EUROPEAN COMMISSION JOINT RESEARCH CENTRE Institute for the Protection and Security of the Citizen Technological and Economic Risk Management Natural Risk Sector I-21020 Ispra (VA) Italy NEDIES PROJECT Guidelines on Flash Flood Prevention and Mitigation Alessandro G. Colombo, Javier Hervás and Ana Lisa Vetere Arellano 2002 EUR 20386 EN ii NEDIES Series of EUR Reports on Lessons Learnt Alessandro G. Colombo (Editor): NEDIES Project - Lessons Learnt from Avalanche Disasters (pp. 14), Report EUR 19666 EN (2000). Alessandro G. Colombo (Editor): NEDIES Project - Lessons Learnt from Recent Train Accidents (pp. 28), Report EUR 19667 EN (2000). Alessandro G. Colombo (Editor): NEDIES Project - Lessons Learnt from Tunnel Accidents (pp. 48), Report EUR 19815 EN (2001). Alessandro G. Colombo and Ana Lisa Vetere Arellano (Editors): NEDIES Project - Lessons Learnt from Storm Disasters (pp. 45), Report EUR 19941 EN (2001). Chara Theofili and Ana Lisa Vetere Arellano (Editors): NEDIES Project - Lessons Learnt from Earthquake Disasters that Occurred in Greece (pp. 25), Report EUR 19946 EN (2001). Alessandro G. Colombo and Ana Lisa Vetere Arellano (Editors): NEDIES Project - Lessons Learnt from Flood Disasters (pp. 91), Report EUR 20261 EN (2002). iii iv ABSTRACT The NEDIES project is being conducted at Ispra by the Institute for the Protection and Security of the Citizen (IPSC), formerly the Institute for Systems, Informatics and Safety (ISIS), of the EC Directorate General Joint Research Centre (JRC). The objective of the project is to support the Commission Services of the European Communities, Member State Authorities and EU organisations in their efforts to prevent and prepare for natural disasters and accidents, and to manage their consequences. A main NEDIES activity is to produce "guidelines" on natural disasters and accidents. The Natural Risk Sector was asked by the Civil Protection and Environmental Accidents Unit of DG Environment to prepare these guidelines on flash flood prevention and mitigation, based mainly on three DG Environment-funded projects (Drau-Fersina, Geul and PREMO ’98). Furthermore, flash floods are also one of the priority areas within the DG Environment Major Project on Prevention of Natural and Technological Disasters. These guidelines are mainly envisaged for decision- makers in the field of flash flood management, but can also be of interest to practitioners and the general public. v ACKNOWLEDGEMENTS The NEDIES Team would like to thank Ernst Schulte of the Civil Protection and Environmental Accidents Unit of DG Environment for his comments and suggestions. The members of the EC Major Project on Prevention of Natural and Technological Hazards are also kindly acknowledged for their support. vi CONTENTS ABSTRACT v ACKNOWLEDGEMENTS vi CONTENTS vii LIST OF TABLES xi LIST OF FIGURES xii 1. INTRODUCTION 1 2. THE ASSESSMENT OF RISK 3 2.1 Characterisation of the Area Concerned 3 2.1.1 Information domains 3 2.1.2 Tools 4 2.2 Hazard Analysis 4 2.2.1 Choice of the map scale and hazard intensity scale 4 2.2.2 Definition of hazard scenarios and construction of a basic map of Hazard 5 2.3 Assignment of Probability to each Reference Scenario 5 2.4 Hazard Assessment 6 2.5 Vulnerability Analysis 7 2.6 Risk Assessment 7 3. MEASURES TO COPE WITH RISK 10 3.1 Background 10 3.2 Structural Measures 12 3.2.1 Catchment-wide interventions 12 3.2.1.1 Agriculture and forestry actions 13 3.2.1.1.1 Cultivated lands 13 3.2.1.1.2 Turf forming plants 14 3.2.1.1.3 Woods 14 3.2.1.2 Water control works 14 3.2.1.2.1 Check dams 15 - Timber check dams 15 - Timber and loose stones check dams 15 - Dry check dams 15 3.2.1.2.2 Drainage ditches 15 - Timbered ditches 15 - Ditches made out of timber and stones 15 3.2.1.2.3 Surface drainage 15 - Drainage by fascines 15 - Filtering wedges 16 vii 3.2.1.2.4 Wattlings 16 3.2.1.2.5 Bench-terraces 16 3.2.1.2.6 Stone retention walls 16 3.2.1.2.7 Revegetation 16 3.2.1.2.8 Road system 16 - Channels 17 - Cross gutters 17 3.2.2 River training interventions 17 3.2.2.1 Transversal protection works 17 3.2.2.1.1 Check dams 17 - Cemented stone or concrete dams 18 - Gabion dams 18 - Wood or rocks and wood dams 18 - Dry stone/wall dams 18 3.2.2.1.2 Sills 19 - Concrete or stone sills 19 - Gabion sills 19 - Sills made of blocks or blocks anchored to ground or wood and rocks 19 3.2.2.1.3 Beam dams and screen dams 20 - Screen dams with vertical steel or concrete bars 20 - Beam dams with central pylon bars, vertical opening and horizontal steel bars 20 3.2.2.1.4 Groynes 21 - Concrete or cemented stones 21 - Gabions 21 - Prefabricated or natural blocks 22 - Rocks and cuttings or wood and cuttings 22 3.2.2.1.5 Channel-lining 22 - Concrete, cemented stones 23 - Gabions 23 - Wood and stones or wood 23 3.2.2.2 Longitudinal protection works 23 3.2.2.2.1 Artificial banks or dikes 23 - Earthfill embankments 24 - Cemented bricks, stones or concrete embankments 24 3.2.2.2.2 Bank erosion protection 25 - Base stones 25 - Anterior stakes 25 - Sunken fascine rolls 25 - Wattle works 26 - Log cribwall with branchlayers on one bank 26 - Live brush mattresses 26 - Cuttings/stakes 26 - Brushlayers 26 - Crib groynes with cuttings 26 3.2.2.2.3 Maintenance of the hydraulic cross section 27 - Cutting of vegetation 27 viii - Reshaping of the hydraulic cross section 27 3.2.2.2.4 Maintenance of watercourses 28 3.2.3 Other flood control interventions 29 3.2.3.1 Passive flood control 29 3.2.3.2 Storm water retention basins 30 3.2.3.3 River corridor enhancement, rehabilitation and restoration 32 3.3 Non-Structural Measures 35 3.3.1 Risk acceptance 35 3.3.1.1 Toleration 36 3.3.1.2 Emergency response systems 36 3.3.1.3 Insurance 36 3.3.2 Risk reduction 39 3.3.2.1 Prevention strategies (Basin area management) 39 3.3.2.1.1 Delimitation of flood areas and securing of flood plains 39 3.3.2.1.2 Implementation of flood areas regulation 39 - Flood zonation and buffer zones 40 - Incentive policies on limited building in flood-prone areas 40 - Introduction and expansion of the legal basis of basin committees 40 - Introduction of Integrated Water Resources Management (IWRM) 40 - Improvement of legislation regarding dams 40 3.3.2.1.3 Application of financial measures 41 - Financial support to individuals and local Communities 41 - Subsidisation of basin committees 42 - Subsidisation of farmers applying the "Principles of sustainable farming" 42 3.3.2.2 Mitigation strategies 43 3.3.2.2.1 Reduction of discharge through natural retention 43 3.3.2.2.2 Actions based on Monitoring, Warning and Response Systems (MWRS) 43 3.4 Policy of Public Sensitisation 43 3.4.1 Advice regarding the public 45 3.4.1.1 Risk communication 45 3.4.1.1.1 Public alert and warning 45 - Regarding coordination of alert/warning Infrastructure 46 - Regarding information systems of alert/warning infrastructure 46 - Regarding flood warning messages 47 - A tool to facilitate public alert and warning 48 3.4.1.1.2 Public awareness campaigns 49 3.4.1.2 Public education schemes 50 3.4.1.2.1 Teaching at schools 50 3.4.1.2.2 Drills 51 ix 3.4.1.3 Public participation: towards an integrated flash flood management approach 51 3.4.1.4 Self-help information against flash floods 51 3.4.1.4.1 Precautions to be taken in the event of an imminent flood 52 3.4.1.4.2 Precautions to be taken during a flood 52 3.4.2 Specific advice regarding farmers 52 3.4.2.1 Management of agricultural areas 53 3.4.2.2 Management of pasture areas 53 3.4.2.3 Management of forestry areas 53 3.4.2.4 Management of areas near water courses 53 3.4.3 Advice regarding the media 53 3.5 Prioritisation of Measures 54 4. CLOSING CONSIDERATIONS 57 REFERENCES 59 ANNEX 1 - List of terms linked to flash flood management 61 ANNEX 2 - A suggested classification of flash floods 63 x LIST OF TABLES Tab. 2.2.1.a Hazard intensity scale, from Kienholz, 1996 (Fig. 3.5 in Ref. Drau-Fersina, 1999, modified) Tab. 2.3.a Probability level scale of a hazard scenario. Flooding of the Drau river (Fig. 3.6 in Ref. Drau-Fersina, 1999, modified) Tab. 2.4.a Hazard level scale. Drau river, Greifenburg - Bruggen Stretch (Fig. 3.8 in Ref. Drau-Fersina, 1999, modified) Tab. 2.5.a Vulnerability level scale as a function of land-use categories. Drau and Fersina rivers (Fig. 3.11 in Ref. Drau-Fersina, 1999, modified) Tab. 2.6.a Risk level scale. Drau river, Greifenburg - Bruggen Stretch (Fig. 3.16 in Ref. Drau-Fersina, 1999, modified) Tab. 3.1.a Structural and Non-Structural Measures Tab. 3.1.b Holistic approach to flood management Tab. 3.2.3.3.a Measures to enhance, rehabilitate and restore river corridors (Table 3.3.4.4 in PREMO'98, adapted from Saraiva, 1999) Tab. 3.3.1.1.a General Guidelines for an Emergency Plan (p. 453, Nicolau in Horlick-Jones et al., 1994, modified) Tab. 3.3.1.1.b Typical Structure of a Disaster Plan (p. 454, UNDRO, 1984, in Horlick-Jones et al., 1994) Tab.