Norwegian University of Department of Hydraulic and Science and Technology Environmental Engineering 0 NTNU ASSESSING INFRASTRUCTURE VULNERABILITY TO MAJOR FLOODS By Lars Jenssen A Dissertation Submitted to the Faculty of Civil Engineering, the Norwegian University of Science and Technology, in partial fulfilment of the requirements for the degree of Doctor Engineer Trondheim, Norway, May 1998 IVB Report B2-1998-2 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Abstract A modem society is complex and depends on several important utilities and types of physical infrastructure, e.g. power supply, transport systems, drinking-water supply, etc. Major floods have devastating effects on society in the flooded areas, and impacts on the infrastructure on which we depend will be felt far outside the inundated area. The objective of this work has been to suggest a method of assessing the direct effects of serious floods on a physical infrastructure or utility. The information was believed to be potentially useful for contingency planning and for designing of structures that are liable to be damaged by flooding. The first part of this work offers a review of a number of topics that are needed to assess the effects of flooding: 1) methods of floodplain management and strategies for mitigating floods, 2) methods of risk analysis which will become increasingly important in the field of flood management, 3) methods for hydraulic computations, 4) a variety of scour assessment methods and 5) several applications of GIS to the analysis of flood vulnerability. Three computer programs were developed to facilitate the practical use of the findings from the review: CULVCAP for computing the headwater level for circular and box culverts; SCOUR for assessing riprap stability and scour depths; and FASTFLOOD, which prepares input rainfall series and input files for the rainfall-runoff model used in the case study. The road system in Orkdal municipality in S0r-Tr0ndelag County in central Norway was chosen for exploring how to analyse the flood vulnerability of an infrastructure. The road system was analysed in two stages. First, a general analysis was performed to assess how major floods effect the road system. The objective of this stage was to identify the parts of the road system that are vulnerable to flooding and to develop methods for assessing the vulnerability of the road system in a specific area. The next stage dealt with evaluating the vulnerability of flooding of the main roads in Orkdal. First, potentially vulnerable objects were identified. Secondly, the objects were investigated for three flood scenarios: the 100-year flood, the 1000- year flood and the Probable Maximum Flood. Finally, a method for analysing the flood vulnerability of physical infrastructure has been proposed. The method involves a general stage that will provide data on which parts of the infrastructure are potentially vulnerable to flooding and methods for analysing them, and a specific stage which is concerned with analysing one particular kind of physical infrastructure in a study area. i 1 Preface This thesis provides a summary of literature reviews, field surveys and calculations carried out between 1994 and 1998, during the development of a method of assessing the potential damage to physical infrastructure from flooding. I have been enrolled as a doctoral student at the Norwegian University of Science and Technology (NTNU), Department of Hydraulic and Environmental Engineering, with a scholarship from NTNU. Special thanks go first and foremost to Professor Dagfinn K. Lysne at NTNU, Department of Hydraulic and Environmental Engineering, under whose supervision the studies in this thesis has been carried out. I thank him in particular for his kindness and support, and for his advice during innumerable discussions. I also wish to thank friends and colleagues at the Department of Hydraulic and Environmental Engineering. In particular I thank Leif Lia, for his friendship and valuable support during the final part of the study, and Hilbjprg Sandvik for always being so friendly and helpful. Last but not least, I thank Ingunn for her absolute support and patience during the long periods when this study took up most of my time and left little time for anything else. Table of content ABSTRACT i PREFACE ii 1 INTRODUCTION 1 2 BACKGROUND. OBJECTIVE AND SCOPE 2 2.1 Some characteristic effects of large floods 2 2.2 Planning for a flood emergency 2 2.3 Objectives 5 2.4 Organisation of the work 5 3 FLOOD MANAGEMENT AND RISK ANALYSIS 7 3.1 Understanding flood impacts 7 3.1.1 The physical destruction mechanisms of floods 7 3.1.2 Direct flood impacts 7 3.1.3 Indirect flood impacts 7 3.1.4 Classification of flood losses 8 3.2 Methods for estimating flood damage 8 3.2.1 The stage -damage curve 8 3.3 Floodplain management 11 3.3.1 Classification of flood mitigation measures 11 3.4 RISK ANALYSIS 14 3.4.1 AN OVERVIEW OF THE RISK ANALYSIS PROCESS 14 3.4.2 Methods for risk analysis 16 3.4.3 PRELIMINARY HAZARD ANALYSIS 16 3.4.4 RISK ANALYSIS APPLIED TO FLOOD RELATED-PROBLEMS 17 3.4.5 The risk and vulnerability (ROS) project in Orkdal MUNICIPALITY 17 4 HYDRAULIC COMPUTATIONS 20 4.1 Computing water profiles 20 4.1.1 Computation of gradually varying flow 20 4.1.2 Data REQUIREMENTS 22 iii 4.1.3 Model output analysis and model verification 23 4.1.4 Modelling bridges 23 4. l .5 G uidelines for bridge modelling with HEC-RAS 26 4.2 Computing culvert capacities 28 4.2.1 Culvert types 28 4.2.2 Culvert hydraulics 28 4.2.3 CULVCAP: A PROGRAM FOR RAPID ASSESSMENT OF CULVERT CAPACITY 33 4.3 Estimating roughness coefficients 35 4.3.1 Engineering judgement 36 4.3.2 Empirical formulae 36 4.4 Summary and recommendations 37 5 SCOUR AND SEDIMENT TRANSPORT 39 5.1 G eneral scour in uniform flow 39 5.1.1 THE shear stress approach to initiation of motion 39 5.1.2 The velocity approach 41 5.1.3 Stream power and erodibility index method 43 5.1.4 Stream bed armouring 44 5.1.5 Cohesive material 44 5.1.6 Erosion ON vegetated slopes 45 5.2 Scour in constrictions 46 5J Local scouring 46 5.3.1 SCOUR AT BRIDGES 47 5.3.2 SCOUR AT CULVERT OUTLETS 55 5.3.3 Scour by overtopping flow 59 5.3.4 Internal erosion 60 5.4 SCOUR - A PROGRAM FOR SCOUR CALCULATIONS 61 5.4.1 G eneral stability of riprap and bed material . 62 5.4.2 Riprap stability at piers 62 5.4.3 Scour depth at piers 63 5.4.4 Riprap stability at bridge abutments 63 5.4.5 Scour depth at culvert outlets 64 5.4.6 Scouring by overtopping flow 65 6 GIS AS A TOOL FOR FLOOD HAZARD ASSESSMENT 67 6.1 AN INTRODUCTION TO GIS 67 6.1.1 Data classes and data structures 68 6.2 The Idrisi GIS 70 IV 6.3 GIS AND HYDROLOGICAL MODELS 72 6.3.1 LUMPED HYDROLOGICAL MODELS 72 6.3.2 Distributed hydrological models 73 6.4 THE USE OF GIS IN FLOODPLAIN STUDIES 73 6.4.1 DATA ACQUISITION AND PRE-PROCESSING FOR FLOODPLAIN STUDIES 74 6.4.2 Preparing input data for the hydraulic model 75 6.4.3 Handling output data from hydraulic models 75 6.4.4 FLOOD DAMAGE ASSESSMENT 76 6.5 GIS APPLICATIONS IN RISK ASSESSMENT AND MANAGEMENT 76 6.5.1 Application of GIS in hazard assessment 77 6.5.2 Applications of GIS in vulnerability analysis 78 6.5.3 Applications of GIS in risk analysis 79 6.5.4 Applications of GIS in decision -making 79 6.5.5 THE USE OF GIS IN EMERGENCY MANAGEMENT 80 6.6 Applications of GIS in the current study 81 6.6.1 DATA ACQUISITION FOR THE CASE STUDY 81 6.6.2 AN OVERVIEW OF GIS APPLICATION OF THE CASE STUDY 83 6.6.3 EXAMPLES OF EXPERIMENTAL GIS APPLICATIONS 83 6.6.4 Identification of road sections vulnerable to debris flows 87 7 METHODS OF ESTIMATING FLOODS 91 7.1 Frequency analysis 91 7.1.1 Single -series flood frequency analysis 91 7.1.2 regional flood frequency analysis 92 7.2 Rainfall - runoff methods 92 7.2.1 THE RATIONAL METHOD 92 7.2.2 THE UNIT HYDROGRAPH 93 7.2.3 THE RAINFALL-RUNOFF MODEL PQFLOM 93 7.3 A ssessing rainfall 95 7.4 Recommendations 96 7.5 The flood model FASTFLOOD 96 7.6 Flood analysis during the case study 98 8 A GENERAL ANALYSIS OF THE FLOOD VULNERABILITY OF ROADS 102 8.1 Involved parties and their need for information 102 8.2 D escription of the road infrastructure 104 8.3 A ssessing undesired events , their causes and consequences 104 8.3.1 A nalysis of historical data on flood damage to roads 104 8.3.2 Preliminary Hazard Analysis 105 8.4 Undesired events and vulnerable objects 109 8.4.1 Undesired events identified during the study 109 8.4.2 Flood -vulnerable objects and their characteristics 112 8.4.3 Developing checklists for vulnerability assessment 113 9 ANALYSIS OF ROADS IN ORKDAT.118 9.1 Objectives of the analysis 119 9.2 Organisation of the work 119 9.3 Selection of flood scenarios 119 9.4 D escription of the road system in Orkdal 119 9.4.1 Subdividing the road system and assigning priorities 119 9.5 Identification of vulnerable objects 120 9.6 The evaluation of flood vulnerable objects 123 9.6.1 Preliminary screening of vulnerable objects 123 9.6.2 In depth hazard evaluation of each object 125 9.6.3 G eneral hazard evaluation of roads 125 9.6.4 Hazard evaluation at culverts 128 9.6.5 Hazard evaluation at bridges 130 9.6.6 Assessment of overall object performance during selected FLOOD SCENARIOS 131 9.7 Examples 131 9.7.1 Hazard analysis of culvert 9-7 Storbekken 131 9.7.2 Hazard analysis of Route 714 close to Skjenaldelv - Object 9-4 140 9.7.3 Hazard analysis of the bridge where Route 710 crosses Skjenaldelv - Object 8-2 143 9.7.4 Assessing road flooding along