A paradigm of extreme rainfall pluvial floods in complex urban areas: the flood event of 15 July 2020 in Palermo (Italy)

Abstract

Abstract. In the last few years, some regions of the Mediterranean area have witnessed a progressive increase in extreme events, such as urban and flash floods, as a response to the increasingly frequent and severe extreme rainfall events, which are often exacerbated by the ever-growing urbanization. In such a context, the urban drainage systems may not be sufficient to convey the rainwater, thus increasing the risk deriving from the occurrence of such events. This study focuses on a particularly intense urban flood that occurred in Palermo (Italy) on 15 July 2020; it represents a typical pluvial flood due to extreme rainfall on a complex urban area that many cities have experienced in recent years, especially in the Mediterranean region. A conceptual hydrological model and a 2D hydraulic model, particularly suitable for simulations in a very complex urban context, have been used to simulate the event. Results have been qualitatively validated by means of crowdsourced information and satellite images. The experience of Palermo, which has highlighted the urgent need for a shift in the way stormwater in urban settlements is managed, can be assumed to be a paradigm for modeling pluvial floods in complex urban areas under extreme rainfall conditions. Although the approaches and the related policies cannot be identical for all cities, the modeling framework used here to assess the impacts of the event under study and some conclusive remarks could be easily transferred to other, different urban contexts.

Detecting Extreme Rainfall Events Using the WRF-ERDS Workflow: The 15 July 2020 Palermo Case Study

“…The return period of both 1 and 3 h durations estimated by [13] using the growth curves obtained by [14] resulted in values greater than 200 years, confirming the exceptionality of this rainfall event. The hyetograph of the rainfall event (not reported here for the sake of brevity) is included in [13] together with some information regarding hourly intensities and instantaneous intensity peaks. Widespread damages caused by urban flooding phenomena were reported in the city area.…”

“…It is worth mentioning that, during that day, the weather radar located in Sicily was not working due to technical problems (Figure 5). According to SIAS (Servizio Informativo Agrometeorologico Siciliano) [12], more than 130 mm of rainfall was recorded in about 2.5 h at Palermo city: this severe storm was the heaviest rainfall event recorded in the city of Palermo during the last 90 years, and it represents the wettest day in July since 1797 [13]. The return period of both 1 and 3 h durations estimated by [13] using the growth curves obtained by [14] resulted in values greater than 200 years, confirming the exceptionality of this rainfall event.…”

“…According to SIAS (Servizio Informativo Agrometeorologico Siciliano) [12], more than 130 mm of rainfall was recorded in about 2.5 h at Palermo city: this severe storm was the heaviest rainfall event recorded in the city of Palermo during the last 90 years, and it represents the wettest day in July since 1797 [13]. The return period of both 1 and 3 h durations estimated by [13] using the growth curves obtained by [14] resulted in values greater than 200 years, confirming the exceptionality of this rainfall event. The hyetograph According to SIAS (Servizio Informativo Agrometeorologico Siciliano) [12], more than 130 mm of rainfall was recorded in about 2.5 h at Palermo city: this severe storm was the heaviest rainfall event recorded in the city of Palermo during the last 90 years, and it represents the wettest day in July since 1797 [13].…”

“…The hyetograph According to SIAS (Servizio Informativo Agrometeorologico Siciliano) [12], more than 130 mm of rainfall was recorded in about 2.5 h at Palermo city: this severe storm was the heaviest rainfall event recorded in the city of Palermo during the last 90 years, and it represents the wettest day in July since 1797 [13]. The return period of both 1 and 3 h durations estimated by [13] using the growth curves obtained by [14] resulted in values greater than 200 years, confirming the exceptionality of this rainfall event. The hyetograph of the rainfall event (not reported here for the sake of brevity) is included in [13] together with some information regarding hourly intensities and instantaneous intensity peaks.…”

Sponge City Program (SCP) and Urban Flood Management (UFM)—The Case of Guiyang, SW China

“…The process of worldwide urbanization has accelerated rapidly [1], and global climate change has also become more apparent in the past century [2]. Urbanization has exacerbated the damaging consequence of climate change, such as more frequent flooding events [3]. For example, flooding events have increased in Chinese cities as a result of climate change and urbanization [4].…”