rules; and
• financial year constraints.

A.2.2 Statutory framework

Risks related to the statutory framework involve possible future changes which might affect the scheme. Such changes might include new legislation, changes in planning procedures and institutional changes. For example, changes in legislation regarding environmental designations could result in previously agreed policies being rejected.

Areas which may be considered include:

• change in policy or legislation;
• change in regulation;
• changes to structure, local and other management plans;
• approvals/permissions/authorisations;
• planning;
• statutory consultees; and
• legal challenge.

A.2.3 Socio-economic framework

Socio-economic risks are those which relate to people, communities, national and regional assets affected by, or having an interest in, the scheme. For example, a change in local government could result in the withdrawal of local authority support for the scheme. The scheme may not be accepted for amenity, visual or other reasons. There may be future changes in local conditions or attitudes which make the scheme less acceptable, for example, the development of tourism, or the development of major new infrastructure.

Areas which may be considered include:

• public perception/expectation;
• public acceptability;
• socio-economic change;
• recreation, amenity, tourism;
• demography; and
• changes in economic base values (eg through market change).

A.3 Project development risks

A.3.1 Management of project development

The management of the project development process is a crucial element in effective appraisal and decision-making. Risks associated with unsatisfactory management include inadequate use of information, lack of communication, and poor quality of assessment, appraisal and decision-making.

Areas to be considered include:

• staff skills and resources;
• management of the analysis and appraisal process;
• approaches to consultation;
• relations between stakeholders; and
• knowledge and understanding of processes.

A.3.2 Strategic risks

Some risks will apply particularly at the strategic level. For example, inappropriate phasing of works within a strategy may have an adverse impact elsewhere, whilst the full effects of a planned strategy may be different from expectations. This may result in a need for mitigation and amendment to schemes elsewhere.

Areas which may need to be considered include:

• interaction between schemes;
• timing and phasing of schemes;
• other resources eg appropriately skilled labour;
• multiple failures;
• large-scale impacts on natural processes;
• interaction with urban drainage systems;
• relationship with land use planning; and
• implications of non-implementation.

A.3.3 Impact of natural processes

Knowledge of river processes is fundamental to scheme design and appraisal but is often limited, particularly in the case of extremes. Events that cause flooding are extremes with a low probability of occurrence. There is, therefore, a risk that the combination of circumstances that cause extreme events may not be predicted by conventional techniques.

For example, the great flood of 1993 on the River Tay was caused by a combination of rainfall and snowmelt. This condition would not necessarily have been predicted using conventional hydrological techniques.

There is also considerable uncertainty regarding the behaviour of sediment under extreme conditions. For example, sediment movement in a river flood may significantly change the river cross section. This will in turn affect flood levels. These examples point to the need to consider an appropriately wide range of conditions when determining likely extremes.

The natural processes that affect flood defence may change during the life of a project as a result of climate or other changes. The aim of risk assessment in relation to such changes should be to determine the robustness of current decisions to a reasonable range of external changes. For localised changes, such as those due to future development, a specific assessment of likely future scenarios may be required. For larger scale impacts, such as those due to global climate change, it will not normally be appropriate to carry out an assessment of variability for each individual project.

Long-term changes in river morphology, whether natural or human induced, may also lead to changes in the risk profile over time.

For each project it is important to determine which impacts are most crucial to the decision-making. They can then be given appropriate priority in the analysis.

Areas to be considered include uncertainty in:

• tidal flows and levels;
• storm frequency, intensity and duration;
• storm surge;
• waves (height, direction, period, transformation);
• wind setup;
• precipitation type, intensity and distribution in space and time;
• runoff, river flow and levels;
• temperature/snowmelt;
• correlation between river flow, tides, surges and waves;
• morphology of rivers;
• extent, depth and duration of flooding;
• speed of propagation of flood waves;
• flood routes and flow velocities; and
• interaction with structures.

A.3.4 Performance of existing works

Schemes often depend on the performance of existing defences and other engineering works, yet there is uncertainty over the way in which existing flood defence structures and other works will perform in the future. Further, the condition of a structure is likely to deteriorate with time, resulting in an increase in risk.

Areas to be considered include:

• condition and performance of existing structures (eg residual life);
• probability of failure and changes with time;
• inter-relationships between structures; and
• hidden weaknesses.

A.3.5 Ecology, heritage and amenity

Many risks associated with natural habitats, archaeology, recreation and amenity need to be considered. Often the actual impact on the environment of a scheme may differ from that predicted. In addition, measures that are intended to enhance the environment may not behave as intended.

There is also a risk that new information may become available at any stage of project development and implementation. For example, a new protected species may be discovered or project excavation may reveal previously unknown archaeological evidence. These can cause delay and can result in the need for radical alterations to a scheme, or even abandonment. Areas to be considered include:

• ecology;
• habitats;
• fisheries;
• landscape;
• implications of enhancements;
• Sites of Special Scientific Interest, etc;
• archaeology; and
• potential for discovery of unknown features.

A.3.6 Human intervention

In addition to natural processes, the scheme may be affected by human activity. The development of other schemes may affect conditions, for example, the construction of a storage reservoir in a river catchment will increase the response time and, therefore, the characteristics of the critical flood. Other developments and land use changes in a river catchment may also affect runoff, and hence, the downstream flow regime. Areas to be considered include:

• dredging;
• barriers and barrages;
• flood protection schemes;
• loss of floodplain storage/conveyance due to development; and
• runoff from new development.

A.3.7 Design parameters

Hydraulic conditions are one of the key parameters in design of flood prevention schemes, with design conditions usually defined as extreme values extrapolated from shorter term data sets. Depending upon the length of the data record and methods employed, there will be differing degrees of accuracy in the predicted values. Risks to be considered include the sensitivity of scheme response, and subsequent consequences to variations in these values. Other risks include over-estimation of conditions if the joint likelihood of combined events is not considered, although the potential for greater inaccuracy in predictions is currently inherent in the methods available to perform this analysis. Because knowledge of hydraulic conditions is fundamental to all that is subsequently designed and constructed, a significant risk exists if insufficient attention is given to providing appropriate accuracy.

The use and analysis of field data is also essential for understanding defence problems and developing appropriate solutions. Appreciation of site conditions in a mobile environment is particularly important.

Areas to be considered may include:

• adequacy of site investigation;
• sensitivity to uncertainty in input parameters and design assumptions;
• joint probability of different loads (eg waves and water levels);
• site conditions (eg ground conditions);
• choice of design conditions;
• accuracy of extrapolation or derivation; and
• data quality and length of record.

A.3.8 Knowledge of principles/methods

The risk of inadequate design exists, particularly with new techniques and approaches. There are areas where knowledge and the ability to produce quantified results continue to be limited without extensive work (for example, overtopping rates for different construction forms).

Areas to be considered include:

• new techniques and approaches;
• limited knowledge (data or understanding); and
• experience of designer.

A.3.9 Scheme performance and response

There are considerable uncertainties over the way in which new flood prevention schemes will respond in the future. Hazards to be considered include seepage, breaching and overtopping of a structure (including that from events which exceed the design standard). The condition of a structure is also likely to deteriorate with time, resulting in an increase in risk during its lifetime.

Scheme response must also be considered in combination with changes to natural processes. If, for example, river flood conditions worsen with time, the risk of structure overtopping or failure will also increase.

The nature of failure of any structure or scheme influences the risks that exist. Different failure modes may exist for the same scheme depending upon the circumstances that induce failure. Depending upon the nature of the scheme, and these circumstances, such failure may be either instantaneous or progressive. This has potentially different consequences.

A distinction should also be made between functional failure and structural failure, which again have potentially different consequences. For example, flooding arising from overflow of an embankment when design levels are exceeded may be very different from structural failure, which may be potentially catastrophic, leading to breaching and more severe consequences.

Areas to be considered include:

• performance of new structures and schemes;
• impact of events larger than the design event;
• flood defence structure response (eg overflow, overtopping, seepage, or

breaching);

• degree or extent of failure;
• nature of failure (instantaneous or progressive);
• change in probability of failure over time;
• factors of safety and freeboard allowances; and
• interaction with other structures.

A.4 Project implementation risks

Dealing with risks at the project implementation stage is largely a matter for risk management. There are many standard texts that deal with all aspects of construction risk management, which is outside the scope of this guidance. Nevertheless, a number of issues which arise in the implementation phase should be considered during project appraisal.

A.4.1 Procurement and construction

To allow the identified risks to be assessed by the contractors at procurement stage, they must be communicated in a clear manner, facilitating appreciation and understanding.

The contract provides for the allocation of risk between the various parties. The contract form adopted should therefore be appropriate to the nature of the risks associated with the scheme. In principle, responsibility for each of the risks should be allocated to the party best able to manage them.

'Buildability' issues should be addressed during scheme development and design. They should not be left to the contractor, though the opportunity should be given to develop alternative proposals. While it is the contractor's responsibility to design and mitigate risks of temporary works and working methods these should be adequately considered in the design stage. Similarly, the possibility of adverse weather or storm occurrence during construction deserves appropriate attention.

In many cases, river defences will be constructed on recently deposited alluviums or other weak strata. It is important to ensure that their strength characteristics are taken into account when considering feasible approaches to construction of the works and access to the site.

Environmental sensitivities will be identified in scheme development and appraisal. These must be adequately communicated to the contractor and site managers if appropriate measures are to be adopted during construction. Practical methods of dealing with any restrictions identified in the environmental assessment should be considered carefully.

Areas to be considered include:

• risk communication and documentation;
• contract terms and conditions;
• funding and financial budgets;
• contractor experience and resources;
• health and safety issues;
• buildability;
• flooding or storm conditions during construction;
• ground conditions;
• access;
• accommodating environmental requirements; and
• potential environmental impacts of construction.

A.4.2 Operation and maintenance

Assumptions about the philosophy and approach to long-term project operations and maintenance will often have to be made at the appraisal stage. These will have to be communicated effectively to those responsible for the operation and management of the scheme, to ensure it is managed appropriately in accordance with the design.

The different risks associated with different structure types will influence the monitoring regime that is adopted, and the intervention thresholds that need to be set for maintenance or repair. For example, for some defences the failure mode may be sudden and complete, whereas others may deteriorate in a more gradual and progressive manner.

The available maintenance resources will affect the operation and management practices that are likely to be implemented. Limited access, resources and material availability may mean that a slow speed of response must be assumed with consequent risks. Alternatively, it could be decided that fast response systems are essential for some specific risk areas, which will have cost implications for the management regime required. For defences with mechanical components, or other high maintenance elements that are potentially subject to failure, duplication or back-up systems may need to be considered.

Flood warning may be a valid response strategy for some residual risks. However, there are many risks associated with any flood warning system such as the communication of the warnings, public response and the ability of people to respond. These need to be considered in the development of emergency response plans. In making assumptions about operation and management procedures it is important that the risks associated with any flood warning system are taken into account.

Areas to be considered include:

• risk communication;
• characteristics of structure response;
• risk appreciation and understanding;
• frequency and nature of monitoring;
• intervention thresholds;
• failure mechanisms and consequences;
• maintenance regime; and
• flood warning systems and their effectiveness.