1 Introduction

Urbanization, which is a universal trend, negatively affects the local water cycle, since it inevitably leads to sealing of a large percentage of the soil. The impervious surfaces (houses, conventional pavements, road surfaces, etc.) favor surface run-off of rainwater over infiltration and evapotranspiration. Moreover, new direct and indirect pollution sources appear (Tsihrintzis and Hamid 1997), together with urban heat island effects. Additionally, in many cases, trespassing has led to reduction of urban stream areas, while in others, poorly planned development has resulted in complete substitution of urban stream sections by underground closed conduits. The situation is usually worse in densely built areas, which suffer from lack of green and open spaces, and are consequently aesthetically and socially degraded (Mumm et al. 2022). Climate change, rendering extreme events, such as heavy rains, more frequent and more intense, aggravates the rainwater management problems (Willems et al. 2012).

The conventional way to handle rainwater in urban areas is through sewer networks, which can carry it to sewage treatment facilities or to surface water bodies (lakes, rivers, the sea). The usefulness of these networks is undeniable. However, they are often not sufficient to fully protect the cities, and their improvement is costly (Butler et al. 2018).

Integrated rainwater management combines the aforementioned basic approach with ecological (or low impact) techniques, such as rain gardens, green roofs and permeable pavements, which are also known as Sustainable Drainage Systems (SuDS). Such techniques have already been used in very different areas of the world (Sadeghi et al. 2019; Palermo et al. 2023; Salamanca et al. 2023). It is worth mentioning that integration of SuDS into urban drainage systems can lead to net financial benefit, due to possible decrease of sewer network upgrading and maintenance costs, and of sewage treatment facility operational costs, which are related to rainwater runoff peak and total volume (Nowogoński 2021). Moreover, rain gardens and other low impact techniques contribute to the environmental and aesthetic upgrading of their surroundings (NY State Stormwater Management Design Manual 2015).

In densely built areas, the need to upgrade available free spaces, even small ones, to benefit the residents, is imperative. This can be achieved by transforming them to “pocket parks”, namely to small-scale green spaces, which are created in irregular pieces of land between the building blocks and the road network of an area. Pocket parks make the most of public space and change the daily life of citizens for the better (Abd El Aziz 2015; Armato 2017).

Building on previous work (Bagiouk et al. 2022), we highlight a proposal, which aims to transform a cemented space in a densely built neighborhood of Thessaloniki, Greece, into a new pocket park, that can serve as a small green lung and a new meeting and interaction point for residents. In addition, it will combine the aforementioned socio-environmental gains with mitigation of flood phenomena, through ecological rainwater management techniques. In this paper, we focus more on financial issues, on design options and on technical details, stressing features and conclusions of general applicability.

2 The Study Area

Thessaloniki, which was founded in 315 B.C., is currently a major port of southeastern Europe, the second largest city in Greece, the capital of Central Macedonia Region and home of three Universities. Its location is marked on the map of Fig. 1 (left) with a red circle. Its historical center, which is sloping towards the sea, is densely built.

Fig. 1
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Left: Thessaloniki (red circle) in Southeastern Europe; Right: The greater area of Thessaloniki (based on Google Earth)

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The site under consideration is located at the junction of L. Iasonidou and Evripidou streets, inside the city’s historical center. An aerial view of the wider area is shown in Fig. 2; it is characterized by impermeable surfaces, narrow sidewalks and relatively steep ground slope. All these result in street inundation by rain runoff, even during rainfalls of moderate intensity.

The specific lot belongs to the Municipality of Thessaloniki. Its shape is triangular and its area is about 180 m2. It abuts two apartment buildings on one side, while its limits on the other two are the sidewalks of L. Iasonidou and Evripidou streets. Most of its surface is covered with impermeable materials, mainly concrete. The only green elements are 5 trees, which are not in good condition. As it remains unused, waste and debris are often deposited on it. Moreover, its space is encroached upon by cars and two-wheelers for parking. In the photo of Fig. 3, some of the problems mentioned can be observed, as well as the general poor condition of the place.

Fig. 2
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(source: Google Earth)

Aerial view of the study area.

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Fig. 3
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Situation of the studied lot

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3 Application

The proposed pocket park will upgrade the neglected piece of urban land described in the previous section. As shown in the plan of Fig. 4, it will include a zone of approximately 3.0 m in its upslope part, covered with impermeable material, to protect the basements of the adjacent buildings. A zone of permeable materials will follow, while the rain garden, with an area of ​​approximately 80 m2, will be located in its lower part. Moreover, suitable urban equipment will be placed, to facilitate and attract visitors, as discussed in Sect. 3.4. Thus, it will contribute to the environmental, social, economic, and aesthetic rehabilitation of the neighborhood, which is located at a medium to low-income area of the city center (namely at a city area in need of rehabilitation and able to sustain it). It will also contribute to the mitigation of the disturbance and/or damage caused by rains, even of medium intensity.

Fig. 4
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Plan of the proposed pocket park

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3.1 Contribution to Ecological Rainwater Management

As mentioned above, a special feature of the proposed pocket park, which differentiates it from others of its kind, is the integration of ecological rainwater management techniques in its design, namely the rain garden and the permeable surfaces. Combining SuDS is generally considered beneficial for storm water management (Butler et al. 2018; Ferrans and Temprano 2022). However, in our case, the main reason for choosing the aforementioned combination is to create a small green oasis, i.e., the rain garden, which is easily accessible by pedestrians through the permeable surface part, while exploiting almost all of the park’s surface for rainwater infiltration.

3.2 Rain Gardens

Rain gardens appeared, or rather were described and scientifically analyzed for the first time, during the nineties in the United States. They can be briefly described as “enhanced” gardens that can collect rain runoff from their surrounding area, including adjacent buildings. To this end, they are equipped with suitable inflow structures, which lead rainwater to the garden’s ponding area; they are also equipped with an outflow structure, to allow flow of water surplus towards the local drainage system (Bray et al. 2012). So, they contribute to the reduction of total volume and of peak rainwater runoff through temporary storage, infiltration and evapotranspiration (Osheen and Singh 2019; Ouédraogo et al. 2022) and to the improvement of its quality through various physical, chemical or biological processes (adsorption, filtration, biological degradation, oxidation, etc.) that reduce the pollutant load (Zhang et al. 2020; Sharma and Malaviya 2021).

Rain gardens, of different size and shape, have been already applied in many parts of the world with different climate conditions, e.g., Poland (Burszta-Adamiak et al. 2023), Serbia (Greksa et al. 2023), India (Shreewatsav and Sheriff 2022), Japan (Zhang et al. 2020), China (Zhang et al. 2023). It is worth mentioning that efficiency of rain gardens depends on the features of each selected location; nevertheless, they have been applied successfully even to locations with not favorable characteristics, e.g., soils with low permeability (Chen et al. 2023). In Thessaloniki, Greece, many sites suitable for rain garden construction have been spotted (Katsifarakis et al. 2015).

3.2.1 Basic Design Features of the Rain Garden

Following the simplest configuration for such installations, the proposed rain garden will include a vegetated ponding basin, inflow and outflow structures. Its area will be approximately 80 m2 and it will receive rainwater runoff from the upslope surface of the park, the two adjacent buildings and from Evripidou Street, which has the steeper slope.

Inlet structures to channel street runoff to the rain garden should combine simplicity with safety for the pedestrians. Since the street is narrow, we propose two inlets, in the form of curb openings, which will not interrupt the sidewalk. One of the inlets should be constructed upslope of the upper edge of the rain garden, to allow for gravity flow of diverted runoff towards the whole garden ponding area. The sketch of an inlet is shown in Fig. 5. The small threshold along it aims at diverting very small flows from the rain garden towards the sewer system. The combination of the inlet with a tree-box, suggested by Basdeki et al. (2017) for another location, could be used only in the framework of a more radical and expensive rehabilitation of the broader neighborhood, requiring, among others, local traffic adjustments. It should be mentioned, though, that the proposed works can be integrated into the more expensive rehabilitation project, if it is implemented at a later stage. Actually, they will facilitate its application, since: (a) the required additional funds will be lower; and (b) the expected additional benefits will be more obvious.

Fig. 5
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Sketch of the inlet structure (side view)

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Moreover, the rain garden will receive part of the runoff from the two apartment buildings, with which it borders. This requires reconstruction of the respective gutters, which end currently on the sidewalks of Evripidou and L. Iasonidou streets, adding to the nuisance of the pedestrians. As the two building walls towards the park do not have balconies or any other elements, the system of new gutters could contribute, if properly designed, to the aesthetic upgrade of the area. The main constraint in such cases, is lack of funds, as the building owners may not be able (or eager) to spend money for such “luxurious” interventions, while public spending on private property is bureaucratically cumbersome.

The shape of the garden’s ponding basin is shown in Fig. 4. It will be fenced by a low metal railing, which will not hinder surface rainwater runoff. The characteristics of the soil layers within it are summarized in Table 1, while a sketch of a typical section is shown in Fig. 6. Based on this preliminary design, the storage capacity of the catchment is a function of the following terms:

  • the active porosity of the gravel substrate.

  • the active porosity of the soil material (at this preliminary stage, an approximate and rather conservative value for sandy soil is used).

  • the useful storage space created by the surface level difference of the soil material and the perimetric sidewalk level.

Table 1 Characteristics of rain garden layers
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Fig. 6
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Cross-sectional sketch of rain garden layers

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Taking into account the above characteristics, the volume of water (Sk), that can be stored in each square meter of the rain garden, is calculated using the following equation:

$${S_K} = \sum {b_i} \cdot {n_i} = 0.2 + 0.5 \times 0.2 + 0.3 \times 0.3 = 0.39{\text{ }}{m^3}/{m^2}$$
(1)

Furthermore, with the rain garden area designed at 80 m2, the total volume of rainwater (VTOT) that can be retained by the proposed rain garden is calculated by the following formula:

$${V_{TOT}} = 80 \times {S_K} = 80 \times 0.39 = 31.2{\text{ }}{m^3}$$
(2)

3.2.2 Rain Garden Plant Selection

The selection of rain garden plants is generally based on the following parameters (NY State Stormwater Management Design Manual 2015):

  1. 1.

    The climatic and soil conditions that prevail in the area.

  2. 2.

    The functional and aesthetic needs.

  3. 3.

    The plant maintenance requirements and adaptation to the local conditions. According to this criterion, endemic plants that do not require special maintenance, should be preferred.

  4. 4.

    The plant resistance to temporary flooding.

  5. 5.

    The plant ability to retain rainwater and direct it to the underground.

Rain gardens are not wetlands, as they are designed to drain in 48 h. Due to the large variation of rainfall in Greece, suitable plants should be drought-resistant and able to withstand short periods of flooding, as well. Based on our research, we have compiled a list of suitable plants, which are classified in three categories, as shown in Table 2. This list can serve as guide for plant selection in many North Mediterranean regions, or other areas with similar climatic conditions. Despite the small area of the rain garden, polyculture planting would be advantageous, both from the hydraulic (Morash et al. 2019) and the aesthetic point of view.

The arrangement of the selected plants should be based on their tolerance to moisture. Moisture tolerant plants should be planted at the deepest part of the rain garden (zone B), which retains water for longer periods of time, while dry soil-resistant plants should be used near the garden’s border (zone A). Installation of a surface drip irrigation system throughout the planted area is recommended, to water the plants in dry periods. The five existing trees (Koelreuteria paniculata) can be preserved and integrated to the new garden, since they have survived at that location for some years already.

Table 2 Recommended plant species by category
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3.3 Permeable Surfaces

Another feature of the wider intervention and regeneration is the construction of a zone with permeable surface (Sambito et al. 2021; Kourtis et al. 2021), with an area of ​​approximately 37 m2. The materials that will be used are porous concrete elements. Appropriate wide joints will be left between them, which will be filled with sandy soil and grass. Both the joints and the concrete elements will allow rainwater to infiltrate through them and to be stored in the underlying soil layers. Assuming that the width of these layers is 1.0 m and their active porosity is 0.20 (equal to that of the sandy soil layer underlying the rain garden), the total rainwater volume (VTOT1) that could be temporarily stored is given by the following formula:

$${V_{TOT1}} = 37.0 \times 1.0 \times 0.2 = 7.4{\text{ }}{m^3}$$
(3)

As mentioned in Sect. 3.1, the main asset of this zone is that it will facilitate the use of the park by its visitors, while contributing to the rain runoff management. Rainwater that will reach its surface will either be retained or end up to the rain garden, located downslope.

3.4 Urban Equipment Placement

Along the sidewalks and the permeable pavement zone, the placement of micro-trash bins and seating-benches is proposed to attract and serve the visitors. Following an international trend (and the example of some other Greek Municipalities, e.g., in Crete and Attica), some of these benches could be “smart”, i.e., equipped with solar panels, so that they have smart lighting and support the charging of electronic devices with USB ports, as well as internet connection.

In Fig. 7, a three-dimensional photorealistic image is presented to highlight the proposed application in its entirety and its main features.

Fig. 7
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Axonometric photorealistic image of the proposed pocket park

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3.5 Expected Benefits

Thanks to the temporary storage of rainwater in the vegetated ponding area of the proposed rain garden and the contribution of the permeable pavement zone, the peak flow towards the existing sewage network will be reduced. Moreover, the nuisance from the rapid overland rainwater runoff along Evripidou Street, will be less intense.

4 Cost Analysis

Cost-effectiveness is a major prerequisite for the application of SuDS (Chui et al. 2016; Singh et al. 2020). To check whether the construction of pocket parks integrating ecological rainwater management techniques is a feasible proposal, a typical cost estimate has been performed. Results are summarized in Table 3, which includes unit prices per work, together with the required quantities and the total cost (per work).

Table 3 Project cost analysis
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In comparison, the construction cost for 50 m of a typical storm sewer network in Greece is about 27,000 €, excluding VAT, unexpected cost items and contractor’s profit. While the price values of Table 3 may vary from country to country, the comparison with the sewer network construction cost leads to the more general conclusion that the construction of pocket parks that are equipped with SuDS is, comparatively, cost-efficient.

5 Conclusions

Pocket parks, such as the one presented, can contribute to the upgrading of densely built areas, where the problem is not only the lack of free spaces, but also the poor condition of some of the existing ones.

We have pointed out that supplementing these parks with SuDS, in particular rain gardens and permeable pavements, can be an efficient way to combine aesthetic and social upgrading of neglected urban areas with mitigation of the nuisance and damages caused by rainfall events.

The technical solutions (such as the inlet structures discussed in this paper) can be adapted in densely built areas of any city, while the list of plants can serve as a guide for rain garden construction in other North Mediterranean regions (or other areas with similar climatic conditions). Moreover, our cost analysis shows that SuDS are cost-efficient, compared to sewer network construction cost. Finally, their design and cost could be adjusted to the available funds, while step improvements are possible.