Flood risk management

Flooding is most commonly caused by heavy rainfall, but can result from other phenomena such as storm surges, tropical cyclones, tsunamis or high tide as well.

Furthermore, non-natural hazards such as dam failure can also cause intense flooding.

These various causes result in a series of different flood types, such as river floods, flash floods, urban floods and floods from the sea in coastal areas (the assessment and management of flood risks, Directive 2007/60/EC). Of these types, river flooding has the largest impact, affecting 21 million people with a cost to society of USD 96 billion in Gross Domestic Product (GDP) each year. By 2030, these numbers could grow

up to 54 million people and USD 521 billion in GDP annually (Deltares, 2015). This rise in flood risk can be attributed to population growth, urbanization and poor land use practices in flood prone areas (Keating et al., 2014). Climate change and socio- economic development will further modify the frequency, intensity and regularity of floods and other hazards, especially in already vulnerable regions (UNDRR, 2019).

In the past, decision makers have tried to prevent flooding completely. By implementing technocratic interventions such as dikes and levees, the rainfall was sent downstream as quickly as possible. This approach requires a very high economic investment and does not assure a complete protection against inundations (Vanneuville et al., 2005).

Therefore, the focus of flood risk management needed to shift to protecting as much people and infrastructure as possible for a reasonable cost (Vanneuville, et al., 2003).

When the investments in flood defense infrastructure no longer lead to a proportional decrease in expected damages, the economic optimum has passed and the total cost of damages and investments increases (De Nocker et al., 2004). This is shown in Figure 5.

Figure 5 Economic optimum in a cost benefit analysis for water infrastructure (Vanneuville et al., 2005).

A complete flood risk management focuses not only on this optimal protection and prevention, but on preparedness as well. This approach was set forward by the European Flood Risk Directive of 2007 as the 3 P’s. Measures that prevent and reduce damage to human health, the environment, cultural heritage and economic activity are combined with giving rivers more space and maintaining and restoring natural floodplains (the assessment and management of flood risks, Directive 2007/60/EC).

As such, by considering the water system at large, this approach acknowledges that water is a resource before being a threat (UNDRR, 2017b). In more recent literature, this concept is often described as a multi-layer safety system or multi-level flood governance (Dieperink et al., 2016; Kaufmann et al., 2016; Kolen & van Gelder, 2018; Lopez, 2009).

In the Netherlands, for example, flood measures are classified in three safety layers:

(i) measures for the prevention of flooding such as dykes and storm-surge barriers; (ii) sustainable spatial solutions for the mitigation of losses such as flood proofing or the relocation of buildings to safer places; and (iii) disaster management and emergency management measures such as evacuation (Tsimopoulou et al., 2013).

INTRODUCTION

1.2.3.1 Flood risk assessment

In this dissertation, flood risk is defined through the lenses of the main terms of the risk equation: hazard and vulnerability. When applying the risk definition to flood risk assessment, the term hazard is commonly limited to the probability of flooding as a single event in the study area (Deckers et al., 2009). The types of vulnerability taken into account in flood risk assessment can differ, depending on the available data and the specific aim of the assessment.

In the same manner as other natural hazards, flood hazard assessment models the initiation event – usually rainfall – and its evolution physically and statistically (UNDRR, 2017b). In the case of fluvial flooding, the evolution is modelled using a hydrological model to assess the routing of precipitation from rainfall to runoff and a hydraulic model to evaluate the spatial extent of floodable areas (Arseni et al., 2020). The result of this model is a set of flood hazard maps that cover the geographical areas which could be flooded according to preset scenarios (the assessment and management of flood risks, Directive 2007/60/EC). For each scenario, following elements are commonly shown on the flood hazard map: (i) the flood extent; (ii) water depth; and, where appropriate, (iii) the flow velocity.

In comparison to other types of risk, flood suffers from a very strong imbalance in the level of maturity in assessing the different elements of the risk: whereas hazard modelling is well advanced, vulnerability analysis is underdeveloped, and therefore, the weakest link (UNDRR, 2017b). Especially when a quantitative vulnerability assessment for floods is wanted, data availability and the level of accuracy remains a challenge. The type of vulnerability assessment depends on the type of vulnerability under investigation. Flood vulnerability can be defined as the sensitivity of a community or people to flooding considering the social, economic, environmental, physical and cultural components (Munyai et al., 2019). When a flood event occurs, each of these vulnerabilities can result in losses. Losses are quantifiable measures, often expressed in either monetary terms for physical assets or counts such as number of fatalities (GFDRR, 2014). Other losses, for example the destruction of culturally significant sites or ecosystems or psychological consequences, are more difficult to quantify. Such losses are often described as ‘intangible losses’ and are rarely taken into account in disaster risk assessments (GFDRR, 2014). Direct losses refer to the immediate physical and structural impact caused by a flood, such as the destruction of infrastructure.

Indirect losses are the secondary results of the initial destruction, such as business interruption losses (University of South Carolina, 2014). Figure 6 gives some examples of direct and indirect as well as quantifiable and non-quantifiable losses. While losses are described as a measure of the damage or destruction caused by a disaster, a flood can have a much further reaching impact, including longer-term social and economic effects in education, health, productivity or in the macro economy. The impact does not only cause losses, which are by definition negative effects, but can also generate gains for some people and economies, for example the construction industry (UNDRR, 2015a).

Figure 6 Examples of direct and indirect losses as well as quantifiable and non-quantifiable losses (University of South Carolina, 2014).

By combining flood hazard and vulnerability, flood risk is calculated and visualized in flood risk maps. These maps show the potential adverse consequences associated with the flood scenarios – shown in the flood hazard maps – and are most commonly expressed in the number of people affected or killed and the economic consequences.

Other information, such as the pollution potential, is sometimes shown as well if it is an added value for the area under study (the assessment and management of flood risks, Directive 2007/60/EC).