Flood prevention schemes: guidance for local authorities

SECTION 1: INTRODUCTION

Scope of guidance

1.1 This chapter provides guidance for Scottish local authorities on the risk assessment aspects of project appraisal for flood prevention schemes promoted under the Flood Prevention (Scotland) Act 1961. It aims to enhance decision-making for investment in such schemes through improved methods of risk assessment. No fundamentally new concepts are involved.

1.2 The guidance assumes that the reader has prior knowledge of general risk and sensitivity analysis techniques within project appraisal, and should be read in conjunction with the other chapters. It is not intended to be followed rigidly, or to cover every aspect of risk analysis. In particular, the guidance is not a methodological manual for risk assessment. Appropriate specialist advice should be sought as necessary.

1.3 The primary topic covered is risk assessment and its part in the project appraisal process. Risk management aspects are also discussed, but only where these are relevant to project appraisal. Various methods and techniques are outlined, and these are covered more fully in the references.

1.4 The document is based on the procedural guide, "Flood and Coastal Defence Project Appraisal Guidance, Approaches to Risk", published by the Ministry of Agriculture, Fisheries and Food, now the Department of Environment, Food and Rural Affairs (reference 24).

Background to risk assessment

1.5 Historically, design practice for flood prevention schemes has been largely deterministic, with some sensitivity analysis, but little application of probabilistic approaches. In a purely deterministic analysis, a unique set of input parameters is used in a model to obtain a unique value for the system response. However, there is now a move towards risk-based design within a probabilistic framework.

1.6 In a probabilistic analysis, a full range of input values is tested, each one weighted by the probability of it being encountered. This approach takes account of both the consequences and probability of failure. It allows for uncertainty, rather than the approach where data and prediction/design methods are assumed to be known precisely.

1.7 Risk assessment methods are available at different levels of sophistication - from broad brush approaches using risk registers (section 2) to more specialised techniques such as event/fault trees and multi-criteria tools (section 3). The methods should be soundly based, but not over-complicated. In particular, they should be cost-effective, appropriate and targeted.

Advantages of risk assessment and risk management

1.8 Risk based methods enable the consideration of end consequences. For example, they consider not just the likelihood of high water levels against flood defences, but also the probability of their failure, and the consequent harm to people and property.

1.9 Risk assessment facilitates decision-making - hazards, risks and assumptions are documented, and can be communicated to relevant stakeholders. It provides a basis for risk to be recognised explicitly, and reduced, shared, transferred or accepted. It also reduces the chance of surprise .

1.10 Risk management provides a common approach for the comparison of different responses to uncertainty - for example, choices between provision of increased safety margin; development of contingency plans; adoption of a different (more robust) option; or deferment to carry out more studies and collect further data.

General aspects of risk

1.11 Risk is concerned mainly with (i) the likelihood of future events, and (ii) their possible consequences or impacts. When combined or multiplied together, these 2 aspects give a measure of the scale or significance of the risk.

1.12 Risk is often taken to refer to a harmful or detrimental outcome. However, both negative and positive impacts should be considered. The guidance treats risk holistically, embracing all stages in the implementation of flood prevention schemes.

1.13 It is useful to think of risk in terms of sources, pathways and receptors. The source is generally a hazard such as a storm event, and the receptors include people and assets potentially at harm from associated flooding. The pathway is the set of mechanisms that could trigger the harm. For a community behind a sea wall, a pathway might be overtopping, followed by overland flow, ponding, and consequent flooding. There may be several pathways. For example, an embankment may be overtopped or breached or both, and the location of the breach may be uncertain.

1.14 Probabilistic representations of risk have been used for many years: engineers and managers are familiar with the concept of 'return period' as a measure of the likelihood that a given parameter, such as river level, will be exceeded. Sound decision-making, taking into account a large number of uncertainties, over a wide range of scheme types, requires a more rigorous assessment of risk.