Understanding of and response to severe flash flooding Literature review Science Report: SC070021 Product code: SCHO0509BQAR-E-P The Environment Agency is the leading public body protecting and improving the environment in England and Wales. It’s our job to make sure that air, land and water are looked after by everyone in today’s society, so that tomorrow’s generations inherit a cleaner, healthier world. Our work includes tackling flooding and pollution incidents, reducing industry’s impacts on the environment, cleaning up rivers, coastal waters and contaminated land, and improving wildlife habitats. This report is the result of research commissioned by the Environment Agency’s Science Department and funded by the joint Environment Agency/Defra Flood and Coastal Erosion Risk Management Research and Development Programme. Published by: Author(s): Environment Agency, Rio House, Waterside Drive, Ben Cave, Liza Cragg, Jo Gray, Prof Dennis Parker, Aztec West, Almondsbury, Bristol, BS32 4UD Katherine Pygott, Sue Tapsell Tel: 01454 624400 Fax: 01454 624409 www.environment-agency.gov.uk Dissemination Status: Publicly available ISBN: 978-1-84911-055-6 Keywords: © Environment Agency June 2008 Flash floods, rapid response catchment, public understanding, response, warning All rights reserved. This document may be reproduced with prior permission of the Environment Agency. Research Contractor: Halcrow Group Ltd, Burderop Park, Swindon, The views and statements expressed in this report are Wiltshire. SN4 0QD those of the author alone. The views or statements expressed in this publication do not necessarily Environment Agency’s Project Manager: represent the views of the Environment Agency and the Jacqui Cotton, Flood Risk Science Environment Agency cannot accept any responsibility for such views or statements. Collaborator(s): Ben Cave Assoicates, Flood Hazard Research Centre This report is printed on Cyclus Print, a 100% recycled Middlesex University stock, which is 100% post consumer waste and is totally chlorine free. Water used is treated and in most cases Science Project Number: returned to source in better condition than removed. SC070021 Further copies of this report are available from: Product Code: The Environment Agency’s National Customer Contact SCHO0509BQAR-E-P Centre by emailing: [email protected] or by telephoning 08708 506506. Science Report – Understanding of and response to severe flash flooding: literature review ii Science at the Environment Agency Science underpins the work of the Environment Agency. It provides an up-to-date understanding of the world about us and helps us to develop monitoring tools and techniques to manage our environment as efficiently and effectively as possible. 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Steve Killeen Head of Science Science Report – Understanding of and response to severe flash flooding: literature review iii Executive summary About the report This report presents the findings of the literature review phase of a research project looking at the public’s understanding of and response to severe flash flooding, which the Environment Agency commissioned the Halcrow Group, the Flood Hazard Research Centre at Middlesex University and Ben Cave Associates to undertake in March 2008. The overall objective of this research project was to understand people’s perceptions of severe flash flooding and what their needs are in order to make an appropriate and effective response. Investigation into the meteorological and hydrological conditions that cause flash flooding and flood forecasting and modelling systems are outside the scope of this research project. The next phase of the project is qualitative research using focus groups and one-to-one interviews with professionals involved in a response operation to a recent flash flood. Background The UK has recently experienced a number of flash floods, such as those that occurred in Boscastle in 2004 and Helmsley in 2005. Flash floods are characterised by very short times between rainfall and consequent flooding. They also present an extreme danger to life, property and infrastructure because of the suddenness in the rise in water level, the flow velocity and debris. Consequently, predicting, preparing for and responding to flash floods poses great challenges. There is some evidence that the hazard of flash floods is growing due to climate change (Intergovernmental Panel on Climate Change (IPCC), 2007). Some researchers have also noted that human vulnerability to this hazard is growing because of increased development (Adams and Hooke, 2001). Key findings Key findings of the literature review relating to each of the five themes of the research project are as follows. Understanding of the term ‘flash flooding’: • Key characteristics of flash floods based on professional agencies’ definitions are the short lead in time involved (usually defined as less than six hours), the link to heavy rainfall, dam failure as a possible cause, the volume and velocity of water involved, the danger presented by debris, the potential to cause material damage and the urgent threat to life. • There is little research into how the public understands the term ‘flash flood’ and on awareness of the danger presented by such flooding. • Analysis of print media reporting of four flash flood case studies shows a good understanding of the key characteristics of flash floods. However, a broader analysis of print media articles indicates that the media also use the term more generally to describe sudden flooding associated with heavy or excessive rainfall. • The media have a key role to play in disseminating hazard messages and raising public awareness. However, print media articles rarely give specific public education messages with instructions about what to do in the event of a flash flood. Science Report – Understanding of and response to severe flash flooding: literature review iv Appropriate and effective responses before and during flooding: • Risk management agencies and the public have different ways of judging whether an action is appropriate and effective or not. • Mortality and injury depend on individual flash flood characteristics, characteristics of the area and people’s individual characteristics. However, avoiding risky behaviour, including walking and driving through floods, and climbing to higher ground are the cornerstones of effective response. • Drivers are especially vulnerable and difficult to warn and often underestimate the power of floodwaters. • The evidence suggests that evacuation does not always constitute an appropriate and effective response to flash flooding. Getting those at risk to realise and respond to this type of flood risk • Public information and awareness-raising activities are key to effective flood response. They should focus on the causes and signs of flash flooding, how to stay safe during a flash flood and when and how to evacuate. • Local knowledge and involvement in preparing for flash floods is crucial but there is some evidence to indicate that local knowledge is being undermined. • Whatever the source of a warning, the more times it is received the more likely it is to be believed. Therefore, several different delivery methods should be used, although the message must be consistent. • Vulnerable groups may not receive a warning unless it is a targeted at them. False alarms • Empirical evidence on the public’s tolerance of false warnings is difficult to find, with some saying that they would prefer to receive a warning even if there is no subsequent flood, while others state that they would begin to take warnings less seriously if they are received too often without flooding. There is some evidence that response to future warnings will not be diminished by false alarms where the basis of the false alarm is clear to the public. • Evidence from operational settings indicates that professional partners would prefer to be put on standby at an earlier stage, even if they were subsequently stood down. However, this evidence is limited. Lead time for warnings • The evidence suggests that longer lead times for flood warnings may result in increased benefits for health and reduced damage, but the evidence on damage reduction is not sufficient or robust. However, floods for which long warning times are not possible, such as flash floods, may be associated with reduced benefits from warnings whatever the lead time. • Evacuation may not always be the safest form of action. The characteristics of individual floods, of building type and structure, as well as the warning