Assessing flood risk at the global scale: model setup, results, and sensitivity

Abstract

Globally, economic losses from flooding exceeded $19 billion in 2012, and are rising rapidly. Hence, there is an increasing need for global-scale flood risk assessments, also within the context of integrated global assessments. We have developed and validated a model cascade for producing global flood risk maps, based on numerous flood return-periods. Validation results indicate that the model simulates interannual fluctuations in flood impacts well. The cascade involves: hydrological and hydraulic modelling; extreme value statistics; inundation modelling; flood impact modelling; and estimating annual expected impacts. The initial results estimate global impacts for several indicators, for example annual expected exposed population (169 million); and annual expected exposed GDP ($1383 billion). These results are relatively insensitive to the extreme value distribution employed to estimate low frequency flood volumes. However, they are extremely sensitive to the assumed flood protection standard; developing a database of such standards should be a research priority. Also, results are sensitive to the use of two different climate forcing datasets. The impact model can easily accommodate new, user-defined, impact indicators. We envisage several applications, for example: identifying risk hotspots; calculating macro-scale risk for the insurance industry and large companies; and assessing potential benefits (and costs) of adaptation measures.

The impacts of climate change on river flood risk at the global scale

“…They also truncated the damage functions at defined return periods to represent the effect of flood protection, with the protection standard assumed to depend on country GDP. Ward et al (2013) use a generic global stage-damage function, which is almost linear between a depth of 0 and 5 m.…”

“…New attempts to estimate current global flood risk (e.g. Ward et al 2013) use hydrological models at a much finer resolution (of the order of 1x1km 2 ), but these have not yet been applied to estimate future risk.…”

Defining high-flow seasons using temporal streamflow patterns from a global model

“…In this study, we evaluate simulations of daily streamflow over the period 1958-2000 taken from Ward et al (2013) (Van Beek and Bierkens, 2009;. Although the PCR-GLOBWB model is not calibrated, and simulations may contain biases and uncertainty at course spatial resolution, the long time series of streamflow provided globally has been deemed sufficient to estimate long-term flow characteristics with spatial consistency .…”

“…Additionally, this model has been validated in previous studies in terms of streamflow and terrestrial water storage at stations along major rivers in the world. The model's extreme discharges are also evaluated by Ward et al (2013) with fair to good performance at stations with large drainage area (≥ 125 000 km 2 ), corresponding to 24 % of GRDC stations used in this study, excepting overestimation in several arid regions. Note that for the simulations used in this study, the maximum storage within the river channel is based on geomorphological laws that do not account for existing flood protection measures such as dikes and levees.…”

“…To specifically address flood disasters from a global perspective, understanding of global-scale flood processes and streamflow variability is important (Dettinger and Diaz, 2000;Ward et al, 2014). In recent decades, studies have investigated global-scale streamflow characteristics using observed streamflow from around the world McMahon, 1992;McMahon et al, 2007;Peel et al, 2001Peel et al, , 2004Poff et al, 2006;Probst and Tardy, 1987) and modeled streamflow from global hydrological models McCabe and Wolock, 2008;Milly et al, 2005;Ward et al, 2013Ward et al, , 2014 to investigate ungauged and poorly gauged basins (Fekete and Vörösmarty, 2007). Despite this broad attention to annual streamflow and its connections to global climate processes and precursors, there has been relatively little attention paid to the intra-annual timing of streamflow, emphasizing the need for analysis of seasonal streamflow patterns to further improve understanding of large-scale hydrology and atmospheric behaviors in the main (flood) streamflow season globally (Dettinger and Diaz, 2000).…”