City greening by rain gardens - costs and benefits

Category	Articles
Sub-Categories	n/a
Year	2018
Author(s)	Ewelina Siwiec Anne Maren Erlandsen Haakon Vennemo
	https://www.researchgate.net/publication/325382664_City_Greening_by_Rain_Gardens_-_Costs_and_Benefits
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Environmental Protection and Natural Resources
Ochrona Środowiska i Zasobów Naturalnych Vol. 29 No 1(75): 1-5
DOI 10.2478/oszn-2018-0001	
1	
Ewelina Siwiec*, Anne Maren Erlandsen**, Haakon Vennemo***
City greening by rain gardens - costs and benefits	
* Institute of Environmental Protection – National Research Institute, Poland,
** Vista Analyse,
*** Vista Analyse and Business School, Oslo Metropolitan University; 
 e-mail: ewelina.siwiec@ios.gov.pl
Keywords: 
green areas, rain gardens, adaptation to climate change, costs, benefits, Poland
Abstract
In urban areas, the most pressing adaptation challenges are associated with the problem of poor retention of 
rainstorm water. Hence, recommended actions focus on improving rainwater retention in the landscape. One of 
the strategic direction of adaptation to climate change is including green infrastructure solutions like green roofs, 
green walls, water reservoirs along streets – supplying high vegetation, permeable ground cover and rain gardens. 
Due to a variety of proposals, the costs and benefits resulting from the implementation of a chosen solution should 
be regarded when selecting an appropriate action. Considering the above, the article attempts to show the costs 
and benefits of one of the solutions – a green garden. When analysing the cost of green garden implementation, 
account should be taken of the cost of investment and upholding, as well as the alternative cost of land use. Then 
again, the benefits of the rain garden comprise the losses avoided by limiting the effects of rainstorm as well as 
improving the quality and quantity of water in the urban landscape. The cost and benefit monetization makes it 
possible to decide on the financial viability of implementing rain gardens in the city.
© IOŚ-PIB	
1. IntroduC tIon
Cities are particularly sensitive to climate change. For 
example, in Poland, more than 60% of the country’s 
population live in cities [Podręcznik adaptacji dla miast 
– Urban Adaptation Handbook, 2015] The most pressing 
adaptation challenges in cities are associated with problems, 
such as the urban heat island and low retention of rainstorm 
water. Urban areas are flooded as a result of their progressive 
development [Bolund, Hunhammar 1999]. Inadequate 
drainage systems are not sufficient enough to receive 
all runoff after intense raining or snow melting. Besides, 
reduction of ground permeability impedes groundwater 
supply, and this lowers the groundwater table level, followed 
by water shortages. Another issue associated with city 
water management is limited biological water purification 
on account of removal of biogenic compounds by plants 
[Kosmala, 2003]. Recognizing the severity of climate change 
impacts, Poland’s five medium size cities, that is, Nowy Sącz, 
Tomaszów Mazowiecki, Ostrołęka, Siedlce and Bełchatów, 
joined the project Climcities, in the framework of which, 
draft climate change adaptation strategies were prepared 
for the above partner cities.	
1	
 1 The cities of Ostrołęka, Bełchatów, Nowy Sącz, Tomaszów 
 Mazowiecki and Siedlce are partners of the Institute of Environmental
 Protection – National Research Institute in the project ‘Climate change	
Recommended adaptation measures for the cities focus 
on improving rainwater retention by including green 
infrastructure solutions. These solutions include green 
roofs, green walls, small water reservoirs along streets – 
supplying high vegetation, permeable ground cover and 
rain gardens. Due to a variety of proposals, the choice 
of the right design solution should take into account 
both its costs and the benefits of its implementation. 
Considering the above, the article deals with the issues of 
costs and benefits of the design solution such as the rain 
garden, proposed in climate change adaptation strategies 
prepared for the partner cities.
 
2. GrEEn ArEAS In CItIES
Green areas improve the life comfort of city residents 
directly and indirectly. Plants uptake water from the soil, 
transport it up to their branches and leaves, and then release 
in the process of evapotranspiration [Suchocka 2011]. As a 
 adaptation in small and medium size cities’, co-financed by the EEA
 Financial Mechanism 2009–2014, under the Bilateral Cooperation
 Fund, and realized by the Institute in cooperation with the Norwegian
 Partner Vista Analyse, based on the agreement with the National Fund
for Environmental Protection and Water Management.

2
result, plants become temporary water reservoirs involved 
in slow water release. According to the Federal Interagency 
Stream Restoration Working Group (FISRWG), in urban areas 
with deficient vegetation cover and limited permeable 
areas, just about 55% of precipitation is drained by the 
rainwater drainage system.	
2 In contrast, within the city areas 
with natural ground surface cover, approximately 10% of 
precipitation is discharged into the drainage. The remaining 
rainfall water is absorbed by the soil or is evaporated by 
vegetation. Accordingly, urban greenery reduces the effects 
of rainstorms and flooding. Furthermore, green areas 
improve thermal comfort during hot days and increase air 
humidity, which helps to alleviate effects of the urban heat 
island. In periods of heat wave, temperature in suburban 
forests can be as much as 12°C lower, when compared to 
the city centre [Wagner et al. 2013]. Consequently, urban 
greenery not only prevents flooding, but also mitigates 
microclimate and improves the life comfort of urban society.
The most commonly used indicator to assess a level of 
greening is the share of parks, and other green areas in 
the city area. Among the partner cities in the framework of 
Climcities, the highest share of parks and urban green areas 
is in the city of Ostrołęka (index value 4.3%), and the lowest 
– in Nowy Sącz (1.6%). In the period from 2014 to 2016, the 
index value increased slightly in the case of Nowy Sącz and 
Tomaszow Mazowiecki, while in Ostrołęka and Bełchatów, 
the index value decreased. A detailed comparison of the 
index values for partner cities is presented in Table 1.
In Poland, according to MojaPolis and BIQdata (2015), the 
most green city is Chorzów with the highest greenery index 
of 22.2%. The next in the order are the cities Siemianowice 
Śląskie with the index value 9% and Bydgoszcz having the 
index value over 8%. The low index values obtained for 
urban green areas and data on further damage caused 
by rainstorms and flooding indicate that the potential of 
green areas is not fully utilized in the partner cities.
Green measures can reduce the pressure on the drainage 
system in cities by infiltrating or retaining water during the 
peak of rainstorms. The suitability of the different measures 
will depend on the local conditions of the area. Firstly, the 
measures vary in their capacity to infiltrate/retain runoff 
and so, it is necessary to analyse the extra capacity needed 
in the area to reduce the risk of future floods. Secondly, 
the costs of implementing different measures vary with 
conditions such as the type of ground cover and terrain, 
the value of the land, and the characteristics of the built 
environment in the area. Green measures comprise green 
roofs, open streams, ponds, wetlands as well as rain gardens	
3. r AIn GArdEnS AS A WAY oF GrEEnInG 
 CItIES
The rain garden is an area planted with native plant species, 
located in the ground or in a container, most often in the 
 2 Based on the assumption that there is 75–100% impermeable ground cover. vicinity of storm sewers [Długozima 2009, Śmietańska 
2012, Domanowska, Kostecki 2015]. Properly selected 
plants eliminate pollutants from rainwater flowing down 
from city streets, squares and roofs [Śmietańska 2012] and 
limit the discharge of water from impermeable surfaces to 
the sewage system [Domanowska Kostecki 2015]. These 
also increase water retention, which is of great importance 
in the context of adaptation to climate change and the 
reduction of damage caused by intense rains. Below photo 
one of the first rain garden in Poland (Photo 1).
Rain gardens are also characterized by a specially designed 
structure of the substrate (medium) filtrating rainwater, 
which holds back runoff flow into the drainage system and 
gradually infiltrates water to the ground. According to the 
Low Density Residential Bioretention, rain gardens retain 
in the ground about 30% more water than lawns [Kosmala 
2003].	
4. r
AIn GArdEn CoSt S And BEnEFIt S
Adaptive solutions such as rain gardens should be viewed 
in the context of costs and benefits of their implementation. 
A detailed summary of costs and benefits is presented in 
Table 2.
The cost of implementing a rain garden depends 
on several factors, including its size, soil conditions, 
location, design, time of implementation and the types 
of plants and materials used. The costs of rain gardens 
can be categorised into investment costs, operation and 
maintenance costs, and the opportunity cost of land use. 
Information about costs of rain gardens in Poland is limited. 
According to the Fundacja Sendzimira, the total cost of rain 
gardens is about 2,000–2,500 PLN per 2 m	
2. Nevertheless, 
for specific installations of rain gardens being considered, 
the responsible firm should be able to provide an estimate 
of the costs required for the investment and operation/ t able 1. The share of parks, green areas and green spaces 
in the Climcities partner cities in %.	
City 201420152016
Nowy Sącz 1.61.91.9
Siedlce 3.33.13.3
Tomaszów Mazowiecki 2.42.52.6
Ostrołęka 4.34.34.1
Bełchatów 2.42.32.2
Source: Own development; based on the Local Data Bank
Ewelina Siwiec et al.

3	
maintenance of their proposed rain garden. According 
to the Minnesota Stormwater Manual, the surface area of 
a rain garden should be 5–10% of the area from which it 
infiltrates stormwater runoff [MPCA 2008]. In addition to 
the size of the runoff-area, the infiltration capacity of the 
filtering layer, the intensity and duration of precipitation 
and local requirements to the maximum allowed amount 
of runoff into the sewerage system or streams determine 
the appropriate size of a garden bed [Paus and Braskerud 
2013]. Rain gardens that require drainage are generally 
more expensive than rain gardens where the soil at the 
location can be used as filter medium. This will depend 
on the ability of the local soil to infiltrate water. Clay soil 
is generally unsuited for infiltration and rain gardens 
implemented in the areas with clay soils will always need 
drainage [Paus and Braskerud 2013]. Costs related to the 
operation and maintenance of rain gardens depend on 
a range of different factors, for example, a necessity to 
replace the soil underneath and the type of vegetation 
used.
Upkeeping the rain garden is important after it has been 
created to ensure that vegetation is established. Once 
vegetation is established, the need for maintenance is 
similar to that of parks with watering during dry periods, 
mechanical weed control and fertilisation as needed. Slurry on the filter surface may also need to be removed 
[Vista Analyse 2015]. It follows that if the rain garden 
is implemented in a park it would be reasonable to 
assume that the rain garden would not require additional 
operation and maintenance cost above what is needed to 
maintain the park.
The implementation of a rain garden will require some 
area of land. This has a cost because the area could be used 
for something else if the rain garden is not implemented. 
A relevant question then is: What is the value of this area of 
land in its best alternative use? To answer this question in 
practice, one can generally look at what the area would have 
been used for in the absence of the rain garden. If the rain 
garden was implemented on a vacant block that otherwise 
could have been used to build properties, a reasonable 
measure of the opportunity cost of the land use could be 
the average land value (per m	
2) in the area. However, if the 
rain garden is implemented in a park where the current use 
of land is very similar to that of the rain garden (vegetative 
cover), one could assume that there is no opportunity 
cost incurred from the implementation of the rain garden. 
An important benefit of investing in the rain garden is 
the avoided damage from future floods. The expected 
avoided damage is calculated as the difference between 
the expected damage with and without adaptation. The 
alternative scenario (rain garden implementation) would 
presumably have lower costs associated with damage from 
floods, compared to the baseline scenario, depending on 
the capacity of the rain garden to absorb and delay the 
flow of water, and this translates into flood risk reduction. 
Additionally, the potential damage from floods would also 
depend on the rain garden’s ability to avoid contamination 
of water bodies. The difference in damage cost between 
the alternative scenario and the baseline scenario may 
amount to the full amount or a portion of the damage 
cost. The below sections describe factors impacting a 
rain garden’s ability to control the quantity and quality of 
stormwater runoff. A rain garden’s ability to control the 
quantity of water depends on its capacity to infiltrate water 
as well as its ability to delay the flow of water entering the 
drainage system. A rain garden with underdrain – leading 
water to the drainage system – infiltrates less water than 
a rain garden with no underdrain [MPCA 2008]. The latter 
allows water to infiltrate to the ground and is also better 
suited to restore groundwater supplies. Whether drainage 
is necessary depends on the properties of the local soil. For 
example, drainage is needed in the areas with clay soil. 
The capacity of a rain garden to infiltrate water is a complex 
process influenced by the initial water content of the 
bioretention media, the capillary suction, and the pressure 
head from the ponded water [Paus et al. 2016].	
 They find 
that the infiltration capacity can be reasonably predicted, 
based on the following parameters:
–	 Surface area
–	 The value for saturated hydraulic conductivity of the 
of the bioretention media
–	 The maximum level of water on the surface	
Photo 1. One of the first rain garden in Poland (Marki).
Source: Fundacja Sendzimira
table 2. Rain garden costs and benefits.
Costs Benefits
•	 Investment costs
•	 Operation and 
maintenance cost
•	 Opportunity cost of 
land use •	
Avoided damage
•	 Water quality 
control
•	 Water quantity 
control
•	 Co-benefits 
Source: Own development
City greening by rain gardens - costs and benefits

4
Higher levels of the above factors would result in a higher 
fraction of runoff infiltrated. Also, site-specific properties 
such as catchment size and slope, surface types, and the 
time of concentration have large impacts on the infiltration 
capacity [Paus et al. 2016].	
 Stormwater runoff 
carries various pollutants such as oil, fertilisers, pesticides, 
sediment and chemicals [Riverlink no date]. If left 
untreated, stormwater runoff could contaminate streams, 
rivers and lakes as well as potentially affect recreational 
activities, such as swimming and fishing, injure aquatic 
plants and animals and contaminate drinking water. 
A rain garden can improve the water quality by filtering 
pollutants from stormwater runoff. Particles in stormwater 
are withheld via sedimentation on the surface of the rain 
garden and filtration through a filter medium. The effect 
from sedimentation can be increased by ensuring slow 
inflows of water into the rain garden. The filter medium’s 
ability to filter pollutants depends largely on its grain 
distribution properties. The properties of the filter medium 
also determine the type of pollutants that can be withheld 
[Paus 2016]. According to Paus [2016], a rain garden could 
reduce:
–	 particles by around 90%
–	 oil by around 90 %
–	 dissolved heavy metals (Cd. Cu. Pb. Zn) by around 80–
90%
–	 PAHs (Polycyclic Aromatic Hydrocarbons) by around 
70–90%
–	 phosphate by around 63%
The benefits of the reduction of pollutants depend on 
where these pollutants would have ended up in the 
absence of the rain garden and the impact this could have 
on humans and aquatic life. The plants and vegetation of rain gardens can provide 
ancillary benefits (co-benefits) in addition to the primary 
purposes of stormwater quantity and quality control. 
These co-benefits include;
– 
noise reduction
– improved air quality
– CO2 capture and storage
– local climate control
– biodiversity
– pollination
– visual amenity
– restoration of groundwater supply
The degree to which these co-benefits are realized, and 
their significance will depend on several factors, including 
the size of green area provided by the rain gardens.	
5. SuMMAr Y And ConCL uSIonS
The potential of green areas in the cities is not fully utilized. 
Therefore, the recommended adaptation measures focus 
on expanding urban green areas through a variety of 
design solutions (green roofs, green walls, water retention 
reservoirs, etc.). The rain garden gaining popularity 
worldwide is one of the solutions. When analysing rain 
garden implementation costs, account should be taken 
of investment and maintenance costs, as well as the cost 
of alternative land use. On the other hand, however, the 
benefits of the implementation of rain gardens include 
losses avoided by limiting the effects of rainstorm and 
improving the quality and quantity of water in the 
urban landscape. The cost and benefit monetization 
makes it possible to decide on the financial viability of 
implementing rain gardens in urban area.	
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City greening by rain gardens - costs and benefits