Report No: AUS0001219 . Central America Strengthen DRM+Resilience of CA cities Guidelines for Flood Risk Modelling in the Sula Valley, Honduras. November, 2018 . URS . Document of the World Bank . © 2017 The World Bank 1818 H Street NW, Washington DC 20433 Telephone: 202-473-1000; Internet: www.worldbank.org Some rights reserved This work is a product of the staff of The World Bank. The findings, interpretations, and conclusions expressed in this work do not necessarily reflect the views of the Executive Directors of The World Bank or the governments they represent. The World Bank does not guarantee the accuracy of the data included in this work. The boundaries, colors, denominations, and other information shown on any map in this work do not imply any judgment on the part of The World Bank concerning the legal status of any territory or the endorsement or acceptance of such boundaries. Rights and Permissions The material in this work is subject to copyright. 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THE WORLD BANK GUIDELINES FOR FLOOD RISK MODELLING IN THE SULA VALLEY - HONDURAS Draft Report Maria Carolina Rogelis 11/5/2018 Guidelines for flood risk modelling in the Sula Valley – Honduras Draft Report CONTENTS 1 Introduction ........................................................................................................................ 3 2 Definitions ........................................................................................................................... 4 3 Study Area .......................................................................................................................... 7 4 Planning for flood risk assessment ........................................................................................ 9 5 Methodological approaches ................................................................................................ 11 5.1 Data ........................................................................................................................... 11 5.1.1 Background site data ............................................................................................. 11 5.1.2 Terrain Data .......................................................................................................... 13 5.1.3 Infrastructure data ................................................................................................ 18 5.1.4 Rainfall data .......................................................................................................... 19 5.1.5 Stream flows ......................................................................................................... 21 5.1.6 Historical data ....................................................................................................... 22 5.1.7 Land Use and land cover ........................................................................................ 25 5.1.8 Assets data ........................................................................................................... 25 5.1.9 Demographic and socio-economic data ................................................................... 26 5.2 Flood Hazard assessment and flood mapping ................................................................ 27 5.2.1 Hydrologic analyses ............................................................................................... 29 5.2.2 Hydraulic modeling ................................................................................................ 32 5.3 Exposure ..................................................................................................................... 43 5.4 Vulnerability ................................................................................................................ 43 5.5 Risk ............................................................................................................................ 50 6 Floodplain Zoning .............................................................................................................. 56 7 Maps ................................................................................................................................. 62 8 Visualization and dissemination of hazard, vulnerability and risk information ......................... 65 9 Community consultation ..................................................................................................... 67 10 Debris flows risk assessment guidelines ............................................................................. 67 10.1 Debris flow hazard modeling....................................................................................... 69 10.2 Vulnerability curves .................................................................................................... 72 10.3 Risk analysis .............................................................................................................. 74 11 Required team for flood risk assessment ........................................................................... 76 12 Guidelines on reporting .................................................................................................... 76 References .............................................................................................................................. 78 2 Guidelines for flood risk modelling in the Sula Valley – Honduras Draft Report 1 Introduction Honduras is highly exposed to climate-related hazards (hurricanes, tropical storms, floods, droughts and landslides). The most damaging event so far is the Hurricane Mitch (1998) that destroyed and estimated 70% of the country´s crops and infrastructure, causing more than 10000 deaths and $ 3 billion in damage (USAID 2017). Floods are the one of the disasters that affects the most people out of all hazards in Honduras (Thieme and Jacobs 2012). One of the areas heavily affected by floods is the Sula Valley that concentrates 24.5% of the population of the country and where 5 of the main cities of the country are located. This is the area with the most rapid economic growth of the country with 80% of the manufacturing and textile industry (Caballero et al. 2008). Flood risk assessment is an essential component in the disaster risk management and climate change adaptation planning process, and constitutes a crucial element in the sustainability of Honduras and in particular of the Sula Valley. The purpose of a risk assessment is to define the nature of the risk, answer questions about characteristics of potential hazards (such as frequency, severity), and identify vulnerabilities of communities and potential exposure to given hazard events. Risk evaluation helps in the prioritization of risk management measures, giving due consideration to the probability and impact of potential events, cost effectiveness of preventative measures, and resource availability (The World Bank 2011). Development and implementation of flood risk studies and flood mapping impacts a wide-range of key users in areas as diverse as land use planning, emergency management and community awareness. Communication of this information helps to build flood resilience within the community and informs and agencies to manage flood risk into the future (BMT WBM Pty Ltd 2017). This document compiles good practices and guidelines for flood risk assessment in the Sula Valley, that are intended to promote future studies meeting the regional and local needs, for an effective approach to Integrated Flood Management. Despite the variety of flooding sources in the Sula Valley, this document if mainly focused on fluvial floods. Coastal and debris flows are also addressed although in a more general manner. 3 Guidelines for flood risk modelling in the Sula Valley – Honduras Draft Report 2 Definitions The following are the definitions of the main concepts used in this document. Floods Floods can be defined as a temporary covering of land by water outside its normal confines (Schanze et al. 2006). Figure 1 shows a classification of floods according to the origin. Figure 1. Basic classification for the type of floods according to their origin. Source: (Díez- Herrero et al. 2009) Some types of floods are shown in Figure 4. Their detailed definition is as follows: • Tidal flooding: Both sea and river defences may be overtopped or breached by a combination of low pressure weather systems and peak high tides. Storms with high wind speeds cause tall and powerful waves and low pressure fronts cause sea levels to rise above normal levels (RIBA 2009). • Fluvial Flooding: Flooding occurs in the floodplains of rivers when the capacity of water courses is exceeded as a result of rainfall or snow and ice melts within catchment areas further upstream. Blockages (RIBA 2009). • Pluvial Flooding: Surface water flooding is caused by rainwater run-off from urban and rural land with low absorbency. Increased intensity of development in urban areas has given rise to land with a larger proportion of non-permeable surfaces, a problem