Urban Flood (Risk) Management

INTRODUCTION

CAUSES AND IMPACTS OF URBAN FLOOD RISK

U NDERSTANDING URBAN FLOOD HAZARDS

• Local floods
• Riverine floods
• Flash floods
• Coastal floods
• Climate change and flooding

13 Floods in urban areas can be attributed to one or a combination of the above types. 26 As a result of global warming, many subsystems of the global water cycle are likely to intensify, resulting in an increase in the magnitude and frequency of flooding in many regions [7].

Table 1: Factors contributing to flooding

I MPACTS OF U RBAN F LOODS

• Exposure
• Vulnerability

Cities located in estuaries therefore have to bear the combined impacts of riverine and coastal flooding due to storm surges and tidal effects. Coastal areas are prone to sea erosion, which is particularly likely with the increase in ocean roughness due to climate change. Many million more people are expected to be inundated each year due to sea level rise in the 2080s.

One of the most important factors for the increase in urban flood damage is simply the increasing number of people and assets physically exposed to floods in cities. In developed countries, for aesthetic reasons, there is a preference to have living closure to rivers, seas and other bodies of water. In small islands, sea-level rise is expected to increase exposure to flooding from storm surges and erosion, thereby threatening infrastructure and people's livelihoods.

In particular, the islands with high dependence on tourism, with expected decline in water resources, are likely to lose their clientele and livelihood due to increasing risks and flooding of beach areas. 33 However, urban growth does not necessarily lead to intensification of risks if it takes flood risks into account in the land use (REF) planning processes. What matters is whether urban growth factors flood risks in the development process or not.

Figure 4: Urban population growth [9]

U RBAN F LOOD D AMAGES

47 The ultimate goal of integrated urban flood risk management is to minimize human losses and economic damage, while at the same time utilizing natural resources for the benefit and welfare of the population. 54 Based on the geographical definition of exposed areas, it is possible to make an initial assessment of the potential damage. It is important to be aware of the root causes of these vulnerabilities in order, when possible, to mitigate the underlying causes and not just the consequences.

97 Another approach to reducing economic vulnerability is the diversification of economic activities and sources of income of vulnerable communities. Flood risks must therefore be taken into account in the various sectoral strategies and policies of the country. 103 Urban flood risk management therefore needs to be implemented within a well-defined framework that recognizes and utilizes the potential synergies that could be achieved through coordinated action and addresses the conflicting demands that often arise.

124 To ensure that all stakeholders have the opportunity to be involved at some level in the decision-making process, it is critical to the success of the participation process to carefully identify stakeholders. Special means must be found to exploit the benefits and address the shortcomings. 128 The complexity of the evolution process of flood risks requires a clear understanding of the construction of the typology of different components of flood risks and the factors that mitigate or reduce them.

Figure 6: Categorization of Flood Losses [14]

INTEGRATED MANAGEMENT OF URBAN FLOODS

A IMS AND APPROACHES

This can be achieved either by reducing the flood risk to an acceptable level or by containing, sharing or transferring the flood risk through appropriate measures. 49 This first step in the risk management process is a comprehensive understanding, analysis and assessment of flood risks before flood mitigation measures are planned or implemented. This is the only way to provide urban planners with information about which risk component dominates in which area.

It can help answer questions about the types of measures that are likely to reduce flood risks most efficiently in a given neighborhood. Ultimately, to continuously improve flood risk management plans and associated measures, it is mandatory to evaluate the performance of the implemented measures and reassess residual risks and address them in turn. 50 The following sections discuss the basic steps towards the integrated development of a flood risk management plan.

After considering the flood risk assessment, the focus is on the review of urban flood risk management measures, exposure and vulnerability.

R ISK A SSESSMENT

In combination with data on flood frequency and magnitude, economic risk can be calculated and expressed, e.g. 55 Risk assessment products are risk maps that enable users to clearly identify the most vulnerable areas and neighborhoods. In addition to identifying areas of risk, the advantage of such comprehensive risk assessments is the fact that it is possible to compare risk components in quantitative terms, thereby identifying those risk components that contribute most to the creation of risk.

The latter links risk assessment with risk management measures by indicating the areas where preventive measures can reduce risks most effectively. 56 Risk assessment should not be seen as an exercise in obtaining numbers that indicate risk. Second, a risk assessment that only considers economic aspects does not provide information about society's vulnerability in social and environmental terms, nor does it adequately express society's ability to deal with emergency situations.

57 Normally such qualitative aspects are noted in the margins of risk assessments, but rarely are efforts made to operationalize them and integrate them systematically into the risk assessment. Obviously it is difficult to find indicators that satisfactorily reflect social vulnerability or social resilience, but there are risk assessment methodologies that offer promising suggestions on how to take these issues into account. Furthermore, these indices can be combined into one comprehensive risk index. , which represents the overall risk level of a given location and its inhabitants [17][18].

R ISK M ANAGEMENT M EASURES

• Measures to mitigate hazards
• Preventing exposure
• Reducing vulnerability: building resilience

The latter concept proposes a number of so-called "source control measures" intended to either retain or reduce stormwater runoff in order to prevent overflow of the drainage system and to mitigate the generation of flood hazards downstream. The central purpose of the "dual drainage system" is therefore to model and plan the main system to prevent the location of vulnerable infrastructure in the main system's flow paths and to ensure that there is an overland flow path of reasonable capacity. Polluted flood waters constitute one of the most serious post-flood problems in many urban areas.

77 Different possible flood risk mitigation options to manage urban flood risks are summarized in Box-2. Furthermore, upstream soil erosion and the resulting rate of sediment transport in watercourses can be important factors in the generation of urban floods. Experience shows that large-scale risk transfers can lead, especially in the urban context, to high total costs due to the combination of the cost of risk transfer and the cost of coping downstream.

Typically, when conflicts arise over urban land use, the potential for short-term benefits from development prevails over arguments of the need for "water space" in the context of flood prevention. Practically this means the definition of zones which determine the degree of exposure and the corresponding type of land use allowed to minimize the increase of exposure in the process of urban development. It is essential for the successful implementation of land use plans to have a continuous dialogue and communication conveying the necessity of targeted measures and to ensure public support by raising awareness.

96 In relation to reducing economic vulnerability, flood insurance can be an effective way, especially in the industrializing societies. 99 In all cases, it is essential to adapt the way in which information is presented to the respective socio-cultural environments, composition of the targeted population as well as to include local or traditional knowledge in the creation of information material.

Figure 9: Risk Management Cycle [19]

CONCEPTUAL FRAMEWORK FOR URBAN FLOOD RISK MANAGEMENT

• M AINSTREAMING FLOOD RISKS IN URBAN PLANNING
• Integrated Flood Management
• Total Water Cycle Management
• Land-use Planning
• S URFACE WATER MANAGEMENT PLANS
• O RGANIZATIONAL REQUIREMENTS
• Present shortcomings
• Institutional arrangements
• P ARTICIPATORY P LANNING OF M EASURES

107 Figure 18 illustrates some key components of urban hydrological balance that need to be taken into account if urban flooding is to be managed in an integrated manner. 112 A surface water management plan, based on input from a river basin flood management plan and an assessment of flood risks from local and potential coastal flooding, is an essential element for a comprehensive and coordinated approach to flood risk management. 114 The preparation of flood risk management plans and the implementation of the respective measures should not be regarded as a one-off task, but as a continuous learning process.

Measure assessment is a fundamental step in flood risk management and is closely related to risk reassessment. Decisions on new development (property and infrastructure) are often made without a full understanding of flood risks. Both overlapping responsibilities and gaps of irresponsibility, within institutions or between them, hinder effective urban flood risk management.

The previous chapters provided an overview of some aspects and considerations that should be included in flood risk management plans. 122 All the concepts underpinning the urban flood risk management framework are based on participatory principles. 123 These shortcomings can be overcome by establishing a participatory planning process as a basic principle in the risk management of urban floods.

Figure 18: Major components of the urban hydrologic balance [35]

CONCLUSIONS AND RECOMMENDATIONS

Megacities – Megarisks: Trends and Challenges for Insurance and Risk Management, http://www.munichre.com/publications/302-04271_en.pdf. 3] Glossary of the UN International Strategy for Disaster Reduction 2004, http://www.unisdr.org/eng/library/lib-terminology-eng%20home.htm. Social Aspects and Stakeholder Engagement in Integrated Flood Management, http://www.apfm.info/pdf/ifm_social_aspects.pdf.

5] http://www.nrcs.usda.gov/technical/stream_restoration/Images/scrhimage/chap3/fig3-21.jpg [6] http://www.uwsp.edu/geo/faculty/ritter/images/ lithosphere/fluvial/urban_hydrograph.jpg. Summary for Policymakers, http://www.ipcc.ch/pdf/assessment-report/ar4/wg2/ar4-wg2-spm.pdf. Economic aspects of integrated flood management, http://www.apfm.info/pdf/ifm_economic_aspects.pdf.

Presentation on Community Based Disaster Risk Index – Lessons from Indonesia, www.adrc.or.jp/unu/UNU%20Indicator%20Kobe.ppt. Stormwater Management Guidelines for the Province of Alberta, http://environment.gov.ab.ca/info/library/6786.pdf.