The DANA which caused the death of more than 200 people in Valencia once again reminded the world that floods are the natural disaster that causes the most deaths. Urban expansion and population growth have transformed cities into dangerous spaces in the event of sudden downpours, partly due to urban planning that ignores the risks. But not all areas present the same threat. Some more than others, especially those with a particular orography.
Analyzing which places in a specific city are most exposed to flooding is what the Chair of Disaster Risk Reduction at the University of La Laguna (ULL) has done with Santa Cruz de Tenerife. The capital of Tenerife is the Canarian municipality most affected by this type of disaster. This century alone has seen three episodes: in 2002, during which eight people died, in 2010 and in 2014. Furthermore, three of the eight areas with significant potential flood risk on the island are located in its capital , according to the ministry. for the ecological transition and the demographic challenge.
The research was published in the journal Advances in disaster science a few weeks before the deadly DANA which hit the Iberian Peninsula. The authors used a unique methodology in which, in addition to the traditional method of locating flood-prone areas, they used techniques that make it possible to take into account, among other things, poor management of wastewater or the occupation of old canals, to clarify the damage in urban areas.
“We combine these two issues: the problems that can arise in ravines due to the overflow of rivers, with those that have already been detected in urban areas”, summarizes Nerea Martín-Raya, researcher of the aforementioned chair of the ULL and graduated in geography. and territorial planning.
The results suggest that 16.5% of Santa Cruz de Tenerife, without taking into account more rural areas, such as that occupied mainly by the district of Anaga, is threatened by flash floods. This percentage is equivalent to more than 1,400 hectares of land. Of this total, 19% corresponds to a moderate-high danger, 22.3% to a low danger and 57% to a very low danger.
According to Martín-Raya, the areas adjacent to the ravines are the most dangerous, but there are also others further away from the canals, such as the town of El Chorrillo, near Añaza, which is highly threatened in the event of flood. . Other towns in the southeast of the capital, located next to the ravines of Ofra, Santos, Tahodio and Bufadero, are also exposed.
The researchers were able to specify these locations thanks to a detailed analysis of the hot spots and impact zones in the event of torrential rains envisaged in the General Urban Plan (PGO) of Santa Cruz de Tenerife. According to this analysis, there are 1,023 problematic enclaves in this sense: 639 are linked to the lack or insufficiency of drainage capacity; 208 with incorporation of sediments; 96 with obstruction of the evacuation network and 80 for danger by overflow.
The study simulated a precipitation event that released a total of 280 liters of water per square meter, higher than that recorded during the 2002 disaster (232 mm in two and a half hours), to validate the risk map flood-prone areas. The results show that, in an urban drainage network, the main watercourses (such as ravines) and those that have disappeared due to urbanization are the main places susceptible to flooding and in which the emphasis must be focused on risk reduction actions.
Martín-Raya believes that since the Tenerife floods more than two decades ago and the tropical storm Delta, measures have been taken to resize the infrastructure and improve the protection system, but he believes that “there would still be things to improve.” especially everything related to regulations that require the suspension of non-essential economic activities in the event of a flood alert and the preparation of the population so that they know how to react to them.
“Prevention must focus on training and education, both for the population and for the actors involved, whether civil protection, public administration, emergency services… all “, summarizes the expert.
The publication of the ULL Chair on disaster risk reduction specifies that flooding due to torrential rains “will continue to increase in intensity and scale in the current context of climate change”. Martín-Raya adds that the “accumulations of precipitation” will be increasingly important, leaving unprecedented records, such as that observed in the Valencian Community, where maximums of 460 liters per square meter were recorded on October 29.
Other studies carried out in the Canary Islands concluded that, although annual rainfall has decreased, daily concentrations have shown an increasing trend in recent years, “which is an important issue to consider due to severe flooding and other geomorphological processes.
Beyond the climate variable, the “most important” factor that warns against the risk of flash floods in cities is the increase in exposure and vulnerability, underlines the research, which indeed supports that “There is a strong correlation between urban expansion and the “increase” of this threat, particularly due to the inability of cities to absorb water and the decrease in natural infiltration.
In Santa Cruz de Tenerife, Martín-Raya points out, the urban growth of the 1960s and 1970s “developed in a context of practically non-existent territorial planning” and the urban fabric grew massively “without taking into account the risks”. The study he signed with other colleagues confirmed that this “bad” planning caused the appearance of homes in dangerous spaces, not only because of the simple occupation of flood zones, but also because of “the reduction in the flow threshold due to the impermeability of the soil”.
