CLIMATE PLAN EXECUTIVE SUMMARY URBAN FLOODING

Transcription

1 CLIMATE PLAN EXECUTIVE SUMMARY URBAN FLOODING

2 The risk of flooding in the city, conditioned by the terrain profile, the high ratio of impermeable surfacing and the artificial development of natural water channels The City of Barcelona s configuration and planning is, in one way or another, conditioned by water. Its location between the Collserola mountains, the sea and two rivers, the presence of natural water channels and dry riverbeds and channelling for supplies have all contributed, over time, to its layout. Its terrain profile, with steep slopes in the higher part of the city near Collserola, its hills with gentle slopes in the lower part of the city, added to the high ratio of impermeable surfaces and the presence of artificial features in natural water channels, are just some of the aspects that make Barcelona especially vulnerable to torrential rain episodes, causing the flooding of some areas of the city. Furthermore, the city s extensive impermeable surface area means that a greater amount of rainwater is turned into runoff and this affects the type of run-off and its duration. From 1956 to 2009, the impermeable surface area of Barcelona increased by over 2,800 ha, rising from 45 to 72% of the municipality s total surface area. This effect is increased by the characteristics of the city s climate. On a large scale, the Mediterranean climate is characterised by the concentration of most of the year s rainfall in a few high intensity precipitation episodes. In particular, the proximity of the sea to the Collserola mountain range and the fact that it is a large city are elements that favour the formation of precipitation and make the removal of water even more difficult. Today (Current situation) Tomorrow (Mid-century) % Impermeable surface area % Impermeable surface area COMMITTED SCENARIO PASSIVE SCENARIO 2

3 Climate change could increase the existing risk of flooding The general trends observed indicate that, among other things, there are two impacts associated with climate change that may affect flood risk: changes in rainfall patterns and rising sea levels. Regarding the former, most projections indicate that there may be an increase in rainfall intensity and a greater concentration of extreme episodes. Increased rainfall intensity may cause certain areas of the city to flood, as it will increase the flow of circulating water and test the city s drainage system to its limits. The second aspect is higher sea levels, which may condition the functioning of the drainage system in extreme situations. Under extreme conditions, an increase in sea level would mean that the overflow channels would not work properly, causing higher water levels in the network that could cause overflow at the surface, and therefore flooding in the street. The risk of flooding would affect some areas of the city more than others Identifying the areas of the city that have the greatest risk of flooding can be tackled in different ways. A simplified approach has been taken, based on three factors: drainage-system capacity, terrain gradient and catchment area (surface area that brings water to the specified area). The areas most at risk will be those where, in the case of intense rainfall, there may be overflow from the drainage channels to the surface, the area s capacity for draining run-off water is low, there is a lower gradient. Therefore leading to greater depths of water, and where a larger quantity of water can also potentially accumulate. By contrast, in areas where the drainage network has sufficient capacity and can absorb run-off water through street drains, which have a steep gradient and do not accumulate much water, there will be less risk of flooding. 3

4 A simulation of the sewer network has been conducted for the current condition and for the two climate change scenarios, and the results have been compared. The climate change scenarios analysed were the committed scenario, in which overall emissions are reduced according to the Paris Agreement (scenario equivalent to RCP 4.5), and the passive scenario in which current emission patterns would continue (scenario equivalent to RCP 8.5). For current situations, the conclusion is that for rainfall with a return period of 10 years (T10), the drainage system can function correctly (although it functions under pressure in some sections), but the two future scenarios under analysis both forecast flooding in some locations. The fragility of the system in Poblenou, around Diagonal, the Sant Andreu, Badal and Sant Antoni neighbourhoods is therefore shown. Level of flood risk. Passive scenario (A2 or +/-RCP 8.5) Greater risk Lower risk Source: Barcelona Regional,

5 It is necessary to act locally in order to minimise the risk of flooding Although at a local level, the degree of incidence on the factors associated with climate change is limited, it is important to consider that extensive work can be done to promote measures that minimise their effects. Specifically, in order to minimise the risk of flooding, the city needs to work mainly on increasing infiltration, maximising the absorption of run-off water by the drainage system, carrying out actions on the drainage system aimed at optimising its performance, and using tools that increase our knowledge about the city s response to future flooding and improve its management, as well as evaluating the effects on critical infrastructures and other urban services. Urban water system diagram Source: Barcelona Regional, 2017 In recent years, various projects have been initiated to work on this area. The new Comprehensive Master Plan for Sanitation in Barcelona (PDISBA) is being reviewed, with the aim of including measures of this type, among others. For more information, consult the complete content of the chapter on the Climate Plan website. 5