Floods in Lisbon with more frequent Storm Tides
The risks of floods in Lisbon are high and will get worse over the course of the centuryđ§đ· This is what all the data and studies carried out on climate change in the city show â and which want to prevent all plans to combat them â namely the Municipal Strategy for Adaptation to Climate Change in Lisbon.
That’s what Carlos Moedas, the president of the Chamber, said about this week’s events: âClimate changes are heavy. We have to fight to change this situation and that’s why today [quarta-feira] In the Municipal Assembly, we approved the entire budget that will make it possible to build the large drainage tunnels.â
The truth is that the CML Strategy against Climate Change was presented in 2017 â five years ago â and already pointed to âan increase in the frequency of intense or very intense rain events, simultaneous to strong winds with gustsâ â which, along with drought and excessive heat, it represents the most certain thing the city can count on.
This week, it will be just one example. At the end of the afternoon and evening of December 7th, 40 mm of precipitation fell in a few hours, i.e., practically one third of the average of what it normally rains throughout the month of December in Lisbon (126.2 mm) â in the same height as the so-called preia mar, that is, full tide. That was enough for the city to remain flooded, especially the lower areas of the city â as well as the municipalities of Oeiras, Sintra, Cascais, Loures and Odivelas.
It will have reduced this phenomenon of Storm â Storm Tide â something you see when the water level of the Tagus rises due to the tide and this is associated with a storm. And something that in Lisbon has âspecial acuity, given the geomorphological characteristics of the city â a narrow riverfront delimited in its entirety by the system of hills that define a series of drainage basins that flow into the estuaryâ, as explained in the document of the Climate Change Adaptation Strategy.
And he warns: taking into account that all climate scenarios for the 21st century âproject a worsening of the rise in mean sea level, this phenomenon may have aggravated impacts in the future â overtopping the coast, floods, affecting mobility, parking, effects on structures and infrastructureâ.
What to do?
The CML Plan stated that it was important âto monitor tidal levels (time and height) along the entire riverfront to redefine the tidal effect level â also associated with episodes of heavy rain â in order to scale prevention and adaptation options â. This week the orange warning was lifted, and a request for ânot to leave the houseâ.
But the measures that will have to be implemented have to do with infrastructure issues â which has already been done in the Plan for Parque das NaçÔes and more recently, the Detail Plan for Boavista Poente.
For the rest of the week, the IPMA maintains the warnings, and says that the values ââwill be reached and âmay reach locally between 60 and 120 mmâ. The damage will be evaluated later, but there was one death, of a woman who was trapped in a cave, in AlgĂ©s.
How to prevent the main risk: rising waters
It’s not possible to listen to the rain falling and not wonder if this won’t always be like this now, or worse. O Climate Action Plan de Lisboa â which was presented in 2021 â describes the tragedies that may be about to happen to us, and which required immediate action.
- Worsening of the magnitude of episodes of strong wind/gust;
- Increased heavy rainfall, both in frequency and magnitude;
- High temperatures, aggravating its effect with the decrease in precipitation;
- Cold snaps â high risk for the increasingly vulnerable elderly population;
- Increase in the average sea water level and extreme precipitation events, with the increasing occupation of the territory, greater risk of flooding in Lisbon.
O report by the C40 Cities Network, of which Lisbon has been part since 2019identified that one of the issues more urgent to Lisbon it is the one that protects us from rising sea waters and floods.
Lisbon has a large margin of floodable land, with âan extensive and narrow riverfront. There is a strip between the voices and the Tagus estuary that is very vulnerable to the risks of rising tidesâ, said Cristina Lourenço, responsible for PAC, at CML, in her presentation.
To control floods in the lower areas of the city, the Lisbon General Drainage Plan which provides for the construction of two tunnels with a diameter of more than 5 meters for drainage, strengthening the capacity of collectors and âimproving the capture of surface runoff and the improvement of discharges by increasing outlets in the Tagus Riverâ.
Other city hazards
In Lisbon, the CML strategy warns of the âextensive soil sealingâ with âincreased vulnerability to floods â runoff, and the fact that âsome of the main urban axes coincide with these areas of vulnerability will imply an aggravation of the risk for the location of economic activities or other values ââto be safeguarded, as evidenced in the intended mapping exercisesâ.
This is what happened this week with so many stores on the ground floor flooded â because there are âsignificant concentrations of commercial establishments (figure 4.8) and hotel facilities (figure 4.9) on these axes, demonstrating the need for these facilities to accept accommodation measures in view of the risk of inundation”.
Another important fact â the growing occupation of the underground, which, according to the CML, does not have detailed cartography: âinfrastructures (including tunnels) and underground cellars (public and private use). These occupations have strong skills both in terms of vulnerability to flooding and in the effectiveness of managing the water cycleâ.