Urban Flood Management

Introduction - 1st International Expert Meeting on Urban Flood Management

Andras Szollosi-Nagy, Chris Zevenbergen

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351405973
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Over the last decades the world has witnessed a growing number of floods in urban areas. Climate change and rapid urbanization will exacerbate this trend. Flooding incidents in urbanized catchments and low-lying areas, such as polders, can lead to great public concern and anxiety, and their economical impact is severe. Apart from well-known flood prevention strategies, new approaches to the accommodation of floods are needed to create robust and sustainable solutions that enable us to cope with the ever-increasing urban pressure on flood-prone areas and the uncertainties created by climate change. Urban Flood Management comprises a multidisciplinary survey of recent developments in this field. Subjects like spatial and urban planning, flood insurance, flood resilience, flood proofing techniques, risk perception and preparedness and flood forecasting are treated by authorities from Brazil, India, the USA and Europe. Urban Flood Management will provide anyone active in the fields of water, risk and urban management with the latest information and insights that were obtained with a global and multidisciplinary approach.

1

Introduction

Andras Szöllösi-Nagy

UNESCO Natural Sciences Sector, Paris, France

Chris Zevenbergen

Dura Vermeer Business Development BV, Hoofddorp, The Netherlands

1.1 URBAN FLOODS

Floods and droughts have always been the natural disasters causing the greatest loss of human life and economic damage. As a result of an increase in vulnerability, flood impact statistics show an exponential growth in the number of flood events and affected populations. The greatest potential for flood disasters exists in the populous urban and peri-urban settlements of the globe. The characteristics of flood disasters evolve with the process of urbanisation and industrialisation. Gradually more and more potential flood disaster areas are developing. Some urban areas, however, are more vulnerable to floods than others, depending on both the sensitivity to floods and the ability to absorb the flood impact.

The magnitude of the flood risk depends on a combination of natural factors influencing the frequency of floods and human-related factors influencing both flood frequency and flood impact. The increase in vulnerability of cities to flood disasters arises predominantly from the following factors (Gladwell and Sim, 1993):

– systematic degradation of natural ecosystems;

– increased urban migration;

– unplanned occupation and unsustainable planning and building practices.

It is increasingly recognised that apart from these factors, climate change will intensify the already significant threat posed by flood disasters to human settlements, most particularly in the rapid urbanising third world. A striking transformation can be seen in the countries in this area, where unplanned urbanisation and poverty are dramatically increasing vulnerability to floods.

Urbanisation is a process that has multifaceted effects on the hydrology of a catchment. For instance, increased urbanisation of many cities in delta regions has caused considerable portions of cities to subside due to ground water abstraction. As a direct consequence, land subsidence will exacerbate the urban flood hazard and the impact of flooding. Catchment urbanisation also leads to the replacement of previously natural surfaces with impermeable ones, a reduction of soil infiltration and an increase in run-off volumes. Urban drainage is increasingly viewed as the “Achilles heel” of flood management systems (Ashley, 2003). Urban flood management strategies have traditionally focussed on water originating in the city itself and on protection against local flooding through the provision of a hydraulically efficient drainage system to accommodate the expected increase in storm water discharge (Maksimovic, 2001).

1.2 FLOODS AND DAMAGE

Since 1990 devastating floods were registered in virtually every part of the world, e.g., Mozambique, India, Central America, China, Poland, Germany and the Czech Republic. In this period, the material losses of 30 floods exceeded one billion US$ and/or the number of fatalities was greater than 1000. According to MunichRe (2001), floods accounted for 50% of economic losses caused by natural disasters in 2000. Most of the economic losses are sustained in the developed and industrialised countries, but expressed in relative terms (losses by percentage of GDP) it is the developing countries which show the highest losses. The opposite is the case for the number of people affected (fatalities, homelessness, diseases). During the last decades the reported number of disasters caused by floods in Europe has dramatically increased, from 31 in the period 1973–1982 to 179 during the last decade (see Figure 1.1). In the entire period floods caused a total of 264 disasters. According to Hoyois and Guha-Sapir (2003) the total amount of reported damage from disastrous floods in Europe increases from 5.7 billion in the first to 48.6 billion Euro in the last three decades. The sum total of direct economic loss in Europe in this entire period amounted to 72 billion Euro.

Figure 1.1. Total number of floods reported in Europe (modified after Hoyois and Guha-Sapir (2003).

The impact of floods on cities can devastate national economies and industrial markets at an international level. Due to urban concentration of the population the greatest potential for flood disasters exists in the most populous cities. For instance, in Japan where about 50% of the population and about 75% of the assets concentrate in urban agglomerates on flood plains. It is important at this point to note that in Japan over the last three decades, the number of flood disasters and the amount of economic loss has increased, while the number of fatalities has decreased. This decrease is due to improved early warning systems and increased preparedness, among other factors. An illustration of this trend is the most severe flood disaster, which hit the Tokai area with 2.1 million residents in September 2000. This flood caused 10 deaths and 978.3 billion Japanese yen in direct economic losses. As noted earlier, however, it is expected that in developing countries population growth and increased urban migration will further increase the number of deaths and the economic losses in cities, where nowadays more than 90% of natural disaster related deaths occur.

1.3 DEVELOPMENTS IN FLOOD MANAGEMENT

The way in which flood management is approached has evolved over time. Historically, there have been four successive approaches to flood hazard management (Green et al., 2000):

– Indigenous flood adaptation: communities have occupied flood-prone areas for many generations. Local adaptations develop to make them more resilient to floods: houses are constructed on stilts (e.g., in New Guinea, Benin, Thailand) and on higher ground and mounds (e.g., in The Netherlands, Bangladesh) to raise them above anticipated flood levels. In Bangladesh houses are even temporarily dismantled in times of high water. There are numerous examples of indigenous adaptations to floods, however, mainly due to rapid urbanisation and economic growth in conjunction with a growing faith in structural measures to protect communities from floods, these approaches have been abandoned. It goes without saying that consideration of these approaches may still prove useful in seeking a solution for today’s problems.

– Flood control and defence: structural measures, such as flood embankments, dikes and flood control dams, were developed in the 19th and most of the 20th century. These large-scale engineering measures appeared to be very effective in controlling rivers and preventing flood water entering communities located in flood-prone areas. However, this second generation approach received much criticism in the late 20th century. Provision of flood protection, either in an urban or rural environment has led to cyclical patterns of investment - in the absence of land-use controls, the population density in former flood plains has increased, property values have risen and the pressure to provide a greater degree of flood protection has intensified. The effectiveness of physical flood control measures may change in time depending on the level of maintenance and on physical processes that affect sustainability, for example, changes in river morphology and sedimentation. Even in most European countries with resources for maintenance and more sophisticated flood warning systems, the physical flood protection infrastructure has on occasion failed to provide full protection.

– Non-structural approaches: as an alternative to the engineering solutions non-structural approaches have received much attention in the US and Western Europe. In this approach emphasis is laid on both the behaviour of people and on strategies which influence their behaviour (moving people away from floods, evacuating them using warning systems, etc.) and on land-use planning as a steering instrument to design and plan communities in safe areas or to better adapt them to the impacts of floods. Although public perception may be that physical intervention is sufficient, the reality is that considerable management effort is required and additional attention and resources need to be allocated for non-structural approaches to flood management (flood forecasting, land-use zoning, flood proofing, disaster preparedness, flood insurance) either in parallel or independent of structural forms of flood protection.

– Living with floods: the recurrence of disastrous floods have shown that many structural and non-structural strategies have failed to be effective. New options emerge, in which flood-prone areas are kept free from urbanisation and transformed into “green belts”, through which flood waters can pass and temporarily be stored to prevent surrounding urbanised areas from flood damage. Simultaneously, these open areas may serve recreational purposes. It is becoming increasingly recognised that there is no single effective strategy, but a variety of strategies based on a holistic approach taking into account both the causes and the impact of floods and flood disasters on the catchment as a whole. The choice of measures to be implemented may also be dominated by local economics and social factors. The level of protection a city or region can afford and are willing to accept remains a principle question and imposes complex constraints, which cannot be ignored. A holistic approach thus considers the catchment as a unit for planning and management, in which sustainable water management is the guiding principle in the planning process.

1.4 URBAN FLOOD MANAGEMENT

Urban floods may have wide impact on natural, economic, social and cultural systems. Urban flood management approaches that fail to enhance the capacity of individuals and society to cope with floods are, therefore, likely to be counter productive in the longer term (IRMA, 2001). It is increasingly recognised that urban flood management calls for a holistic approach as outlined in the previous section, considering measures to reduce flood probability and flood impact (see Figure 1.2). The selection of measures including flood protection, flood preparedness and flood mitigation should address different spatial scales and support sustainable development of the entire catchment, while maximising the economic efficiency of land use. The latter implies that, for instance, urban drainage solutions should be at best integrated into flood management plans for the entire catchment (Maksimovic, 2001). Measures to reduce flood impact aim to reduce the risk of urban flooding at a local scale using a portfolio of adaptation strategies. These strategies are directed at increasing flood resilience of the built environment by adopting sustainable planning, pollution source control, urban drainage and flood proof building practices.

Figure 1.2. Urban flood management: measures.

Measures to increase flood resilience are developed with the objective of reducing the impact of flooding on an urban community. Financial benefit in terms of flood damage reduction is an important criterion for the evaluation, ranking and selection of appropriate options (Price and Mclnally, 2001). However, the lack of data and analysis of the technical performance and the economics of implementation and maintenance hampers promotion of flood proofing and urban drainage systems.

1.5 FLOOD PROOFING

Flood-proofing techniques comprise an arsenal of engineering options to protect buildings and infrastructure from flood damage and to keep flood water away from damageable properties.

Flood-proofing techniques have been developed to the greatest extent in the US. Some of these techniques originate in indigenous flood adaptation approaches. Two categories of flood-proofing techniques can be distinguished (Green et al., 2000):

– permanent flood proofing; the building is constructed or transformed so as to reduce flood damage;

– contingent flood proofing; action is taken (e.g., installation of barriers across all openings) immediately preceding the onset of the flood.

Flood-proofing techniques have been widely applied specifically to protect historical buildings from flood damage and to allow low density urban development in flood prone areas. Its application, however, has been restricted to the developed world only. With the growing pressure to obtain urban land due to population growth and rural-urban migration, flood-proofing solutions may introduce a new and valuable option to city development projects located in risk areas in developing countries. Reducing the cost of flood proofing housing, however, using new or improved construction techniques, the application of low cost building technology and the use of appropriate materials is one of the prerequisites for the introduction of flood-proofing techniques in developing countries.

The following are examples of permanent flood-proofing techniques (Green et al., 2000):

– Buildings on fill and artificial mounds: buildings are constructed on fill or artificial mounds raised above the design flood level. This method does not require design modifications, and if the design flood water level is exceeded, the depth of water over the fill will be shallow and of short duration. It is not, however, a practical alternative for protecting existing buildings.

– Floating and amphibious buildings: floating buildings may provide an alternative to elevated buildings in permanently inundated areas (e.g., detention basins). These houses can tolerate a fluctuating water level and are therefore well protected against flood water. Floods and subsequent dike reinforcements in the catchment basin have led to the development of amphibious houses in The Netherlands: i.e., houses that float during floods. To enable the houses to move with the water level, they are built on floating concrete bodies with a coupling construction. At low water levels, the houses rest on a concrete foundation. They have a wooden frame construction in order to keep them as light as possible. To prevent the houses from floating off during a flood, they are anchored to flexible mooring posts. A difference in water level of up to about 5 metres can be accommodated.

– Buildings on piers, piles, columns or bearing walls: for many generations elevated houses have been constructed in delta areas and along rivers. Elevating structures may provide reliable protection ag...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• List of contributors
• 1. Introduction
• 2. Land use and urban floods in developing countries
• 3. Cities, lakes, and floods. The case of the Green Hyderabad project, India
• 4. Co-operation within Europe on flood management and spatial planning
• 5. Spatial measures and instruments for flood risk reduction in selected EU countries – a quick scan –
• 6. Risk perceptance and preparedness and flood insurance
• 7. The role of private insurance companies in managing flood risks in the UK and Europe
• 8. New strategies of damage reduction in urban areas proned to flood
• 9. Flood resilience in the built environment: damage and repair
• 10. California climate change, hydrologic response, and flood forecasting
• Conclusions
• Author index