Lisbon Master Plans and Nature-Based Solutions

Abstract

Sustainable drainage approaches differ from traditional design approaches to manage flooding risks, where runoff is regarded as a nuisance, instead of considering surface water as a valuable resource that should be managed for maximum benefits. Nature-based solutions (NBS) may deliver numerous services including supporting biodiversity, climate regulation, flood control, and water purification and supply. Nowadays, Lisbon city, in Portugal, has a strong commitment to sustainable land use, with particular focus on establishing green infrastructure networks, to counteract the effects of climate change, such as drought, extreme heat, and storm flooding. In this chapter, the Lisbon Drainage Master Plan (PGDL 2016–2030) and the Lisbon Strategic Plan for Water Reuse (LiSWaR) are presented, with an emphasis on green solutions, complementing the structural gray interventions that were included in PGDL 2016–2030 and are presently being implemented. Lisbon is proof that sustainability, rather than representing an extra cost, can deliver long-term savings and economic growth, while fostering social inclusion, with multiple benefits to its residents.

1. Introduction

Lisbon, the capital of Portugal, has a Mediterranean climate with typically long dry periods in spring and summer, and frequent intense rainfalls in the wet season (October–April). In 2022, for example, after a serious drought that lasted practically from the beginning of the year until late November, affecting all of the country, Portugal has suffered, particularly, in its capital, from serious flooding events (8th and 13th December 2022), with 1 fatality and estimated losses of about 49 million EUR [1].

Other major flooding events occurred in the Lisbon region in the last decades, namely in: November 1967, 110 mm in 5 hours (h), with 700 fatalities; November 1983, 70 mm in 6 h, with 10 fatalities; February 2008, 86 mm in 6 h; October 2010, 23 mm in 6 h, September and October, 34 mm in 1 h, and 90 mm in 6 h, respectively; and December 2022, 90 mm, in 6 h [1].

Water scarcity and flooding events in the urban area were the drivers for revising and updating the previous Lisbon Drainage Master Plan in 2015 (updated version entitled PGDL 2016–2030) [2] and developing the Lisbon Strategic Plan for Water Reuse in 2019 (LiSWaR) [3], contributing, in certain aspects, to the successful application of the Lisbon Council to the European Green Capital Award for 2020 [4]. Both plans are presently being implemented in the city.

PGDL 2016–2030 includes a set of gray centralized interventions such as asset rehabilitation and construction of new sewers, improving inlet gutters, minimizing local head losses, and diverting flows from critical vulnerable areas. The plan has also considered decentralized blue-green solutions (e.g., stormwater source control techniques), such as the detention ponds of Alto da Ajuda and Ameixoeira, and involved not only physical construction measures, but also non-physical actions, such as knowledge acquisition through research [5], on capacity building, a monitoring and warning system, and public awareness campaigns. As major interventions for flooding control, two stormwater drainage tunnels have been built since 2022, both with 5.5 m diameter that will divert the stormwater flows of important tributaries areas of the main catchments directly to the Tagus River [6]. These tunnels will prevent the city downtown and most of the riverfront from frequent flooding events.

In addition, to address water scarcity across Europe, the European Union is calling for a more efficient management and preservation of water resources [7]. In this context, Lisbon municipality has decided, jointly with Simtejo (presently Águas do Tejo Atlântico, AdTA), the utility that manages the bulk wastewater system, to develop a strategic plan for water reuse at the city scale that is under implementation. It is considered that the reuse of treated wastewater poses a reliable alternative for various purposes [4], contributing to a transition to a more circular economy and increasing the city’s resilience to extreme climate scenarios.

2. The city and the existing drainage system

The area covered by the PGDL 2016–2030, 10,239 ha, includes a territory where altitude varies between 2 m, in the riverside area, and 250 m, in Monsanto and the higher areas of the Alcântara drainage basin, located in Amadora municipality. Figure 1 presents the altitude distribution in the area covered by the PGDL 2016–2030 in the municipality of Lisbon, where the letters identify each main catchment. The average value is 80 m, with about 75% of the area below 100 m and 3% below 5 m. Average annual precipitation in Lisbon is about 590 mm [8].

Figure 1.

Altitude distribution in Lisbon city.

Lisbon is served by a wastewater system with separate, partially separate, and combined sewers, including more than a dozen pumping stations, inverted siphons, and a significant number of overflows. In Figure 2 the main infrastructures of the Lisbon drainage system are presented. The sewer system is about 1400 km long including assets of very different ages (some are more than 250 years old), built with different construction materials (including stones, bricks, concrete, or PVC), presenting different cross-section types (namely circular, egg-shaped, or rectangular), and implemented typically as branched networks, although some looped networks exist in the downtown ancient city.

Figure 2.

Main catchments and infrastructures of the Lisbon drainage system.

The old Pombaline sewers from the 18th and 19th XIX centuries are called “saiméis” and are made of stone. Examples of these infrastructures can be observed downtown, in catchments J and L (Figures 2 and 3). As a rule, they are in a better state of conservation than the later “cascões”, which do not guarantee water tightness and self-cleaning conditions. A large part of the Lisbon drainage network, built after the end of the nineteenth century, integrates egg-shaped sewers of stone masonry. Subsequently, with the use of materials such as concrete, stoneware, or plastic (e.g., PVC, HDPE, and PP), pipes with oval, arched (of the type of the “Caneiro de Alcântara”), rectangular and circular sections were built.

Figure 3.

Flood risk vulnerability map of the city of Lisbon (https://websig.cm-lisboa.pt/MuniSIG/visualizador/index.html?viewer=LxInterativa.LXi).

Flooding events are frequent, particularly in the low and flat areas of the city, downstream of large catchments of Alcantara (E) and Chelas (O) (Figure 1). In these close to the estuary areas, under high tides, the available energy for outflow discharges during rainfall events is rather limited, and the stormwater in excess surcharges through manholes and gutters, contributing to increased flooding risks affecting people and goods. The situation has a tendency to worsen due to the increasing impervious upstream areas and to climate change effects, namely sea water rise and increasing frequency of extreme rainfall events, which challenges were addressed in the Lisbon Drainage Master Plan. Figure 3 presents the flooding risk vulnerability map of Lisbon, which is being updated due to the new construction works.

3. Main challenges

Lisbon consumes annually 50–60 million m³ of potable water, of which half is consumed by the domestic sector [3]. In the non-domestic sector, the Lisbon Municipality (CML) is the biggest consumer, with a share of 15%. Half of CML’ share accounts for park irrigation, which consumes 4–5 million m³ of potable water annually, while the second biggest water consumption is street cleaning, using 1–2 million m³ of water yearly [3]. The latter is especially frequent in popular districts with intense nightlife activities.

Within the city’s water efficiency program, the freshwater demand has been reduced dramatically by controlling water loss in the distribution network from 23.5% in 2005 to 7.9% in 2013 [9]. Another noteworthy water consumption reduction strategy, the creation of a green infrastructure that is resilient to drought (based on rainfed meadows and Mediterranean climate-adapted species), has been implemented. In addition, sustainable irrigation for green public spaces, particularly in summer months when city temperatures are higher, has been promoted, e.g., using cultures that do not need so much water, with irrigation taking place late afternoon or at night.

The complexity of the systems in ancient European cities, like Lisbon, which includes separate and pseudo-separate sewers, branched and looped networks, overflows, inverted siphons, storage tanks interceptors, and a number of wastewater treatment plants (WWTP), makes the problem of controlling flooding and pollution risks, particularly demanding. In fact, the main challenges associated with the drainage system include controlling flooding events, and the number and volume of overflows, in a sustainable way (e.g., at a reasonable economic, social, and environmental cost), if possible with added value, through sub-products resource recovery.

Commonly, the traditional techniques to solve these challenges are based on increasing inlets and sewers’ capacity, building storage tanks, controlling and pre-treat overflows, or rehabilitating wastewater assets. These approaches involve significant investments and present high social and environmental costs, associated with the inconveniences caused by construction works in urban areas.

There is a trend in the development of wastewater systems in modern cities based on principles that promote infrastructure’s advanced management, combining centralized gray solutions (sewers and storage tanks) with decentralized source control techniques (or best management practices, BMP, or sustainable urban design, SUDS techniques), which seeks to promote retention and infiltration upstream of the sewerage system. Advanced management advocates the installation of monitoring equipment and the implementation of real-time modeling and management tools, so as to take advantage of online storage and maximize the wastewater volume conducted to the WWTP, thus reducing the risks of flooding and contamination of the receiving waters.

Furthermore, the expansion of urban areas (increasing impervious surfaces) and climate change’s effects on the sector may be seen as special challenges. Climate change, one of the biggest threats facing humanity, involves many dimensions (science, economics, society, politics, and moral and ethical questions), constituting a global problem.

Given the scale of climate change and the fact that it will affect many areas of life on earth, adaptation also needs to take place on a greater scale. Modern economies and societies, particularly in cities, need to become more resilient. This will require large-scale efforts. New sewers and storage tanks may be required to withstand more intense storms, and urban planning should consider climate change’s impacts and challenges. Some coastal cities, like Lisbon, will have to implement solutions that help prevent more common floods, namely in properties and underground transport systems, using retention valves to control saline inflows and, in extreme situations, resorting to pumping stations.

In general, responding to climate change involves two complementary approaches: a) mitigation, e.g., reducing emissions and stabilizing the levels of heat-trapping greenhouse gases in the atmosphere; and b) adaptation, e.g., adapting to the climate change already in the pipeline.

In the context of this chapter, mitigation means controlling stormwater flows at the source, typically through decentralized practices such as nature-based solutions (NbS), which promote infiltration and storage upstream of the physical sewer network (“source control techniques”). Adaptation means accepting the design flows and planning and designing urban drainage infrastructures (namely new sewers, tunnels, or storage tanks) to accommodate and transport those design flows with minimum adverse impacts.

It is considered that the implementation of the Lisbon Drainage Master Plan provides opportunities for a better use of resources, exploring the relationship with other critical services, namely through facilitating water reuse corridors and communications lines [2].

4. Preparing Lisbon to the twenty-first century—the vision and the plans

4.1 General aspects

Since 2008, important interventions have been carried out to divert dry weather flows (domestic wastewater) from the combined system, conducting it to WWTP, and the receiving water quality in the city has improved significantly. It is worth highlighting the interventions in the drainage fronts between Largo Chafariz de Dentro and Terreiro do Paço, between Terreiro do Paço, Cais do Sodré and Alcântara, and between Algés and Alcântara, which included the construction of weirs for flow control, the installation of pumping systems and the execution of stormwater discharges. In addition to these important interventions in the drainage fronts of the lower city areas, several rehabilitations of existing infrastructures were carried out. Flow meters and rain gauges were also installed, constituting a monitoring system that has allowed for a more adequate management of the drainage system.

The improvement of the Lisbon drainage system’s performance, in order to control direct domestic effluent discharges during dry weather, was practically finished more than a decade ago.

As so, the main objectives of PGDL 2016–2030 were developing integrated flood control solutions for an uncertain future, and providing Lisbon city with a set of works and actions that would be able to cope with twenty-first century’s challenges. In summary, the PGDL2016–2030 basic guiding principles were the following:

1. Emphasis on minimizing flooding risks, while taking into account overflows control.

2. Developing integrated solutions with minimal impacts from a social point of view, minimizing intervention in the most urbanized and constrained areas of the city.

3. Optimizing the solutions, combining centralized gray interventions with decentralized blue-green techniques, and complementary non-physical actions in order to take the most benefit from the adopted approach and associated investments.

4. Defining priority interventions as those that present the greatest benefits with controlled costs.

Within the scope of the PGDL 2016–2030, several alternative solutions were studied for the different catchments, based on different types of structuring interventions, supplemented with a set of smaller complementary actions. These complementary actions are fundamental to the success of the Lisbon Drainage Master Plan and, globally, do not represent significant investments.

Under this Master Plan, the main physical and nonphysical interventions were expected to take place over 15 years, between 2016 and 2030, with an estimated total cost, in 2015, of around 175 million € (the updated value will be around 250 million €). A part of the work is already accomplished, but the two tunnels under construction are only expected to start operating in 2025.

4.2 Gray interventions

The PGDL 2016–2030 includes the following main physical gray interventions:

• flow diversion from the critical areas downstream through two large tunnels, with 5,5 m diameter: one nearly 5 km long (Campolide-Santa Apolónia tunnel, including deep shafts along its length; another nearly 1 km long (Chelas-Beato tunnel);

• rehabilitation and sewer capacity up-grading of existing assets and construction of new drainage infrastructures;

• complementary interventions for flow control (overflows up-grading);

• construction of storage solutions (e.g., the Alto da Ajuda and the Ameixoeira open-air basins);

• minimization of local head losses;

• increasing the number and capacity of gutters and other stormwater interception devices.

During the PGDL 2016–2030 development, to analyze the hydraulic performance of existing and proposed infrastructures, for different design rainfalls (with return period from 10 to 100 years) and climate change scenarios (sea level rise up to 0,5 m, in 2060), dynamic simulation tools were used. For the city drainage system, a 1D model based on the US EPA Stormwater Management Model (EPA SWMM) was assumed. In the downtown historic center, more prone to risk flooding (the “Baixa”), a 1D/2D model was used, based on SWMM and Mohid Land (developed by MARETEC at the Instituto Superior Técnico of the University of Lisbon, this software allows 2D surface runoff simulation). For some complex inlet infrastructures, a 3D dynamic simulation model was developed using Computational Fluid Dynamics (CFD), namely FLOW-3D software.

In addition, main planned works, such as the headworks of both tunnels and the deep shafts along the Campolide-Santa Apolónia tunnel, were subject to scale model tests at the National Laboratory of Civil Engineering (LNEC), the results crucial to optimizing their hydraulic design and give reliability to the adopted solutions.

Figure 4 presents the layout of the two tunnels (Campolide-Santa Apolónia on the left; Chelas-Beato tunnels on the right). The Campolide-Santa Apolónia Tunnel crosses a series of urban catchments, collecting along the way the stormwater from the upstream areas of these catchments through deep shafts, marked with yellow dots.

Figure 4.

Campolide-Santa Apolónia (left) and Chelas-Beato tunnels layout (right).

In the upstream section of the Campolide -Santa Apolónia tunnel, after being diverted, the initial combined inflow (up to 10 m³/s) is conducted to a 16,000 m³ settling tank. For higher flows, up to 36 m³/s, the stormwater is subject to micro-screening before entering the tunnel. Both the settling tank and the micro screening provide treatment to the overflows, in accordance with the spirit of the proposed revision of the Urban Waste Water Treatment Directive (UWWTD) [10], which sets as a goal the control of runoff pollution from large agglomerations.

Figure 5 illustrates the headworks of the Campolide-Santa Apolónia tunnel: a) its general overview and the green space over the infrastructure; b) 3D drawing of the tunnel headworks, including the settling tank, and the micro screening beds; c) results of 3D modeling at the inlet works; and d) scale model of the inflow chamber of the micro screening equipment, at LNEC.

Figure 5.

Headworks of the Campolide-Santa Apolónia tunnel: Overview, 3D design drawings, 3D modeling results, and scaled model tests run with the micro screening equipment at LNEC.

Since the Campolide-Santa Apolónia tunnel crosses a large part of the Lisbon city, communication lines and a pipe to transport treated wastewater from Alcântara WWTP, to be reused in different Lisbon locations, were installed along the tunnel path. These critical infrastructures are protected and installed close to the bottom of the tunnel, allowing easy access to the delivery water points in the center of the city, and avoiding difficult work for installing the pipes on the streets, with associated social and environmental negative impacts. The tunnels are, therefore, considered multipurpose infrastructures.

In addition to the mentioned large gray interventions, there are minor actions, referred to in the project as “acupuncture interventions” that strategically act on specific points along the drainage system, crucial in energy terms, largely benefiting the overall system performance. In the context of urban drainage and urban metabolism, “acupuncture interventions” include localized interventions conceived to reduce local head losses (e.g., enlarging outfall discharges, aligning flow directions in the sewers, and rounding sharp transitions).

Other minor but relevant actions are the increase of the hydraulic capacity of the inlet devices (e.g., gutters), the concretization of maintenance periodic activities, or the monitoring of the performance of existing infrastructures.

The relevance of these local interventions is particularly high when the flow volume and velocities increase, which typically occurs during major flooding events.

4.3 Blue-green solutions and water reuse

Since 2008, Lisbon has seen a 13% increase in new and renewed green spaces, and the city intends to achieve a 20% increase by 2025 (more than 350 ha). Urban planning and the rezoning of ecologically sensitive areas to curtail construction laid the groundwork for its Green Corridor Strategy, designed to preserve and increase permeable areas and rehabilitate underground waterways. One example is the Vale de Alcântara green corridor, which links the city’s natural amenities, such as Monsanto Park with the Tagus River, giving its citizens greater access to green spaces [9]. Currently, 80% of people live within 300 m of green urban spaces (this percentage was 76% in 2016), and ongoing projects might increase this rate to 93%. Over 55,000 trees have been planted since 2017, directly on streets.

This tendency was also followed by the PGDL 2016–2030, which defines and plans several source control interventions to mitigate urban flooding (i.e., infiltration and storage infrastructures upstream of the sewer network). In fact, Lisbon is implementing several blue-green solutions, incorporating natural systems into urban areas to mitigate increased pressure derived from urbanization, namely flood risks, droughts, urban heat island effects, biodiversity losses, and resource limitations, which are likely exacerbated by climate change. Examples of blue-green solutions already constructed in Lisbon include green roofs, urban allotment gardens, stormwater retention basins, and infiltration trenches.

It should be noted that according to the already mentioned proposed revision of the Urban Waste Water Treatment Directive (UWWTD) [10], reduction of the pollution due to rain waters and overflows treatment will be required in large cities, requiring integrated urban water management plans that propose source control approaches, among other nature-based solutions. In addition, urban wastewater will be subject to more demanding and efficient treatment schemes, increasing the availability of higher-quality treated wastewater. This may promote water reuse projects, namely from an economic perspective.

Under PGDL 2016–2030, the construction of different source control solutions for decentralized stormwater management is being implemented, including NbS that promote retention and infiltration, as presented in Figure 6. In this context, the open-air retention basins of Alto da Ajuda and Vale da Ameixoeira (numbered 1 and 3 in Figure 6, respectively) have already been built. The retention basin of Alto da Ajuda (before and after images shown in Figure 7) will promote infiltration and attenuation of the stormwater peak flows and gross solids retention, preventing them from entering the drainage system, and causing blockages downstream, in the flatter coastal areas. The basin is inserted in a green area of 32 ha, contributing to landscape requalification and territory valorization, with a connection interface to the “Rio Seco” green corridor.

Figure 6.

Lisbon retention basins and other NBS solutions for stormwater control, adapted from [9].

Figure 7.

Alto da Ajuda before (left) and after (right) the construction of the retention basin.

Another example, Parque Eduardo VII (number 9 in Figure 6) presents multiple valences and unique characteristics of high patrimonial, socio-cultural, landscape, and environmental interest. Due to its location and slope, the area presents high potential to contribute to the generation of flash floods that overload the downstream drainage system and aggravate flooding risks in the “Baixa” area of Lisbon. During recent requalification works in the park, infiltration trenches and modular retention infrastructures were used to promote stormwater retention and infiltration (Figure 8).

Figure 8.

Construction of modular retention basins in Parque Eduardo VII.

Lisbon city has also promoted, jointly with Simtejo, the “Lisbon Strategic Plan for Water Reuse” (LiSWaR) [3], with the main objective to irrigate the city parks with recycled water (RW) [9], considering the requirements set by the European legislation [10]. Being a source of natural fertilizers, not dependent on climate, which is particularly relevant in the context of lasting droughts and water crises, the RW use presents several benefits. In the case of Lisbon, the main water source is Castelo do Bode reservoir, about 140 km away, and the opportunity to reduce the pressure in this source, releasing it for human consumption, may be especially important in an uncertain future.

LiSWaR was one of the first city-wide encompassing recycled water plans in Europe and was planned to focus on irrigation of urban parks, though it considers other non-potable urban uses, like street and sewers cleaning, and supplying water for industry cooling purposes. The RW originates from the effluent of the city’s three wastewater treatment plants (WWTPs of Alcântara, Chelas, and Beirolas), after final advanced membrane treatment and chlorine disinfection. The LiSWaR plan involves the construction of a distribution network for recycled water, from the three WWTP to nearby major points of non-potable urban consumption, as shown in Figure 9. Overall, the construction of about 55 km of main pipelines, 13 storage tanks, and 19 pumping stations is planned.

Figure 9.

Interventions planned in the “Lisbon Strategic Plan for Water Reuse” (LiSWaR) [3].

Different uses have been identified depending on the proximity to each of the WWTP, to limit the costs associated with the distribution of RW. For instance, Alcântara WWTP is connected through a water reuse distribution system to the city center, where the reused water can be used to irrigate green spaces and for street cleaning, while Beirolas WWTP provides RW to irrigate the adjacent Tejo Urban Park green area (up to 400,000 m³/year) since March 2022. With the RW sources close to the potential points of demand, the benefits of reusing water within the city are promising.

The RW network will be used to irrigate the city’s green spaces and parks, namely the axis Parque Eduardo VII–Cidade Universitária (numbers 1 to 3 in Figure 9), the Chelas Valley (number 6 in Figure 9) and the Lisbon waterfront (from number 11 to number 7 in Figure 9), and typical areas of nocturnal activity (namely Cais do Sodré, number 5 in Figure 9).

The implementation of LiSWaR constitutes an important process of environmental valorization and protection of natural resources, with the conservation of water and minimization of effluent discharge into the Tagus estuary, and the use of three new water sources, the Wastewater Treatment Plants (WWTP) of Alcantara, Chelas, and Beirolas, for urban irrigation and other non-potable urban uses. The Strategic Reuse Plan network will promote the reuse of 1.6 million m³ of water per year.

5. Conclusions

Lisbon city has a Mediterranean climate with typically long dry periods and frequent intense rainfalls in the wet season. So, water scarcity and flooding events were the drivers to update the Lisbon Drainage Master Plan (PGDL 2016–2030) and to develop the Lisbon Strategic Plan for Water Reuse (LiSWaR). Both water plans are presently being implemented. The PGDL 2016–2030 defines objectives, strategies, and actions in order to meet the current and future challenges of the city’s stormwater drainage, prioritizing the protection of people and goods, within a framework of economic, social, and environmental sustainability. The PGDL includes centralized gray solutions but also a set of decentralized NbS interventions, complementing the green interventions already accomplished under other municipal initiatives. The NbS solutions adopted by the city will contribute to mitigating flood risks and foster social inclusion, biodiversity, and the health and wellbeing of Lisbon residents.

Conversely, LiSWaR was one of the first city-scale European water recycle plans, developed considering mostly landscape irrigation, but also non-potable urban uses (e.g., street cleaning, car washing, construction water) and industrial uses (primarily for cooling and process needs). The implementation of LiSWaR constitutes an important process of environmental valorization and protection of natural resources, promoting additional water sources, for example, the treated effluent reuse from the 3 WWTP of Lisbon, for non-potable consumption and reducing effluent discharge into the Tagus estuary.

The centralized and decentralized approaches proposed in Lisbon Master Plans, the water reuse strategy, and the combined use of gray and nature-based solutions are proof that sustainability, rather than representing an extra cost, can deliver long-term savings and promote economic growth, preparing the city for the future under climate change scenarios.

Acknowledgments

The authors are grateful for the Foundation for Science and Technology’s support through funding UIDB/04625/2020 from the research unit CERIS.

Conflict of interest

The authors declare no conflict of interest.