Flood prevention schemes: guidance for local authorities

SECTION 2: THE APPRAISAL CONTEXT

Defining the problem

2.1 An appraisal should start with a clear statement of the problems to be tackled or the objectives to be achieved. These should be defined without prejudging the solution; for example, it is not acceptable to state that the objective is 'to replace existing flood defences with a 1 in 100 year design standard flood embankment'. Further, all social, environmental and economic issues should be taken into account. Major constraints affecting the options should also be stated.

2.2 An example of such a statement might be:-

'The town is prone to flash flooding with an annual probability of greater than 5% (that is, at return periods of less than 1 in 20 years); the area at risk covers 120 properties including some sheltered housing with a higher expectancy of risk to life. The problem is believed to have increased in recent years because of urbanisation of the catchment. Sections of the river border an SSSI.'

Strategic approach

2.3 Flood prevention schemes must meet certain basic criteria. Specifically, they should be technically sound, economically viable, and environmentally acceptable. To achieve these objectives, a strategic approach is desirable. Once broad solutions have been determined at the strategic level, the range of options to be considered on a project-specific basis will become clear. Guidance on strategic considerations is provided in Chapter 3.

Issues which cannot be incorporated into a benefit-cost analysis

2.4 A limitation of economic analysis is that it involves comparison of options in terms of a single objective, economic efficiency. There may be other objectives relevant to the choice; for example, the achievement of sustainable development and the promotion of sustainable flood management (Chapter 3). Further, there are some impacts that cannot be readily valued in economic terms and others which, for various reasons, may not be given their full weight in the analysis. It is, therefore, crucial to set out project objectives and to compare the options in terms of their contribution to the achievement of these objectives. If the contribution cannot be wholly quantified in economic terms, or does not affect economic efficiency, it is important that it be identified and stated.

2.5 Schemes may be affected by legal obligations under national and international law, which may constrain the appraisal process. For example, where the obligations involve requirements that must be met, it would be incorrect to test that position using economic analysis. Instead, the principles of cost-effectiveness would usually be more appropriate. A benefit-cost analysis may serve only to identify the least costly method of meeting the legal obligations.

Hazard management options

Importance of a good choice of options

2.6 Benefit-cost analysis can only identify the best of those options considered. A good appraisal will, therefore, encompass a wide range of management options, if only to rule out many of these at an early stage. A narrowly defined search may only identify the best of a poor set.

2.7 Options may reduce the risk of an event, or reduce the damages when an event occurs. Institutional or behavioural adaptations, such as relocation of some activities, or the temporary closure of roads when flood warnings are in force, may be considered in addition to the construction of works.

2.8 The form of the detailed analysis also depends on the situation and physical aspects of the land, whether or not there is an existing scheme, and whether there is a statutory requirement to maintain the watercourse. The residual life and standard of protection offered by any existing scheme should also be taken into account. These different conditions determine the appropriate 'do nothing' and 'do something' options (section 3).

Assessing risk to life

2.9 Deaths from flooding in the UK have fortunately been rare. However, this risk is always present. It is sensible, therefore, to assess whether floods in a particular situation pose an unusually high risk to life. Such a threat might occur with a rapid rise in floodwaters, accompanied by high flow velocities and deep water, particularly where this could result in the structural failure of buildings. Consequently, where structures such as flood embankments fail, or where flooding can occur in very small, steep catchments, special attention should be given to managing the risk.

Events exceeding scheme capacity

2.10 There is always the possibility that a more extreme flood than the design event will occur during the lifetime of a scheme. Consequently, appraisals should consider all events, not just those up to the design standard of protection.

Environmental protection

2.11 The water environment provides a range of habitats, which supports a wide variety of species and water-related uses. Changes to that environment, both locally for a scheme and in a wider national context, need to be taken into account in the appraisal process, particularly the ability to meet statutory environmental objectives. Further information is provided in Chapter 7.

Consultation

2.12 Consultation is a necessary part of most schemes. It is good practice to consult early in the scheme design, and to continue the process throughout the design work and implementation. This should enable a comprehensive consideration of the appropriate costs and benefits.

Multi-functional schemes

2.13 The development of strategic approaches to flood prevention may result in the promotion of a multi-functional project. At its simplest, such a project may involve 2 or more different structures, each with a different purpose, but built together to make savings in total construction costs. In this case, it would be reasonable to apportion these costs, and to appraise the flood defence function of the works separately.

2.14 However, the aim of promoting a multi-functional project will generally be to provide a range of facilities at a lower total cost than if each were provided separately. In this situation, when undertaking the benefit-cost analysis, all benefits and costs should be included and the question of who benefits and who pays can be ignored.

2.15 The question then arises of how to share the costs equitably between funders. Because the overall aim is to make more efficient use of resources, it is reasonable to assume that no party should pay more than the whole-life cost of meeting their specific requirements on a stand alone basis. The contributions should usually be based on the costs of the relevant major sections of the work. However, there may be situations where it will be more equitable to divide costs in the ratio of the major benefits, provided these can be determined on a consistent basis (see also paragraphs 5.9 and 5.10).