Barcelona's drainage model stands out for being a unitary system, which means that both sewage and rain water are channelled through the same collectors towards treatment plants.
This unitary model poses a risk at the time when the treatment plants do not have enough capacity to absorb the excess water caused by torrential rains, and then the drainage network drives the water to the marine emitters.. These spillages lower the quality of bathing water, which usually leads to the closing of the city’s beaches, with the corresponding impact on activities linked to Barcelona’s coastline.
Methodology
A seawater-quality model has been created to better understand how these spillages and their repercussions occur and for the purposes of studying the impact of the city’s unitary system discharges (DSU) on bathing areas. The marine model is used for simulating the spatial and temporary distribution of E. coli bacteria in seawater during and after DSU events (figure 1). E. coli is one of the species of bacteria stated in European and Spanish regulations on bathing-water quality.
Figure 1. Example of simulation of seawater quality after a DSU episode (Red = high concentration of E. coli; Blue = low concentration of E. coli).
The original model was developed under the COWAMA (Coastal Water Management) project which provided a computational model in operation since 2007 for real-time simulations of bathing-water quality in Barcelona’s beaches. The marine model includes a new 3D mesh obtained from new bathymetry data gathered in 2016 (figure 2), a new calibration and new validation with data on concentrations of E. coli measured during the 2014-2017 period.
Figure 2. New bathymetry for Barcelona’s maritime model.
The simulation of water quality close to the coast line requires spatial discretisation scales of the order of tens of metres, whereas coastal hydrodynamic processes can occur in scales of hundreds of kilometres. Three nested domains are therefore used for simulating hydrodynamic processes from large regional scale to local scale close to the Barcelona coast (figure 3).
Figure 3. The three levels of nested meshes implemented under Barcelona’s maritime model.
Similar to the analysis conducted for obtaining design rainfalls in the urban-flooding model by producing a series of climate-change data, the USDs are analysed for ordinary hydrological scenarios, although in this case it is concluded that the future precipitation scenarios do not entail a worsening of the current situation, given that climate change will not significantly increase the average rainfall or number of episodes. So, for the purposes of analysing the USDs, both for the current scenario and for the climate-change scenario, the series of rainfall of 2009, where 60 episodes of USDs occurred with a total rainfall of 520 mm (figure 4).
Figure 4. Rainfall series for the average year selected (2009).
Impact on water quality
Concentrations of E. coli in the seawater are obtained from simulations where the urban flood model and the maritime model are applied in coordination during the bathing season (from 25 May to 15 September). The impact is quantified in terms of time of non-compliance with bacteriological pollution values (concentrations of E. coli > 500 cfu / 100 ml) in accordance with the recommendations of Royal Decree 1341/2007 and the EU Bathing-Water Directive for the bathing season taken as a reference (2009) on each Barcelona beach. The following charts show the results for the current and future scenarios (matching) and for the two adaptation scenarios.
The study area in the coastal neighbourhoods with spillage points has been reduced for the purposes of preventing an overestimation (figure 5). The neighbourhoods affected represent 40.5% of the total of Barcelona’s coastal economy.
Figure 5. Map of Barcelona’s coastal area highlighting the neighbourhoods affected by USD.
It can be concluded from the results of the simulation of the current scenario (matching the future one without adaptation) that the Bathing-Water Directive was breached on all Barcelona’s beaches, on average, for a total of 3.22 days (figure 6), the equivalent of 2.82% of the duration of the bathing season. The variation between the various beaches ranges from 2.35% at the Fòrum to 3.40% at Nova Icària.
On average, SUDS can reduce the time to 2.36% in adaptation 1 scenario, whereas the non-compliance time for adaptation 2 scenario (SUDS and structural initiatives) is reduced to 1.8 % (figure 7), a little above the 1.5 % target of the Catalan Water Agency (ACA) circular. In any case, this 1.5% value was merely for guidance, and the proposed initiatives do meet the goals set out.
Figure 6. Non-compliance time (in days) with the bacteriological pollution values set out by the Bathing-Water Directive as a result of spillages into the marine environment through current/future and adaptation 1 and 2 scenarios for Barcelona’s beaches and average value.
Figure 7. Non-compliance time (in %) with the bacteriological pollution values set out by the Bathing-Water Directive as a result of spillages into the marine environment through the current/future scenarios and adaptation 1 and 2 scenarios for Barcelona’s beaches and average value.
It can be seen from figure 8 that the effect of the adaptation measures varies greatly depending on the beach concerned, and is practically imperceptible on the Fòrum beach and, by contrast, causes reductions of over 50% in the case of the SUDS + anti-USD Deposits combination on the Sant Sebastià - Sant Miquel and Barceloneta - Port Olímpic beaches.
Figure 8. Decrease in non-compliance time (in %) with the bacteriological pollution values set out by the Bathing-Water Directive as a result of spillages into the marine environment and the measures implemented in the adaptation 1 and 2 scenarios for Barcelona’s beaches and average value.
On the other hand, spillages into marine ecosystems not only have a negative impact on health and the environment but also affect the area’s economic activities. We have to assume there are economic losses associated with the closing of the beaches as a result of high pollution levels. That is especially important for a city such as Barcelona where tourism and leisure have a considerable impact on its economy, with urban beaches that both the local population and tourists benefit from.
