Potential Contribution of Green Rooftops to Rainwater Management in Urban Areas in São Miguel Island, Azores

Abstract

The Azores archipelago is located in the North Atlantic Ocean, including nine volcanic islands. The Azores climate, temperate oceanic, is influenced by the Azores anticyclone, being characterized by high air humidity indices, low thermal amplitude, abundant precipitation and persistent wind. Like other urban areas, Ponta Delgada, in S. Miguel Island, suffers from high impermeabilization with all the negative environmental consequences that arise. As such, the present chapter intends to discuss the opportunity of greening rooftops in urban areas in S. Miguel Island (performing a survey of the roofs and the corresponding area that could be used) showing the advantages that could arise, regarding the environmental performance improvement (e.g., building water storage and heat balance) and their role in the minimization of extreme weather events effects (e.g., urban wastewater infrastructure). As a first step toward evaluating the contribution of these infrastructures in the urban areas of the Azores, a green rooftop pilot study using plants and local substrates has been designed, to be built in the near future.

1. Introduction

1.1 Aims and scope

This chapter will focus on specific services provided by green roofs (GR) that we consider to be more valuable for the current situation of urban areas in the Azores islands. Water management issues are a major concern, due to the water demand increase associated with tourism and the impacts on water quality derived from high precipitation runoff. Thermal regulation of buildings is an energy consumption process and since in the Azores most of the energy is derived from fossil fuel, improving thermal regulation of buildings by non-energy techniques could contribute to reduce electricity demand. Although concerns about the reduction of building stock energy consumption have been growing in our society, cases of inadequate indoor conditions and high energy consumption are still too common. In order to study strategies for an improved sustainability of the building stock, it is important to evaluate the possibility and impact of using green rooftops. A function of plants in green rooftops is the storage of atmospheric carbon and small-size particles, contributing to improve air quality in urban areas. Since Ponta Delgada is located between an airport and a maritime port, we think that air quality improvement can also be an important green roof service in the local context.

We include a geographical and climatic characterization and a description of the roofing typology in two of the cities with the largest continuous urban areas, Ponta Delgada and Lagoa. Through the design of a pilot study, we explore the potential of the local substrates and of the Azorean flora to be used in green roof building. We end this chapter with a general discussion, emphasizing the need for the construction and operation of the designed pilot scale model, to obtain data to validate the use of green rooftops in the Azores.

1.2 Territorial context

The archipelago of the Azores, composed of nine volcanic islands, is located in the North Atlantic Ocean, between latitudes 37° and 40° N and longitudes 24° and 31° W (Figure 1). In biogeographical terms, it is part of Macaronesia, being the northernmost and the one where the effects of isolation are most felt, due to its remoteness from the North African and European coasts.

Figure 1.

Geographic localization of Azores Archipelago.

The surface area of the archipelago totals 2322 km², with a high territorial disparity: the island of São Miguel (744.5 km²) contrasting with Corvo (17.1 km²), the largest and smallest, respectively. The distribution of the resident population (236,413 inhabitants) reflects the demographic centrality of São Miguel: with 133,288 individuals (179 inhabitants/km²), it corresponds to 56.4% of the population of the Autonomous Region of the Azores [1].

1.3 Urban areas

Since São Miguel is the biggest and most populated island in the archipelago, in this chapter we focus on the urban areas located at the south coast, corresponding to the relatively large urban centers of Ponta Delgada and Lagoa cities (Figure 2). Ponta Delgada urban areas (São José, S. Pedro, S. Sebastião, Santa Clara, Fajã de Baixo and São Roque) present population densities ranging between 3471.4 and 636.5 inhabitants/km², much higher than the urban areas of Lagoa (Rosário, Cabouco, Santa Cruz and Água de Pau), ranging between 829.1 and 167.2 inhabitants/km² [1].

Figure 2.

Urban areas studied in São Miguel Island, Azores.

The impervious areas, including continuous and discontinuous urban fabric, industry, commerce, equipment and infrastructure, road network, port and aeronautical areas, sports, cultural, tourist and leisure facilities, were obtained by intercepting the administrative boundaries of the civil parishes [2] with the Azores Land Occupation Charter [3], using a Geographic Information System. In Ponta Delgada, the highest percentage of waterproofed areas are located at São José (82.0%) and São Pedro (65.2%), and the lowest at Fajã de Baixo (24.6%). Lagoa is much less impervious than Ponta Delgada, with the highest percentage of impervious areas located at Rosário (27.7%), and the lowest at Água de Pau (4.4%).

1.4 Climate: Rainfall, temperature, and relative humidity

The climate of the Azores is defined by its location in the context of the general atmospheric and oceanic circulation of the North Atlantic. The thermodynamic influence of the warm Gulf Stream, the progression of the Polar Front depression cells and the positioning and seasonal intensity of the Azores Anticyclone are the factors that weigh most heavily on regional climatology [4]. According to the Köppen-Geiger classification system, the climate is temperate maritime (Csb), typical of oceanic mid-latitudes, with four well-defined seasons. Unlike the other archipelagos of Macaronesia (Madeira, Canary Islands and Cape Verde), the Azores have climatic conditions without the direct influence of the surrounding continental masses.

The average annual temperature in Ponta Delgada is 17.8°C, ranging from 22.6°C in August to 14.4°C in February [5]. The annual temperature range is just over 8°C. The three summer months are the hottest (21°C) and the winter months the coolest (15°C). The number of hours of sunshine per year is 1737. Annual rainfall totals 1052 mm at sea level, with December being the wettest month (159 mm) and July the driest (28 mm). According to the average distribution of temperature and rainfall (thermopluviometric graph), two dry months (July and August) occur in Ponta Delgada, considering that the precipitation column does not exceed the temperature line (Figure 3). From October to March, about 70% of the annual rainfall is recorded. The relative humidity is always very high, almost always above 80% in all months of the year.

Figure 3.

Monthly precipitation and monthly average temperature in Ponta Delgada (IPMA, 2022).

The dominant winds in Ponta Delgada blow from the north quadrant (20%), followed by the north-east (15.5%) and west (14.4%) quadrants. The average speeds for the same directions reach 12, 13 and 11.7 km/hour [6]. However, despite the apparently low wind values, the Azores is regularly hit by extreme weather events associated with the passage of tropical and subtropical cyclones and hurricanes, with gusts that can reach 150 km/hour.

The most favorable time of year for these storms to progress north is from July to October. According to the US National Oceanic and Atmospheric Administration [7], between 1851 and 2021, 159 hurricanes and storms were recorded in a 400 nautical mile buffer around the Central Group of the Azores Archipelago.

1.5 Stormwaters

One of the effects of extreme storm rainfalls is floods, which represent a potential adverse effect on human health, the environment, cultural heritage and economic activity. Within the framework of the Flood Directive (2007/60/EC, of the European Parliament and Council, of 23rd October) [8], the Management Plan for Water Resources of Azores (PGRH – A, 2022–2027) [9] identified areas of high flood risk in four river basins in São Miguel Island: Ribeira Grande, Ribeira da Povoação, Grota da Areia and Grota do Cinzeiro. The most frequent floodings in the archipelago originated from rapid floods. Usually, a consequence of very intense precipitation for a short period of time, which can be devastating – especially when it occurs in urbanized areas located at drainage basins and constructed at steep slopes. Since the beginning of the settlement, São Miguel Island has witnessed several flooding events, according to the historical data [10]. These events have resulted in the loss of human lives and severe property damage – mainly in Ribeira Grande and Povoação Municipalities [10]. Ponta Delgada and Lagoa urban areas present a lower risk of flooding.

Even without a high flood risk, stormwater drainage discharging in receiving waters is a nonpoint source of pollution. The stormwater runoff is generated by rainfall at impervious surfaces of urban areas, like rooftops, roads and parking lots. The rainfall in those impervious areas generates five times more runoff than a woodland area of the same size [11]. The chemical and microbiological composition of this flow includes a certain number of pollutants, such as [11, 12]: sediments; oil, grease and toxic chemicals from vehicles; pesticides and nutrients from lawns and gardens; viruses, bacteria and nutrients from pet waste; heavy metals from roof shingles, motor vehicles and other sources and thermal pollution from dark impervious surfaces. The concentration of particulate matter is an indicator of other contaminants, like phosphorus, trace metals (Cu, Cr, Pb and Zn) and organic pollutants (PCBs, PAH), because they are sediment-bound. This situation corresponds to discharges from 100% separate sewers, consisting of independent sewers (sanitary sewer and storm sewer), where stormwater does not mix with residential sewage. However, there are other typologies of drainage systems, which include areas with separate sewers with older areas provided by combined sewer (only one sewer to collect domestic wastewater and storm waters). Those systems are designed to conduct all the flow to a treatment facility. However, as a result of certain high rainfall events, the combined sewer overflows and separate sewer overflows discharge untreated urban wastewater into the receiving waters. Lee and Bang [13] analyzed the combined and separate sewer overflow runoff and quality parameters during 34 storms in distinct land use areas, and concluded the average mass loading of pollutants is higher for residential than for industrial watersheds (e.g., SS, and TP, are 1802.3 and 14.8 kg/ha/yr. for residential watershed, and 197.0 and 5.4 kg/ha/for industrial watersheds). Table 1 summarizes the possible effects of runoff in urban areas depending on rainfall (duration and intensity), sewerage typology and water basin characterization (e.g., waterproof areas, slope and traffic).

Possible effects of rainfall in urban areas
Floods	Temporary surface cover with water related to climatic events (extreme rainfalls)
Overflow Combined Sewer; Overflow Separate Sewer	Overflow represents a flow that exceeds the system’s maximum capacity and is discharged in the receiving waters. The overflow occurs at pump stations or in devices before the WWTP (Wastewater Treatment Plant)
Stormwater runoff	Runoff produced by rainfall in impervious areas, collected in separate sewer systems and discharged in receiving waters

Table 1.

Results of rainfall events in urban areas.

Reducing runoff in urban areas contributes to reduce the impacts of stormwater runoff on receiving water quality, and minimize the occurrence of the overflow of combined or separate systems.

1.6 Roofs landscape in urban areas

We based our evaluation of the residential building stock in São Miguel island, namely in Ponta Delgada and Lagoa urban areas, on the Residential Building Stock Survey (RBSS) developed by the University of the Azores in the framework of the Green Islands Project, a project which associated the Massachusetts Institute of Technology and several Universities in Portugal [14]. This RBSS was applied to a sample of 500 households in São Miguel, the largest and most populated island of the Azores. That database shows that 77.3% of residential buildings in Ponta Delgada municipality and Lagoa municipality (corresponding to 295 buildings) were located in urban and periurban areas. From that sample of residential building stock, some statistical results are presented at Table 2.

Parameter	Percentage of buildings (%)
Horizontal rooftops	3.1
Elevated wind exposure	24.5
Hight between 3 to 9 m	78.5
Location at less than 50 m altitude	46.5
Distance to ocean less than 1000 m	56.6

Table 2.

Characterization of residential buildings in Ponta Delgada and Lagoa urban and periurban areas [14].

Lately, with air space liberalization, there has been a major increase in tourism [15], enhancing the rehabilitation/transformation of the existing building stock and a boom of new constructions. This growth is confirmed by the registrations of new building constructions [1], between 2011 and 2021 in peri-urban areas of Ponta Delgada, like Fajã de Baixo (4.42%) and São Roque (3.86%), and much higher in Lagoa, where the increase of building stock varied between 11.93% (Cabouco) and 4.31% (Santa Cruz).

2. Design of the experimental green rooftop pilot study

2.1 Aims and scope

To evaluate the potential of constructing and expanding the use of GR in the urban areas of the Azores, we designed a pilot study, where we access the viability of such initiatives, considering: the existence of structures available to be used in a pilot study; the characteristics of the local substrates and the requirements in terms of plant species, considering the possibility of using the local flora. In this framework, the state of the art reveals important benefits of urban GR implementation and the possibility to include natural materials highly available at the intervention areas (e.g., volcanic substrates) for the constitution of the substrate layer of the GR, with intrinsic characteristics that ensure a high environmental performance. Moreover, GR researchers highlight the need to test different configurations of substrate and plant compositions, and the use of long-term monitoring plans to assess the economic viability of their implementation more accurately, on a large scale, and their real impacts on water management and thermal performance. Therefore, to enhance GR construction and implementation, technical and research studies regarding their design project, economic viability, using native vegetation and innovative substrate composition, are needed. Following this objective, we present the design of a prototype to be installed at the rooftop of the building of the Scientific Complex at the University of the Azores, located at the urban area of Ponta Delgada, São Miguel Island (Figure 4).

Figure 4.

Location of the University of the Azores at Ponta Delgada urban area (yellow line) and the building to install the pilot study.

2.2 Study site

The block now occupied by the University of the Azores was designed as a botanical garden and private residence in the late 19th century, and a collection of native plants from the Americas, Africa, Asia, Oceania, Europe and Macaronesia remains in the area. It is located on the south coast of the island of São Miguel in the vicinity of the historic city center of Ponta Delgada, the largest and most important urban agglomeration in the Azores. The building of the scientific complex, inaugurated in 2001, develops three floors above ground and is oriented in the North–South direction. The south part of the rooftop was considered to install the experimental study (Figure 5a).

Figure 5.

Selected place to install the experimental study of green roof (a), and implantation design (b).

2.3 Local substrates

In the Azores, volcanic rocks predominate, namely basalt and trachyte [16]. Pyroclastic deposits, such as ash, slag (lapilli) and pumice, originated from explosive volcanism, cover considerable areas in most of the islands, including São Miguel. The pomitic soils (ASTM 2487 [17], as well as the volcanic slags, are natural materials that are available in appreciable quantities in the Azores and present some geotechnical characteristics that make them interesting in their application in the structure of the strata of the green roofs: the volumetric weight, permeability, porosity and degree of saturation.

The organic compost produced by the waste management company in São Miguel Island (MUSAMI), is produced in compost piles using green waste from gardens, agricultural and forest activities [18]. This compost corresponds to a Class I soil, with organic matter >30%, pH between 7.5–8 and a balanced composition in Ca, Mg, K, S, Fe and B. The use of this compost, named SO-MUSAMI, should be encouraged in São Miguel Island since it is a local production, is available at an accessible price and contributes to the nutrients recycling.

As shown in Table 3, volcanic soils generally present low values of volume weight in situ, contrasting with high values of water contents and, consequently, high degrees of saturation in fine-grained soils. Volcanic slags have a drainage capacity in which the coarser fraction presents higher absorption than the fine fraction, since in the latter the porosity is predominantly due to isolated pores, hindering absorption, i.e., not allowing water to access the innermost voids of the particles.

Material	γh (kN/m^3)	w (%)	γd (kN/m^3)	e (−)
Pomitic soil	13,0–13,5	50,2–53,5	8,6–8,9	1,97–2,1
Black volcanic slags	19,0–21,7	n.a.	16,4–19,5	n.a.
Red volcanic slags	20,3–27,9	n.a.	14,8–23,4	n.a.
Pumice	9,3–10	65,1–69,5	n.a.	n.a.
Organic compost	7,35	<40	3,92	3,57

Table 3.

Physical characteristics of locally available materials for substrate [18, 19, 20, 21].

Characteristics in situ Table 3 presents a range of values of wet weight (γ_h), water content (w) and dry weight (γ_d) in situ and the voids index (e).

The black volcanic slags present a more uniform granulometry than the red volcanic slags, which present an extensive granulometry.

2.4 Local plants

Vegetation is a key component of green roofs and one of the most important factors affecting the rainfall quantity and quality of green roofs. Many plants have been tested. Plant species, previously of the genus Sedum (Crassualaceae), corresponding to succulent perennials, such as Sedum lineare Thunb., Sedum aizoon L. [accepted name, Phedimus aizoon (L.) ‘t Hart] and Sedum spectabile Boreau [accepted name, Hylotelephium spectabile (Boreau) H.Ohba, see [22]] are usually considered as suitable for green roof applications [23]. However, many other plants have been used.

Liu et al. [24] tested two C4 turfgrass Poaceae species, Eremochloa ophiuroides (Munro) Hack cv. ‘Civil’, a perennial or rhizomatous geophyte, and Cynodon dactylon (L.) Pers cv. ‘Tifdwarf’, a perennial or rhizomatous geophyte; two C3 turfgrass Poaceae species, Poa pratensis L. cv. ‘Midnight’, a perennial or rhizomatous geophyte, and Festuca arundinacea Schreb. cv. ‘Jaguar 4G’ [accepted name, Lolium arundinaceum (Schreb.) Darbysh.], a perennial; and two CAM plant species, the Crassulaceae S. lineare and the Commelinaceae Callisia repens (Jacq.) L., an annual or subshrub. C4 and C3 plants contributed significantly higher runoff reduction, removal of total nitrogen, chemical oxygen demand, total nitrogen, ammonium nitrogen, nitrate nitrogen, phosphates and total phosphorus than CAM plants [24]. Regarding the different green roof cooling effects of the same species, the results showed that different plant species performed differently: cooling with C3 and C4 plant species was mainly from their transpiration; C4 plants with higher transpiration rate originated higher cooling than C3 plants; CAM plants originated the lowest canopy cooling due to their stomata closure and lower transpiration during the daytime, but originating significant night cooling [25].

In other cases, the water needs of two Aizoaceae succulent subshrubs, Carpobrotus edulis (L.) N.E.Br., and Aptenia cordifolia (L.f.) Schwantes [accepted name, Mesembryanthemum cordifolium L.f.] were evaluated under different soil-containing and soil-less water-absorbing substrate amenders humic acid applications [26]. Recent studies also included mixtures of alien and native species, in comparison with conventional roofs, showing increased biodiversity in the former [27]. In a recent review, [28] found that plant diversity on intensive roofs was more responsive to soil texture, area and fertilizer use, when compared to extensive roofs, due to substrate depth increasing potential plant diversity, coupled with a wider variability in design intentions is typical for extensive roofs. Those authors also suggested the definition of new classifications to help bridge the gap between practitioner and ecological knowledge and aid in predicting patterns by emphasizing green roof design intent rather than substrate depth [28].

Within this framework, we see that most plants used in green roofs were alien, in a somewhat restricted number of plant families. However, in some cases complex mixtures have also been used, using local European or Mediterranean plant taxa, including small shrubs, such as Lavandula latifolia Medik. (Lamiaceae), Rosmarinus officinalis L. [Salvia rosmarinus Spenn.] (Lamiaceae), Viburnum tinus L. (Viburnaceae), Calluna vulgaris (L.) Hull (Ericaceae), Santolina chamaecyparissus L., Santolina virens Mill. (Asteraceae) and Erica verticillata Forssk. [Erica manipuliflora Salisb.], as well as plants from other continents, including Cotoneaster dammeri C.K.Schneid. (Rosaceae).

Most of the above-mentioned plant species occur in the Azores, although several are considered as top invaders, such as C. edulis and A. cordifolia [29]. The Azores’ vascular plant flora includes more than 3800 plant taxa, with about 2901 cultivated or at least imported, 322 escaped and 578 naturalized, and no more than 400 native or endemic [30]. Meanwhile, due to the wet climate, succulent plants are probably not appropriate to be used in GRs. Nevertheless, at low elevations, plants of European or Mediterranean origin are very common in the Azores. Thus, at this stage, it would be possible to consider the comparison of two alternative mixtures of introduced shrubs with interest in pollinator insects, L. latifolia, R. officinalis, a common grass, P. pratensis, and two native shrubs, C. vulgaris and Erica azorica Hochst. ex Seub (Ericaceae). The latter has a wide ecological amplitude in the Azores [31], being found from the coast up to high elevations (over 1000 m a.s.l.).

2.5 Pilot study design

The experimental design will evaluate the performance of different native vegetation species, individually and polyculture, and different proportions of organic and inorganic substrates using abundant local materials (volcanic materials and organic compost). The pilot GR will fulfill the characteristics of an extensive one (substrate layer depth between 100 mm and 150 mm). Considering the intended plants and substrate combinations, with replicates, a total of 24 modules is planned to be installed on the 72 m² of roof-top, coupled to a control area to collect rainfall (Figure 5b). A saturated weight of 45 kg/module was admitted, to calculate the additional overload on the building derived from the weight of the saturated boxes, obtaining less than 1 kN/m². Each module will have two drainage pipes at the bottom and an overflow pipe on the surface to collect flows for quantity and quality analysis.

To the quality parameters, a smart multi-sensor probe or laboratory analysis will be performed (e.g., pH, turbidity, conductivity, COD, BOD and total suspended solids), to conclude about water purification or deterioration in samples collected in each module and in the control. Flowmeters will be used to measure the water that runoffs the system and will be collected in the outflow of each module. At the same time, data from the nearby meteorological station will be used to correlate water retention information with climate characteristics, namely precipitation, temperature and air humidity. These collected data will allow to determine the amount of water retained and water running off the systems, especially the runoff coefficient parameter, an important value to be used when GR systems are being planned and dimensioned in the urban environment.

GR substrate mixtures will be characterized regarding their physical and chemical characteristics (granulometry, porosity, pH, water holding capacity, nutrients (N and P), retention capacity, toxicity and percentage of each component present on the mixture). The formulated substrate will then be characterized considering the main physico-chemical characteristics that will influence its performance (porosity, water retention, pH, density, granulometry and presence/absence of toxins).

The vegetation growth during this uninterrupted long-term monitoring plan (2 years) will be measured to assess the most adapted species to the climate and subtract composition. By controlling temperature in the different layers, in the surroundings and inside the building, data will be provided to evaluate the thermal regulation of this GR essay.

3. Discussion and conclusions

Green roofs can contribute to the mitigation of extreme rainfall events through water retention and evapotranspiration [32].

Regarding the urban landscape, the horizontal roofs are the most significant projects in terms of nowadays urban planning. Consulting a legitimate tribute to the contemporary architecture [33], promoted by the Association of Architects of the Regional Delegation of the Azores, it’s possible to have a selective and competent overview of the territory regarding state-of-the-art construction that indicated this evolution.

Horizontal rooftops are a condition to implant green rooftops. Since the available data just covers residential buildings, and other typology buildings, such as hotels, schools and hospitals, in Ponta Delgada and Lagoa adopted horizontal rooftops instead of other rooftops typologies, we believe that at present, the percentage of horizontal rooftops in the urban areas of Ponta Delgada and Lagoa is over 3.1%. Also, the new constructions since 2010 adopt this cover typology more often. Studies must be done to the building’s typology, to get actual percentage of horizontal rooftops.

One of the potentialities of GRs is to be able to reconcile the ecosystem services provided by plants, such as the contribution to urban biodiversity, with other functionalities, such as water storage for use in buildings and the installation of photovoltaic panels for energy production. The storage of water for on-site use in second-line uses has a double advantage, which is to avoid the production of runoff and thus protect the receiving waters from contaminants, and on the other hand to constitute a local water source, which will reduce the consumption of drinking water and thus increase its availability for human consumption. Providing a more resilient city in terms of water availability is a major issue in the actual context of climate change and increase of touristic activity.

The use of indigenous resources in the design of GRs contributes to the sustainability of the proposal, as their construction is not associated with transport costs or the import of new species. As a result of the bibliographic review of existing plants in the Azores with application in GR, five species were selected that can be used individually or in polyculture: two introduced shrubs with interest for pollinator insects, L. latifolia, R. officinalis, a common grass, P. pratensis, and two native shrubs, C. vulgaris and Erica azorica Hochst. ex Seub (Ericaceae). However, the choice of plants depends on the specific objectives of the GR: water retention, thermal regulation, biodiversity (insects) or esthetics, as well as wind exposition and altitude of the building.

The cost and local availability of the identified materials (pomitic soil, organic compost, volcanic slags and pumice) must be assessed before evaluating their use in a substrate composition. For the substrate, the water content and the weight are important properties of water retention and overload of the GR in the building. Also, the nutrient availability to plants growth and toxicity absence must be guaranteed.

Green roofs are not widespread in Azores’ urban areas. Their dissemination will create more job opportunities, since the maintenance of these green rooftops needs special requirements.

In order to start its dissemination, it is necessary to review and characterize the actual build stock in urban areas, and more important, construct and operate the presented GR pilot scale to select the most effective substrates and plants for water retention and building thermal regulation.

Conflict of interest

The authors declare no conflict of interest.