Fewer deaths per event

Since 1975, the global number of flood events has increased over time, albeit with a slower increase since 2000 compared to previous decades. However, floods have become less lethal. The doubling of global population since 1975, partly in flood prone areas along coasts and in river basins, has not increased the total number of annual flood fatalities over time. In fact, since 1975, the average number of people killed and affected per event has decreased. The latter reflects the impact of increased flood protection, better warning, forecasting and early warning communication and other forms of risk reduction over the last decades. The annual number of large events – with over 100 fatalities – has decreased since 2000 (24).

Biggest change for middle-income countries

In their analysis, the researchers also made a distinction in low-, middle- and high-income countries (based on World Bank data). They showed that especially middle-income countries have succeeded in reducing mortality from flooding. For low-income countries mortality increased after the year 2000 whereas no trend was found for high-income groups. According to the authors of the study this may be due to an increase of the number of people living in floodplains. In low-income countries, people moving into floodplains are too poor to adequately protect themselves against flooding. Indeed, as we have shown before, more and more natural floodplains are being converted into agricultural land and built-up areas, globally, increasing exposure to flooding (24).

High mortality flash flooding

The study includes riverine, coastal, and flash flooding. The analysis shows that riverine and flash floods are the most frequent flood type for flood events up to 1,000 fatalities. Around 60 events of these types of flooding occur each year with 5 or more fatalities. The study also highlights the relatively high mortality from flash flooding, consistent with previous findings. Although coastal floods from storms and tropical cyclones occur less frequently, they are dominant for events over 1,000 fatalities. Particularly due to these large events, most flood fatalities – around 85% – occurred in Asia (24).

Floods and overflows occur when runoff or sewer flow thresholds are exceeded. Given that the response of the sewer system to rainfall may be highly non-linear, the changes in the sewer response may be much stronger than the changes in rainfall. And the impact ranges can even by wider when studying the impacts of sewer overflows on receiving rivers. Sewer overflow mainly occurs in summer and as models project the likelihood of lower river flow in summer in northwestern Europe, dilution effects in the receiving water might be less, thus increasing impacts on river water quality and aquatic life (6). Changes other than those related to climate may also occur in urban areas, such as increased urbanization and more pavement surfaces, and affect or strengthen urban drainage impacts (7).

The necessary adjustments can be fit in the city’s infrastructure quite easily. By digging a trench, for instance, alongside the pavement filled with gravel and soil, and planted with trees. The trench acts as an intermediary step between the storm drain and the sewer, a sponge, and slows and purifies the runoff to protect the city. There’s an extra advantage to more trees in the cities: their shade relieves the heat during summers (1).

In addition to these adjustments of the urban drainage system, several other measures can be taken that strengthen these positive effects of reducing urban flooding and increasing water storage in aquifers: pervious paving increase infiltration, and green roofs capture storm water, allowing much of it to evaporate before directing the filtered excess into the city’s drainage system (1).

Green versus engineering: (bio)infiltration

Urban green infrastructure may be effective as an integral component of storm water adaptation measures to mitigate climate change-induced flooding (12). The relative efficacy and costs of green infrastructure and conventional engineering adaptation approaches has been quantified for two communities in the Midwest USA (11). Grey infrastructure approaches include upsizing existing storm water pipes and containing excess flood volume in underground storage chambers. Local-scale green infrastructure practices focus on increasing the proportion of watershed runoff draining to bioinfiltration areas. Utilizing over-curb surface storage in areas where structures would not be impacted was also considered as a non-structural adaptation strategy (11).

Model results show that adaptation strategies incorporating storm water (bio)infiltration may reduce overall adaptation costs while providing a more comprehensive suite of benefits than grey infrastructure alone. For a 10-year design storm, substantial reductions in flood volume were predicted by applying bioinfiltration areas to only 10% of a ‘traditional, built-out urban area’. The flood storage volume of existing green infrastructure was much larger, however, than the flood volume that could be handled by introducing bioinfiltration. This led to the conclusion that for the study site preserving the hydrologic connectivity of green infrastructure was a more robust adaptation approach than engineered infiltration approaches (11).

Vulnerability of urban areas to storm water flooding varies according to regional climate patterns and site-specific factors such as topography, drainage system configuration and the capacity of soils and other natural or engineered storage elements to store excess runoff (13). Therefore, the efficacy of green infrastructure versus conventional engineering adaptation approaches will also vary from one urban area to another. Climate-proofing urban storm water networks calls for tailor-made solutions that fit the specific context of a certain urban area. 

Green versus engineering: green roofs

Urbanization modifies the hydrologic cycle. Rainfall rapidly runs off the impervious surfaces of buildings and roads, and concentrates in in the drainage network. When it rains hard, and long, peak flows in the drainage network may be exceeded and surface flooding results. In most parts of the urbanized world, stormwater runoff increases for two reasons. First, due to the ongoing urbanization and, hence, change of pervious in impervious surfaces. Second, because global warming increases the intensity of extreme rainfall events. A logic response may be to increase the discharge capacity of drainage networks. This is very expensive, however, and municipalities are trying to avoid this. They are seeking for other options to cope with more extreme rainfall events. One of these options is implementing green roofs. 

Whether green roofs are a good alternative to adapt to more extreme rainfall events depends on their performance in stormwater runoff mitigation. This performance has been evaluated for a municipality in Milan, in northern Italy, a city with a combined system, designed to collect both sewage and urban stormwater during rainfall events. The current stormwater infrastructure of this municipality is undersized or non-existent, a situation common in many municipalities globally. For this performance evaluation, the impact of both spatially homogeneous and heterogeneous installations of green roofs was studied. In the latter case, green roofs were concentrated where the drainage network is more prone to high degrees of filling. Calculations were made for different combinations of rainfall intensity and duration (14).

The results show that implementing green roofs can be a valuable strategy to reduce stormwater runoff to the urban drainage networks, both in terms of flow peak and volume. The larger the roof area that is covered with green roofs, the larger the mitigating impact. Also, the impact is larger when green roofs are concentrated near parts of the drainage network that are more prone to high degree of filling. Thus, spatially heterogeneous installations of green roofs may be a more tailor-made solution than homogeneous installations. There is a downside, however. The strategy seems to be most effective for frequent storms of smaller magnitude. For infrequent storms of larger magnitude, green roofs alone are not able to significantly reduce stormwater discharge. For these extreme events, stormwater discharge capacity of urban areas is determined by the characteristics of the sewer infrastructure. It will be necessary to include structural measures such as sewer relining, enlargement of the diameters, or simplifying network layout to cope with these extreme situations as well (14).

The benefits and price tag of greening European cities

Green roofs can be an important measure in the context of the European green deal. The benefits of green roofs include the reduction of storm water runoff by retaining precipitation (15), less energy demand for the cooling of buildings (16), and lower air temperatures in the city on a hot day (as a result of transpiration by the plants on the green roofs) (17). In addition, green roofs enhance sequestration of carbon dioxide and pollutants from the atmosphere (18), reduce noise in buildings (20), increase habitat for especially birds and pollinators (19), and improve well-being for the local population (21).

The opportunities and limitations of green roofs as a tool for sustainable urban development in Europe have been assessed in a model study. The study focused on the period 2070–2100, compared with the reference period 1990–2013, for a moderate (RCP 4.5) and high-end (RCP 8.5) scenario of climate change. In this study, roofs were turned into green roofs at a scale that agreed with 35% of the total European urban impervious surfaces (22).

According to the results, green roofs cool surfaces by between 2.5°C and 6°C, and air temperature in the city by about half of this surface temperature reduction. Cooling energy saving of buildings could be about 92 TWh/year. The combined effect of carbon dioxide sequestration by biomass growing on green roofs, and energy savings can be up to 55.8 Mtons per year. Transpiration of rainwater from these green roofs could potentially reduce urban runoff by about 17.5%, and thus help to avoid urban flooding during rainstorms (22).

There is a price tag attached to this, however. The benefits that could be monetized by the authors cover less than half of the estimated costs of greening. There are additional benefits that might be worth such extra cost, according to the authors. These benefits are related to biodiversity, water quality, health, wellbeing and other aspects (22).