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Article

Hydrologic Effectiveness and Economic Efficiency of Green Architecture in Selected Urbanized Catchment

by
Marcin K. Widomski
*,
Anna Musz-Pomorska
and
Justyna Gołębiowska
Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka St. 40 B, 20-618 Lublin, Poland
*
Author to whom correspondence should be addressed.
Land 2023, 12(7), 1312; https://doi.org/10.3390/land12071312
Submission received: 1 June 2023 / Revised: 22 June 2023 / Accepted: 28 June 2023 / Published: 29 June 2023
(This article belongs to the Special Issue Economic Valuation of Urban Green Spaces)
Browse Figures
Versions Notes

Abstract

:
This paper presents a numerical assessment of the influence of green roofs applied in the urbanized catchment on the rainwater outflow hygrogram as well as costs and economic efficiency analysis of the proposed green architecture application. The campus basin of the Lublin University of Technology, Poland, was selected as the object of the study. Three variants of extensive green roof applications were designed. The numerical model of surface runoff was developed in US EPA’s SWMM 5.2 software. The simulations were performed for three different rainfall events of various intensities and durations. The cost efficiency of the proposed green architecture was assessed by the Dynamic Generation Costs indicator, while economic effectiveness was tested by Benefits–Costs Ratio and Payback Period determined for all assumed variants. The determination of economic efficiency indicators was based on investment and maintenance costs estimation, assumed discount rate, and time duration of assessment. Results of numerical calculations showed up to 16.81% of peak flow and 25.20% of runoff volume reduction possibly due to the green roof application. All proposed variants of green roof applications in the studied urbanized catchment were assessed as generally profitable due to possible financial benefits related to heating and cooling energy savings and avoiding periodical change of bitumen roof cover.

1. Introduction

Climate change, population growth, and urbanization, creating, among others, a significant pressure on the quality and quantity of water resources, are the current challenges for spatial planning and natural resource management, especially in the cities, which are extremely vulnerable to the negative effects of these phenomena [1,2,3]. Many cities are already experiencing climate extremes, such as droughts, floods, and heat waves [2,4,5].
The distorted water balance of urbanized basins, with a significant share of hardly permeable sealed surfaces, strengthened by the actual climate changes, presents, on the one hand, the increased runoff volume and evapotranspiration, and on the other, the limited infiltration and reduced groundwater resource supply [6,7,8,9,10]. Increased runoff during extreme rainfall events also results in an increase in pollutant loads flushed from the catchment-sealed surfaces and delivered through the stormwater system, mostly without any treatment, directly to the rainwater receivers [11,12]. The increased volume of runoff water, in combination with the limited capacity of urban stormwater systems, may lead to frequent flooding.
Sustainable rainwater management in urbanized catchments, considered within the three main circles of sustainability, i.e., environmental, social, and economic [3,13,14,15,16,17], should allow limiting the negative changes in the water balance of the urban watershed, reducing the runoff volume and its peak flows, decreasing flooding risk and antrhoporessure on the waterbody of rainwater receivers [9,18].
One of the ways to adapt to climate change and counteract its negative effects is to increase the green infrastructure in urban areas, among others, by green roof application. A green roof is a roof utilizing vegetation and porous media of a specific construction, including certain layers, such as vegetation, substrate, filter, drainage, and insulation system [19], designed to partially restore the ecology and water balance of urbanized catchment [20,21]. Green roofs can be generally divided into extensive and intensive ones, depending on the thickness of the substrate layer and the type of vegetation. Extensive roofs are characterized by the thickness of the substrate layer of less than 20 cm, and they are planted with self-sufficient species like drought-tolerant grasses, sedums, mosses, and wildflowers, with a weight of 60–150 kg/m2. They can be installed on flat and sloped roofs [22,23,24]. They are often recommended for implementation at the individual level or small scale as this solution provides the best cost–benefit ratio [25]. Intensive roofs are characterized by a thicker substrate ranging from 20 cm to 200 cm [21,23] and a greater weight of 200–500 kg/m2 [22]. Vegetation planted on intensive roofs includes lawns, perennials, herbs, shrubs of various sizes, and even trees, and they require regular care. Due to their specific and heavy construction, they are not suitable for sloping roofs and roofs with small load-bearing capacity [22].
Compared to conventional roofs, green roofs require higher investment and, in some cases, operating costs. In addition, the maintenance of the roof itself may be more complicated and vary depending on the type of vegetation and irrigation needs [24]. However, due to a number of social, environmental, and economic benefits, green roofs seem to be essential for creating sustainable environment in urbanized areas. Moreover, they can contribute to climate change mitigation, as they have the potential to reduce the carbon footprint due to the number of their functions connected with carbon sequestration [26], stormwater management [19,20], and energy saving [24], which is described in more detail in the following paragraphs.
Green roofs can enhance the aesthetic value of building architecture and improve the biodiversity in the municipalities by creating quality habitats for diverse species [19,27]. Moreover, the implementation of green roofs can improve air quality by retaining air pollutants like: CO, CO2, SO2, NO2, PM10, PM2.5 [26,27,28,29]. It was also observed that thanks to the development of green roofs, the temperature in urbanized areas can be decreased by 1–3 °C, depending on the climate [30,31], and the urban noise can be reduced [32].
It has also been proved that green roofs are among the most sustainable and efficient solutions that have positive effects on the local water balance as they are able to reconstruct the natural hydrologic conditions [19,20]. The implementation of green roofs enables the retention of stormwater for a longer period of time in urban areas and increases evaporation, thus it is possible to reduce the peak flow and the rainfall runoff volume from the sealed surface by an average of between 50 and 80%, depending on the roof construction and its characteristics, as well as on the local climate conditions [19,23,33,34].
From the more practical point of view, essential for potential investors, it is important to underline that green roofs provide additional thermal insulation for buildings, allowing to reduce the heating and cooling energy demand [24,35,36,37,38]. The elongated lifespan of green roofs, compared to conventional roofs, accounting for 30–50 years, may also be a crucial factor in the investment decision process [24]. Incidentally, there are reports about green roofs in service for more than 90 years [39]. In addition, there are several incentive policies available, usually on the local level, encouraging potential investors to make future investments: Reduction in property tax or other tax cuts, reduction in stormwater fees [24]. There are financial benefits available for owners of buildings with green roofs applied, including property tax exemption, in selected cities in Poland, including Katowice, Kalisz, and Gdansk [40].
No matter the positive environmental and social effects of different types of sustainable rainwater management, including green roofs of different types, their applicability may be affected by the economic aspects, mainly capital, as well as operation and maintenance costs, closely related to users’ acceptability, willingness to pay, and ability to operate and maintain [41,42,43,44]. Environmental investments are often related to significant investment, as well as operation and maintenance costs required for the construction and successful operation of these systems. Low Impact Development (LID) architecture, including green roofs, similar to water supply, sanitation, and stormwater management systems, is not excluded from this microeconomic issue [23,45,46,47]. Green roof construction requires capital costs, including purchase, transport of various materials (i.e., substrate, geotextiles, drainage filters, hydro insulation, edge and separation metal or polymer stripes, vegetation mat, vegetation cover plants, etc.), and workload necessary for green roof installation and vegetation planting. Under the actual conditions in Poland, the reported capital costs of one square meter of an extensive green roof are estimated as 140–900 PLN (approx. 30–195 Euro) [48,49,50,51,52]. Operational green roofs require watering in the first period after planting or during prolonged dry periods, regular gardening care, servicing and cleaning of drainage and gravel layers, etc.
Moreover, the evident environmental and social benefits of green roof application are not uniform, even in the city limits, and are highly related to land uses, hydrology schemes, population profiles, and LID variants accepted [53].
This paper presents the analysis concerning a case study of hydrologic efficiency and economic effectiveness of green roof application in urbanized catchments under climatic and economic conditions in Lublin, Poland. The aim of the presented research was to relate possible changes in the water balance of the selected urban basin related to green roof application to cost efficiency and profitability of such investment. As it was mentioned above, the possible microeconomic acceptance of green roof designs under various climatic and economic conditions is non-uniform, thus this paper aimed to assess the potential economic feasibility, influencing users’ acceptability and willingness to pay, of proposed designs for a tested urbanized catchment. The hydrologic efficiency of green roofs was assessed by possible reduction in rainwater outflow volume and peak flows, while economic efficiency was related to cost estimation of the investment and possible financial benefits resulting from green architecture installation.

2. Materials and Methods

This paper presents a numerical study concerning the influence of green roof application on rainwater outflow hydrograph and economic efficiency analysis of applied green architecture in the urbanized catchment. This study assumes the application of several variants of extensive green roofs, with different areas, on various buildings within the selected urbanized catchment. Numerical modeling of stormwater outflow from the selected catchment was performed in US EPA SWMM ver. 5.2 software. Calculations were repeated for 3 rainfall events: Two observed by the local weather station and one model assuming the value of rainfall intensity suggested for stormwater management design in Poland [54,55]. The economic efficiency of the proposed green architecture was determined by calculated indicators of Dynamic Generation Cost, Benefits–Costs Ratio, and Payback Period. For this case, the investment, as well as operation and maintenance (O&M) costs, were estimated, together with the possible financial benefits of green roof application. The financial analysis presented in this paper was based on measurable and quantitative microeconomic financial benefits of green architecture application, clear, sound, and understandable for the potential investor.

2.1. Object Description

A part of the Lublin University of Technology campus area was selected for this study. The area selection was determined by the spatial arrangement of the stormwater system and urbanized watershed configuration. The total area of the studied urbanized catchment is 2.72 ha. The object covers nine buildings of different heights, parking lots, roads, pavements, and green areas. Figure 1 presents the orthophoto of the studied area, while its characteristics are presented in Table 1. The total area of roofs was determined as 1.08 ha, roads, pavements, and parking lots 1.44 ha and green area 0.2 ha. Roofs are covered mostly (buildings 1–8) by bituminous waterproofing, while pavements and roads are sealed by various types of concrete paver blocks. Only building No 9 has a different roof cover, a steel roofing sheet. The variants of green roof application for the selected urbanized catchment considered in this study are also presented in Table 1.

2.2. Green Roof and Study Variants

Two types of extensive green roof construction presented in Figure 2, for two different roof slopes, i.e., flat roofs of slope up to 5 degrees and inclined roofs up to 35 degrees, were assumed for our study. The assumed type of green roof design was related to possible gain in water retention and construction, including slope and possible load, of the existing conventional roofs. The main vegetation layer was assumed as extensive substrate [56] consisting of recycled, unused, crushed brick and compost, in agreement with German FLL and GRO UK green roof guidelines [57,58,59]. The particle size distribution of the selected substrate is presented in Table 2, while its water retention curve, presented as pF = log H, where H–pressure head in cm, is shown in Figure 3. The bulk density of the substrate equals 0.83 kg/dm3, and its particle density is 1.79 kg/dm3. The selected thickness of the substrate layer varied between 8 cm for flat roofs and 5 cm for inclined roofs. Additionally, the vegetation layer in the inclined green roofs was strengthened and supported by the crate anti-slip system. The vegetation cover for the flat green roofs was selected as planted perennials and for the inclined, mosses and sedums already planted on the ready-to-application mat.
The filtration layer was designed as a standard polypropylene infiltration geotextile limiting the propagation of fine particles. The excess water collection was designed in the retention-drainage layer consisting of an HDPE (high-density polyethylene) drainage mat. The PP/PES (polypropylene/polyethersulfone) geotextile was selected as a material for the protective layer. The hydro insulation layer was designed as a reinforced PVC (polyvinyl chlorine) membrane. The lowest layer, root protection barrier, was assumed as 0.5 mm LDPE (low-density polyethylene) foil.
Determination of the investment costs of the proposed green roof was directly based on the actual market pricing in Poland and covered all required materials, services, workload, etc. The operational and maintenance costs covered assumed plant watering (the first and the following years after planting), fertilization, conservation, substrate, and plant replenishment [60,61]. The investment, as well as operation and maintenance costs of each variant, are presented in Table 3.
The unit investment cost for 1 m2 of flat green roof (buildings 2–9) was determined as 45.20–53.20 Euro, in relation to the green roof perimeter length. The unit cost of the inclined green roof applied in variant 3 in buildings no. 8 and 9, was, according to the different construction, see Figure 2, higher and reached values 71.22–76.47 Euro. The clear difference in costs between the two types of proposed extensive green roofs results from the polymer substrate stabilization grid, required in the case of the sloped roof, and the assumption of different vegetation layers, i.e., mosses and sedums on a ready-to-apply matt, on the sloped roof. Similarly, the unit mean annual O&M costs for flat and inclined green roofs were also different and reached values of 0.71–1.30 Euro/m2, respectively.

2.3. Numerical Modeling

The numerical model used to assess the influence of the gradual application of green roofs on rainwater outflow hydrograph was developed in the United States EPA (Environmental Protection Agency) SWMM (Stormwater Management Model) 5.2. The total area of the selected basin, 2.72 ha, was divided into 20 subcatchments, including 9 roofs, 8 pavements, parking lots and roads, and 3 green area subcatchments. The stormwater pipeline network applied to the model reflects the existing system of 0.2–0.5 m diameter concrete (Manning’s roughness n = 0.015) pipelines and total length 514 m and consists of 15 nodes and 14 lines. Numerical modeling of surface runoff from the studied urbanized catchment was performed for four cases, all three variants, marked as 1–3, of green roof application (see Table 3) and “zero” variant with no green architecture applied. The developed model of the studied catchment is presented in Figure 4.
Numerical calculations were performed for 2 monitored rainfall events from 2021 and 2023, measured by the local weather station. The third rainfall event assumed to numerical calculations reflected rain commonly accepted to stormwater system design in Poland, with unit outflow 177 dm3/(s∙hectare), with duration t = 15 min, probability p = 20%, and appearance once per 5 years [62], which was fitted to Chicago model rain distribution [63,64]. The assumption of these three rainfall events of different intensities and unit runoff was aimed to test the proposed green roof designs under variable climatic conditions. The assumed rainfall data are presented in Table 4 and Figure 5.
The input data required for the numerical modeling of surface runoff are presented in Table 5. The infiltration rate of the concrete paver blocks surface sealing was determined in situ according to ASTM C1701/C1701M-09 [65] standard with 300 mm polypropylene infiltration rings.
Hydraulic and water retention characteristics of the applied extensive roof substrate, surface, and drainage layers accepted for numerical modeling of green roof efficiency in SWMM are presented in Table 6. The difference between field water capacity (pF = 2.0) and water content for plant wilting point (pF = 4.2) is commonly described as the retention capacity of porous material. Thus, this value for the applied substrate reached level 0.345 m3/m3. The initial saturation of the substrate layer was assumed as 30%.

2.4. Cost-Efficiency and Benefits-Costs Analysis

The assessment of cost-efficiency of studied variants of green architecture application to stormwater management in the urbanized catchment was based on the Dynamic Generation Cost indicator [68,69,70]:
D G C = p E E = ∑ 0 t = n I C t + E C t ( 1 + i ) t ∑ 0 t = n E E t ( 1 + i ) t
where: ICt—annual investment costs in a given year (Euro), ECt—annual operation and maintenance costs in a given year (Euro), t—year of investment time duration, from 0 to n, where n is the last assessed year of investment activity (year), i—discount rate (%), pEE—price of the ecological unit effect of the investment (Euro/m3), EEt—annual ecological unit in a given year (m3).
The popular DGC indicator, commonly applied in the comparison and assessment of environmental designs [68,71,72] determines the discounted cost of the ecological effect of the investment, taking into account investment and O&M costs. Thus, the DGC allows us to compare and assess the cost-efficiency of several designs. The application of DGC is rather simple, the more financially effective method the lower value of the determined indicator is. However, the assumption of the unit ecological effect, typical for each sort of ecological and environmental design (i.e., the volume of supplied water or treated sewage), is required. In this study case, the DGC was calculated for one cubic meter of possible water retention of assumed green roofs as the constant characteristic of each design, not related to the variable climatic conditions. The retention volume of green roofs in each assumed variant was determined, taking into account the area of LID, the thickness of the porous substrate, and its retention characteristics, i.e., water field capacity and water content for plant wilting point. The determined possible retention volume was 53.34, 71.94, and 109.24 m3, for each studied variant, respectively.
The economic efficiency of green roof application was determined using the Benefits–Costs Ratio (BCR) indicator calculated as follows:
B C R = P V b P V c
where: PVb—present value of investment benefits (Euro), PVc—present value of investment costs (Euro).
P V b = ∑ t = 0 n C F b t ( 1 + i ) t
P V c = ∑ t = 0 n C T c t ( 1 + i ) t
where: CFbt—benefits cash flow for a t period (Euro), CFct—costs cash flow for a t period (Euro).
The simple indicator of investment profitability, not including the variable value of money during the investment lifespan, Payback Period, was calculated as follows:
P P = I C N C F
where: PP—Payback Period (years), IC—initial investment costs (Euro), NCF—net cash flow (Euro/year).
The following input data were assumed for cost-efficiency determination: (i) Time duration 50 years, according to [24] (ii) discount rate 5% [42,46,47].
To determine the financial benefits of green roof application required for benefits– costs analysis, the following assumptions concerning only the measurable and quantitative possible direct savings for stakeholders were made:
  • Savings due to the positive effect of green roof on heat and cooling energy consumption, i.e., due to the additional thermal insulation, the mean annual heating energy savings was determined as 8.14 kWh/m2 and cooling energy savings as 0.59 kWh/m2 after literature reports for buildings of similar construction and located in zones of moderate climate of similar mean annual temperature approx. 7–10 °C [23,73,74,75]. The building heating in winter months is based on heat provided by the municipal combined heat and power station. The university facilities’ cooling is based on the electric power provided by the municipal electric network. The electric power (0.21 Euro/kWh) and local heat energy (0.094 Euro/kWh) prices were obtained from the market [76,77].
  • Avoiding change of roof cover (bituminous waterproofing) after 20 years of operation, the total assumed costs of heat-sealable bitumen coating restoration, including materials and workload, was determined as 17.00 Euro/m2 [60].
The project covers the campus of a public state university, so according to biding law in Poland [78], the educational and research facilities are excluded from the property tax, so no extra savings are possible in this case. The CO2 emission costs in Poland are included in energy prices, thus no extra payment is required from the facilities owner/operator [78].

3. Results

3.1. Numerical Rainwater Outflow Modeling

Figure 6 presents rainwater outflow hydrographs for all four variants of calculations for four different rainwater management systems and three rainfall events applied. The statistical description of determined hydrographs is presented in Table 7. It is visible that the application of green roofs alerted the shape of outflow hydrograph curves in all studied cases (Variants 1–3 versus Variant 0, without green architecture). An increase in green roof area resulted in a decrease in hydrograph values, also including their peak flows. The determined modeled decrease in peak flows after application of green roofs was in the range of 1.46–16.81%. The transformed outflow hydrograph also resulted in changes in rainwater outflow from the catchment.
The calculated values of outflow volume for studied scenarios are presented in Table 8. As it could be expected, the non-uniform reduction degree is related to the area of green roofs introduced in the studied urbanized catchment and to the characteristics of the applied rainfall event. The weakest performance in runoff volume reduction was observed for Rainfall No. 1, characterized by the very high depth and the longest observed duration. In the case of two remaining rainfall events applied to simulation, the studied green roofs performed similarly. It is worth noting that in all cases, the decrease in rainwater outflow is not linearly related to the relative increase in the green roof area. The 7.1% of green roofs in relation to catchment area resulted in a 1.9–8.5% outflow reduction, 9.58% of green roofs led to a 2.5–10.6% outflow reduction, and 17.53% of green roofs allowed a 5.1–25.2% of outflow reduction, respectively. The performed one-way Kruskal–Wallis ANOVA test for calculated outflow volume for three applied rainfall events showed that the determined differences are statistically significant.

3.2. Economic Efficiency Calculations

Determined values of Dynamic Generation Cost for 1 cubic meter of retention volume of designed green roofs for the assumed 50 years lifespan of the investment are presented in Figure 7. The highest unit costs were observed for Variant 3, assuming the installation of green roofs also on steep sloped roofs of buildings No. 8 and 9 (see Figure 1). In this case, the increased investment and O&M costs, directly affecting the indicator value, are related to more complex and costly green roof construction.
The results of profitability of economic efficiency determination of the assumed variants of green roof application, based on Benefits–Costs Ratio indicator determination, are presented in Figure 8. It is visible that, in relation to the assumed investment, O&M costs, possible financial benefits (savings), and 50 years of lifespan, all proposed variants of green roofs installation on the selected part of the Lublin University of Technology campus are profitable, with values of BCR greater than 1.0. The lowest value of economic profitability indicator was determined for Variant 3, assuming green roofs also on steep roofs. In this case, the possible savings were also lower, because building No 9 is covered by a steel roofing sheet, for which service and restoration are not required during the assumed time of assessment.
Figure 9 presents determined values of simple profitability indicator, Payback Period, for all tested variants of green roof application in the studied urbanized catchment. In all studied cases, the time of investment cost recovery was shorter than 20 years. The longest duration of Payback Period was determined for Variant 3, assuming the installation of green roofs on steep slopes. In this case, as was presented before, an increase in capital investment and operation costs is required.
It is worth noting that cost-efficiency and economic profitability, measured by DGC, as well as BCR and PP, respectively, of Variants 1 and 2 are nearly the same, without any relation to approx. 35% increase of the applied green roofs of low slope inclination. A decrease in cost-efficiency, determined by an increase in DGC value, as well as a decrease in profitability, described by a decrease in BCR and an increase in PP values, were observed after analyzing green roofs on inclined roofs.

4. Discussion

The performed numerical simulations of the hypothetical application of three variants of extensive green roofs in the selected catchment of the Lublin University of Technology, Poland, showed the possible hydrologic benefits, i.e., a decrease in the volume and peak flows of rainwater surface runoff after three selected, different rainfall events. As it was described earlier, the determined modeled effects, including up to 16.81% of peak flow and 25.20% of runoff volume reduction, were related to rainfall characteristics. This relation was reported in several earlier studies for different locations and climates [19,24,33,79]. The observed modeled runoff peak flow reduction was comparable with calculation results reported for two buildings’ urban catchment with extensive green roofs located in India, where a decrease in flow was determined as approx. 10% for the applied rainfall events [25]. On the other hand, the 61.8–100% peak flow reduction reported for seven extensive green roofs in Beijing, China [80], or average values of 49.0–90.0% for colder climates presented in a survey paper [24] were higher than those presented in this study, however, these values were measured solely for green roofs and standard roofs.
The calculated retention volume for the studied catchment and applied rainfall events was comparable with 33.6–81.5% runoff reduction determined for five years of green roof observations in Wroclaw, Poland [33] or with the annual 27% reduction of runoff volume from nine experimental green roofs in Sheffield UK. A similar paper [81] also presenting research performed in Wroclaw, Poland with two extensive green roofs and 17 rainfall events reported a runoff volume reduction from studied roof panels as 10.5–90% and 27.1–90.0%. Similar, comparable results were reported for 12 months of observation of two extensive green roofs of 75- and 115-mm substrate thickness, for which the annual mean reduction of runoff was 32.9% and 23.2%, respectively. Interestingly, similar results of 10.0–60% runoff volume reduction were presented for a single 663 m2 extensive green roof in Seoul, S. Korea [19]. All the above-quoted data are in agreement with information provided in [82], which means that the retention of extensive green roofs generally reaches up to 50% by volume.
After performing cost-efficiency and economic effectiveness assessment analyses, all three variants of the proposed green roof applications in the selected part of the Lublin University of Technology catchment should be assessed positively. Despite the significant, comparable to reported for different locations [23,45,74], investment, as well as operation and maintenance costs, all proposals, after applying measurable possible financial benefits for the investors, should be assessed positively as financially profitable. The determined cost-efficiency and economic profitability of green roof installation for the selected urbanized catchment are in general agreement with the previous reports. The determined discounted costs of green roof installation and operation are comparable with the results of a study presented by [45], which described the ecologic cost of 1 m3 runoff reduction as approx. 270 US $.
The obtained results of benefits–costs analysis, taking into account different input assumptions considering available financial benefits, were comparable to values presented in [23] for eleven locations in Poland, for which the profitability of the extensive green roof investment was proven by the benefits–costs indicator Net Present Value determination. The results of BCR and PP calculations presented in this study are supported by statements provided in several papers [74,83] that green roofs enable cost savings, in relation to traditional roofs, during their longer lifetime operation, and that green roofs are low-risk investments. Thus, the probability of financial benefits possible after green roof installation is much greater than the profitability of losses.
The presented results of economic analyses were closely related to the available local conditions determining the possible beneficial financial effects of green roof application. In this paper, according to the current legal and financial conditions, the possible benefits were related to sound, measurable, and easy to validate input data, including heating and cooling energy savings and prolonging the lifespan of bitumen roofing cover. The other possible benefits, possible under different local conditions, i.e., property tax or stormwater fee reduction, were not included in this research [23]. Moreover, several possible intangible effects of green roofs described in the literature [23,24,84], such as biodiversity, well-being, aesthetic, and recreational value, were also not included in this study as they are hardly measurable and difficult to quantify for the potential investors.
The presented results of runoff peak flows and volume reduction, as well as cost-efficiency and economic effectiveness of selected designs of variable green roof application, showed that an increase in hydrologic efficiency was related to a decrease in cost-efficiency and profitability. However, in all cases, the economic assessment of proposed designs should be positive. The determined decrease in cost and economic efficiency was, in the case of the studied catchment, related to the introduction of green architecture on steep slope roofs, up to 35 degrees. Due to the construction and required maintenance works investment and O&M costs for such green roof designs are higher, directly affecting their assessment. Thus, in our opinion, the selection of area, type, substrate, plants, and planning of conservation works should always be supported by the cost and financial effectiveness analysis.

5. Conclusions

The performed numerical calculation of hydrologic efficiency and economic effectiveness of the proposed extensive green roof applications in the selected part of the Lublin University of Technology, Poland urbanized catchment allowed the following conclusions:
  • The proposed variants of green roof application of different slope allowed a decrease in peak flows and volume of surface runoff, depending on the area and type of green roof, as well as to characteristics of the applied rainfall event;
  • From the economic perspective, all studied variants of green roof applications were generally assessed as profitable due to possible benefits related to heating and cooling energy savings as well as avoiding periodical change of bitumen roof cover;
  • Thus, the administrator of the studied catchment would financially benefit in the long term from the application of all proposed designs of green roofs;
  • All of the economic benefits mentioned above would also have a positive impact on the building’s carbon footprint;
  • A decrease in cost-efficiency and profitability of green roofs applied in the studied catchment was related to the increase in required investment and operation costs of green roofs on sloped roofs, the increase in the area of flat green roofs, affecting the hydrologic efficiency, had hardly any effect on cost and economic effectiveness;
  • In our opinion in order to sustain the acceptable cost-efficiency and economic profitability of green roof application in the urbanized catchment, the design decision-making process should be supported by economic analyses, including investment and operation and maintenance costs as expenditures and the measurable and quantitative financial benefits of green architecture application.

Author Contributions

Conceptualization, M.K.W., A.M.-P. and J.G.; methodology, M.K.W. and A.M.-P.; validation, M.K.W.; formal analysis, M.K.W. and A.M.-P.; investigation, M.K.W.; resources, A.M.-P., J.G. and M.K.W.; writing—original draft preparation, M.K.W., J.G. and A.M.-P.; writing—review and editing, M.K.W. and A.M.-P.; visualization, M.K.W., A.M.-P. and J.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by internal projects of Lublin University of Technology, Poland numbers FD-20/IS-6/024 and FD-20/IS-6/039.

Data Availability Statement

Data sharing not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Land 12 01312 g001 550
Figure 1. Orthophoto map of the studied catchment.
Figure 1. Orthophoto map of the studied catchment.
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Figure 2. Construction of assumed green roof designs: (a) Flat green roof, (b) sloped green roof.
Figure 2. Construction of assumed green roof designs: (a) Flat green roof, (b) sloped green roof.
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Figure 3. Water retention curve of selected extensive green roof substrate modified after [56].
Figure 3. Water retention curve of selected extensive green roof substrate modified after [56].
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Figure 4. The model of the studied urbanized catchment developed in SWMM.
Figure 4. The model of the studied urbanized catchment developed in SWMM.
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Figure 5. Assumed rainfall event hyetograph: (a) Rainfall no. 1, (b) rainfall no. 2, (c) rainfall no. 3.
Figure 5. Assumed rainfall event hyetograph: (a) Rainfall no. 1, (b) rainfall no. 2, (c) rainfall no. 3.
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Figure 6. Outflow hydrographs for all variants of green roof application: (a) Rainfall no. 1, (b) rainfall no. 2, (c) rainfall no. 3.
Figure 6. Outflow hydrographs for all variants of green roof application: (a) Rainfall no. 1, (b) rainfall no. 2, (c) rainfall no. 3.
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Figure 7. Dynamic Generation Cost indicator of cost-efficiency determined for studied variants of green roof application.
Figure 7. Dynamic Generation Cost indicator of cost-efficiency determined for studied variants of green roof application.
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Figure 8. Benefits–Costs Ratio values determined for studied variants of green roof application.
Figure 8. Benefits–Costs Ratio values determined for studied variants of green roof application.
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Figure 9. Payback Period values determined for studied variants of green roof application.
Figure 9. Payback Period values determined for studied variants of green roof application.
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Table
Table 1. Types and roof areas of buildings in the selected urbanized catchment and assumed variants of green roof application.
Table 1. Types and roof areas of buildings in the selected urbanized catchment and assumed variants of green roof application.
Building No.BuildingRoof Area (ha)Designed Green Roofs Area (m2)
Variant 1Variant 2Variant 3
1–4Dormitory0.04-248.60 *248.60 *
5Administration and canteen0.121277.92 *1277.92 *1277.92 *
6Storehouse0.02---
7Education-research facility0.31381.46 *381.46 *381.46 *
8Education-research facility0.18273.06 *273.06 *273.06 *
242.00 **
9Education-research facility0.25-425.49 *425.49 *
1920.1 **
Total area (m2)1932.442606.534768.63
* Flat (5 deg.) roofs, ** Inclined (15 deg.) roofs.
Table
Table 2. Particle size distribution of selected extensive roof substrate modified after [56].
Table 2. Particle size distribution of selected extensive roof substrate modified after [56].
Particle Size FractionParticle Content (%)
Stones (>8 mm)61.2
Coarse gravel (8–4 mm)28.5
Fine gravel (4–2 mm)1.2
Very coarse sand (2–1 mm)0.5
Coarse sand (1–0.5 mm)0.5
Medium sand (0.5–0.25 mm)1.3
Fine sand (0.25–0.125 mm)1.2
Very fine sand (0.125–0.05 mm)0.7
Silt (0.05–0.002 mm)2.7
Clay (<0.002 mm)2.0
Table
Table 3. Investment and mean annual operation and maintenance costs of proposed green roof application variants.
Table 3. Investment and mean annual operation and maintenance costs of proposed green roof application variants.
VariantInvestment Costs (Euro)Mean Annual O&M Costs (Euro)
190,090.631355.14
2123,382.141827.85
3275,330.774597.91
Table
Table 4. Rainfall events accepted for numerical simulation of runoff hydrograph.
Table 4. Rainfall events accepted for numerical simulation of runoff hydrograph.
Rainfall EventDepth (mm)Duration (min)Calculated Unit Runoff
(dm3/(s∙Hectare))
169.4069516.64
210.005952.80
315.9315177
Table
Table 5. Input data assumed to numerical calculations in SWMM.
Table 5. Input data assumed to numerical calculations in SWMM.
Input DataArea Type
RoofPavementGreen Area
% impervious908020
Max infiltration rate (mm/h)0.125.050
Min. infiltration rate (mm/h)0.041.04.0
Delay constant (1/h)4
Drying time (day)7
Table
Table 6. Hydraulic and water retention characteristics of the selected extensive substrate [66,67].
Table 6. Hydraulic and water retention characteristics of the selected extensive substrate [66,67].
CharacteristicsValue
Extensive substrate
Saturated hydraulic conductivity (cm/min)4.8
Total porosity (m3/m3)0.464
Water field capacity (m3/m3)0.376
Plants wilting point (m3/m3)0.031
Hydraulic conductivity slope (-)41.8
Surface layer
Berm height (mm)50
Vegetation volume fraction0.05 for grass on flat roofs
0.01 for vegetation on sloped roofs
Surface roughness Manning’s coefficient (s/m1/3)0.24 for grass on flat roofs
0.1 for vegetation on sloped roofs
Surface slope (%)5% for flat roofs and
variable (20–30%), depending on roof construction
Drainage layer
Thickness (mm)25
Void fraction (-)0.3
Roughness Manning’s coefficient (s/m1/3)0.2
Table
Table 7. Descriptive statistics of calculated outflow hydrographs determined for three tested rainfall events and all variants of green roof application.
Table 7. Descriptive statistics of calculated outflow hydrographs determined for three tested rainfall events and all variants of green roof application.
Rainfall EventVariantMean Flow (dm3/s)Peak Flow (dm3/s)VarianceSD (dm3/s)Peak Flow Reduction (%)
No 10 (no green roofs)22.84101.05887.7529.80
122.4099.03848.7329.132.00%
222.2499.03841.8929.022.00%
321.7199.57823.2928.691.46%
No 20 (no green roofs)3.3757.6376.818.76
13.0956.6670.488.391.68%
22.9954.7765.908.124.96%
32.6647.9451.117.1516.81%
No 30 (no green roofs)13.98458.164446.4166.68
112.23436.183518.2359.314.80%
211.94428.233355.7857.936.53%
311.47418.13173.2156.338.74%
Table
Table 8. Modeled values of rainwater outflow volume from the studied catchment and percentage outflow volume reduction for tested variants of green roof application.
Table 8. Modeled values of rainwater outflow volume from the studied catchment and percentage outflow volume reduction for tested variants of green roof application.
VariantRainwater Outflow Volume (m3)
Rainfall No. 1Rainfall No. 2Rainfall No. 3
0 (no green roofs)1705.47224.66362.40
11673.60206.78331.70
21662.29200.34323.84
31623.47179.38297.81
Outflow volume reduction (%)
11.98.08.5
22.510.810.6
35.125.221.7
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Widomski, M.K.; Musz-Pomorska, A.; Gołębiowska, J. Hydrologic Effectiveness and Economic Efficiency of Green Architecture in Selected Urbanized Catchment. Land 2023, 12, 1312. https://doi.org/10.3390/land12071312

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Widomski MK, Musz-Pomorska A, Gołębiowska J. Hydrologic Effectiveness and Economic Efficiency of Green Architecture in Selected Urbanized Catchment. Land. 2023; 12(7):1312. https://doi.org/10.3390/land12071312

Chicago/Turabian Style

Widomski, Marcin K., Anna Musz-Pomorska, and Justyna Gołębiowska. 2023. "Hydrologic Effectiveness and Economic Efficiency of Green Architecture in Selected Urbanized Catchment" Land 12, no. 7: 1312. https://doi.org/10.3390/land12071312

APA Style

Widomski, M. K., Musz-Pomorska, A., & Gołębiowska, J. (2023). Hydrologic Effectiveness and Economic Efficiency of Green Architecture in Selected Urbanized Catchment. Land, 12(7), 1312. https://doi.org/10.3390/land12071312

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