MANAGEMENT OF FLASH FLOODS A Tool for Integrated Flood Management ASSOCIATED PROGRAMME ON FLOOD MANAGEMENT May 2012 WMO/GWP Associated Programme on Flood Management The Associated Programme on Flood Management (APFM) is a joint initiative of the World Meteorological Organization (WMO) and the Global Water Partnership (GWP). It promotes the concept of Integrated Flood Management (IFM) as a new approach to flood management. The programme is financially supported by the governments of Japan, Switzerland and Germany. The World Meteorological Organization (WMO) is a specialized agency of the United Nations. It coordinates the activities of the meteorological and hydrological services of 189 countries and territories and is the authoritative voice on weather, climate and water. The Global Water Partnership is an international network open to all organizations involved in water resources management. It was created in 1996 to foster Integrated Water Resources Management (IWRM). Cover photo credits: © FloodSafe/SES/NSW Government – Heavy rainfall led to flash flooding: a torrent of water running down across the highway at swollen Newell Falls, New South Wales, Australia, March 31, 2009. Management of Flash Floods – A Tool for Integrated Flood Management, Version 1.0 WMO/GWP Associated Programme on Flood Management Note for the reader This publication is part of the “Flood Management Tools Series” being compiled by the Associated Programme on Flood Management. The contained Tool for “Management of Flash Floods” is based on available literature, and draws findings from relevant works wherever possible. This Tool addresses the needs of practitioners and allows them to easily access relevant guidance materials. The Tool is considered as a resource guide/material for practitioners and not an academic paper. References used are mostly available on the Internet and hyperlinks are provided in the “References” section. This Tool is a “living document” and will be updated based on sharing of experiences with its readers. The Associated Programme on Flood Management encourages flood managers and related experts engaged in flash flood management around the globe to participate in the enrichment of the Tool. For this purpose comments and other inputs are cordially invited. Authorship and contributions would be appropriately acknowledged. Please kindly submit your inputs to the following Email address: [email protected] under Subject: “Management of Flash Floods”. Acknowledgements This Tool makes use of the works of many organizations and experts, as listed in the references. The APFM Team is grateful to Mr. Robert Jubach of the Hydrologic Research Centre (HRC), San Diego, USA for a general review of this tool. Disclaimer The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. APFM Technical Document No. 21, Flood Management Tools Series © World Meteorological Organization, 2012 Management of Flash Floods – A Tool for Integrated Flood Management, Version 1.0 WMO/GWP Associated Programme on Flood Management Executive Summary 1. The purpose of this document is to provide an overview of the actions that can be taken to manage and minimize the potential impacts of flash floods. This tool aims to: • Provide basic information about flash floods especially focusing on differences from riverine floods; • Identify flash flood risk components and provide guidance to assess its risk; • Provide perspectives of flash flood management strategies on how to reduce risks; and, • Provide guidance to involve communities and individuals in flash flood management and risk reduction. 2. This management tool is written primarily for flood managers at various levels of government, specifically for National Meteorological and Hydrologic Services (NMHSs) and disaster management agencies for improvement of flash flood warning systems. It is not meant to be a comprehensive manual but only to highlight those important aspects for planning and management towards mitigation of the impacts of flash floods. 3. Due to its characteristics, it is difficult to address flash flood issues by traditional flood management approaches as used for low‐land riverine floods. Integrated approaches as a combination of structural and non‐structural measures are essential for successful flash flood management. This tool mainly focuses on the non‐structural measures. 4. This tool is based on available literature, and draws findings from relevant works wherever possible. This Tool addresses the needs of practitioners and allows them to easily access relevant guidance materials. The Tool is considered as a resource guide/material for practitioners and not an academic paper. 5. This tool is based on the APFM Guidance on Flash Flood Management, which reported a flash flood pilot project in Central and Eastern Europe (WMO, 2007a). It is also recommended that the reader consult the Flash Flood Early Warning System Reference Guide (University Corporation for Atmospheric Research, 2010). Management of Flash Floods – A Tool for Integrated Flood Management, Version 1.0 WMO/GWP Associated Programme on Flood Management Table of Contents 1. Introduction.......................................................................................................1 2. Flash Flood Characteristics ................................................................................3 2.1 Flash Flood Definitions ..................................................................................................................3 2.2 Generation Mechanisms of Flash Floods.......................................................................................3 2.3 Dangers of Flash Floods.................................................................................................................5 2.4 Differences between Flash Floods and Riverine Floods................................................................6 3. Flash Flood Risk Assessment .............................................................................7 3.1 Risk Components ...........................................................................................................................7 3.2 Assessing the Risk..........................................................................................................................7 3.3 Assessing Vulnerabilities ...............................................................................................................9 4. Strategies for Flash Flood Management..........................................................11 4.1 Structural Measures to Mitigate Flash Flood Hazards ................................................................12 4.2 Strategies for Exposure................................................................................................................13 4.2.1 Flash Flood Forecasting Considerations and Uncertainties ................................................13 4.2.2 Flash Flood Forecasting Operational Examples...................................................................14 4.2.3 End‐to‐End Forecasting and Warning Systems ...................................................................16 4.2.4 Flash Flood Warnings...........................................................................................................17 4.2.5 Organizational Requirements..............................................................................................18 4.2.6 Spatial Planning ...................................................................................................................18 4.3 Strategies to Reduce Vulnerability ..............................................................................................19 4.3.1 Participatory Planning .........................................................................................................19 4.3.2 Planning Preparation ...........................................................................................................20 4.3.3 Challenges in Community Participation ..............................................................................21 4.3.4 Raising Awareness and Preparedness .................................................................................21 4.3.5 Flood insurance ...................................................................................................................22 4.4 Requirements for a Flash Flood Management in a Legal and Institutional Framework .............23 5. Local Level Activities........................................................................................25 5.1 Step 1 – Evaluation of Community Readiness.............................................................................25 5.2 Step 2 – Creating a Planning Team..............................................................................................25 5.3 Step 3 – Public Consultation........................................................................................................26 5.4 Conditions for Success.................................................................................................................26 6. Conclusions and Recommendations................................................................27 7. References.......................................................................................................29