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NATURE BASED
SOLUTIONS
Using rainwater as a resource to
create resilient and liveable cities
INSIDE THIS WHITE PAPER
Managing rainwater with
Nature Based Solutions
Cross-disciplinary collaboration
in climate adaptation
Urban recirculation
of rainwater
2
Nature Based Solutions
Using rainwater as a resource to create resilient and liveable cities
Version 2.0
February 2021
Front page photo
Climate adaptation at Taasinge Plads in Copenhagen
Photo: Kontraframe
Editor in Chief
State of Green Malene Bering Beitzel, mbb@stateofgreen.com
Technical Editors
Water Vision Denmark / KLAR Utility Hanne Kjær Jørgensen, hkj@klarforsyning.dk
Danish Technological Institute / DNNK, Danish Network for Climate Adaptation Ulrik Hindsberger, uhi@teknologisk.dk
Contributors
1:1 Landskab Jacob Kamp, jacob@1til1landskab.dk
Amphi Consult Lars Briggs, lb@amphi.dk
Aarhus Vand Anne Laustsen, Anne.Laustsen@aarhusvand.dk
Aarhus Vand Pia Jacobsen, Pia.Jacobsen@aarhusvand.dk
BOGL Dorte Hermann, DH@bogl.dk
Byggros Louise Hansen, loh@byggros.com
City of Copenhagen Lykke Leonardsen, lykleo@kk.dk
City of Copenhagen (Skt. Kjelds Kvarter) René Sommer Lindsay, ZT2J@tmf.kk.dk
DHI Morten Just Kjølby, mjk@dhigroup.com
HOFOR - Greater Copenhagen Water Utility Martin Vester, mves@hofor.dk
LYTT Architecture Jacob Fischer, jf@lytt.dk
NCC Brask Michael, MCB@ncc.dk
NIELS LÜTZEN landskabsarkitekter Niels Lützen, nl@nl-landskab.dk
NIRAS Jan Jeppesen, JANJ@NIRAS.DK
NIRAS Jens Brandt Bering, JENS@NIRAS.DK
NIRAS Rikke Juul Monberg, RJMO@NIRAS.DK
Novafos Tina Otterstrøm Jensen, toj@novafos.dk
Pilebyg Vibe Gro Falk, vibegro@pilebyg.dk
Ramboll Martin Zoffmann, MAZO@ramboll.dk
SLA Kristoffer Holm Pedersen, khp@sla.dk
The Climate City Bjarne Rasmussen. Bjarne.Rasmussen@middelfart.dk
THIRD NATURE Flemming Rafn, fr@tredjenatur.dk
University of Copenhagen Marina Bergen Jensen, mbj@ign.ku.dk
Watercare Kristian Bruun Nielsen, kristian@watercare.dk
Wavin Henning Nykjær Stabell, henning.stabell@wavin.com
WSP Jesper Fog, Jesper.Fog@wsp.com
WSP Mads Poulsen, Mads.Poulsen@wsp.com
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Executive summary 3 EXECUTIVE SUMMARY Population growth, increasingly polluted drinking water, floodings and more frequent and longer periods of droughts all encourage an optimised use of rainwater in cities. Climate adaptation seeks to lower the risks posed by the consequences of climate change, including flooding that arises from extreme rain events. However, when approached holistically it can also be used to create synergies between a number of urban challenges by addressing several areas of urban development at the same time e.g. turning urban areas green, whilst at the same time reducing heat islands. In this respect, Nature Based Solutions (NBS) can play a key role in urban water management and contribute to building cities that are attractive for people to live, work and play in. This white paper features lessons learned from a range of Danish stakeholders within rainwater management and NBS. It is supported by state-of-the art case examples demonstrating how rainwater can be used as a resource to create more resilient and liveable cities. Multifunctional solutions create more resilient and liveable cities A large number of synergies and cost-efficiencies can be achieved when rainwater management is integrated into overall urban planning. Cities around the world are increasingly concerned with becoming more resilient to the effects of climate change, including reducing the risk of flooding and rising temperatures. At the same time, many cities are focusing on securing an ample supply of clean drinking water that can sustain a growing population. Denmark is working towards creating multifunctional solutions which – in addition to managing rainwater and reducing the risk of flooding – also create added value for cities as they contribute to maintaining a sustainable water balance and result in new, attractive recreational areas. Find inspiration for your own NBS projects The content of this white paper is intended as a tool for international stakeholders who wish to gain insight into the many potentials of using Nature Based Solutions, gain an overview of how solutions have been implemented in Denmark and around the world, or who are looking for Danish partners to collaborate with on projects in their own country. We hope you will be inspired.
4 FOREWORD
SHAPING TOMORROW’S CITIES
THROUGH NATURE BASED
SOLUTIONS
“Denmark’s experience shows that climate adaptation and rainwater
management can be turned into an opportunity to create not only
more resilient but also more liveable cities”
Lea Wermelin, Minister for Environment, Denmark
As the climate changes and the number create more climate resilient cities are well Based Solutions can contribute to making
and frequency of rainfall events increases, underway in Denmark where we have had cities not only more resilient to climate
so does the need for intelligent rainwater with a national action plan for a National change but also more attractive places to
management solutions. Like many other Action Plan for a Climate-Proof Denmark live.
countries, Denmark has experienced since 2012. Today, all municipalities must
the consequences of a changing climate have a local climate adaptation plan, which The best examples of Nature Based
first-hand. Over the past decade, we have addresses the specific climate change-re- Solutions succeed in solving more than
witnessed extreme rain events that have lated risks in their area. one problem at the same time. Holistic
caused flooding and damages to homes as solutions, however, require close collabo-
well as infrastructure. Nature Based Solutions ration between authorities, water utilities,
More and more Danish cities and water organizations, private companies and
According to the Fifth Assessment Report utilities are looking into managing rain- individuals. Denmark has a long tradition of
from the IPCC, we can expect even more fre- water as close to the source as possible such collaboration. This has brought us far.
quent and more extreme weather events in and diverting it away from the sewerage We are proud of our results and solutions.
the years to come. Finding new and innova- systems and wastewater treatment plants, And we are eager to share our experiences
tive solutions to managing rainwater in our thereby bringing down the risk of combined and lessons learned with other countries.
cities should therefore be a focal point for sewer overflows. Solutions that manage
cities and water utilities around the world. increasing volumes of rainwater span from Join us in Copenhagen for the IWA
hydraulic models for the planning phase to World Water Congress & Exhibition
Increasing pressure on sewerage solutions for local detention of rainwater Denmark will be hosting the IWA World
systems such as subsurface infiltration beds, green Water Congress & Exhibition in September
Increasing precipitation caused by climate roofs and permeable paving, to drainage 2022. I invite you to join us for a week of in-
change is putting greater pressure on our solutions such as separate sewers for teresting discussions on how we can shape
sewerage systems. However, rather than rainwater and sewage water as well as local our water future together.
simply replacing existing pipes with larger rainwater treatment via roadside infiltra-
ones, benefits can be achieved by focusing tion beds etc. Until then, I hope you will be inspired by the
on more intelligent rainwater management many solutions in this white paper.
which detains the rainwater in existing Multipurpose solutions enrich the city
structures or distributes it to areas where By combining climate adaptation measures
it creates the least damage. Efforts to with exciting urban development, Nature
INDEX 5
INDEX
1. From climate adaptation to green urban development...................................................6
Using rainwater as a resource
2. Managing rainwater with Nature Based Solutions..........................................................8
NBS a tool for climate change adaptation and increasing urban biodiversity
3. Cross-disciplinary collaboration in climate adaptation ................................................10
Creating synergies and saving costs through collaboration between different stakeholders
4. Nature Based Solutions in simulation models................................................................12
Assessing the urban water cycle and the impact of NBS
5. Breaking through the surface..........................................................................................14
Managing rainwater in densely populated areas with impermeable paving
6. Increasing urban biodiversity.........................................................................................16
Creating green corridors and resilient cities by integrating nature into urban life
7. Nature Based Solutions and treatment of runoff ...........................................................18
Improving the water cycle by ensuring an appropriate quality of rainwater runoff
8. Urban recirculation of rainwater.................................................................................... 20
Using rainwater harvesting to create a city in water balance
9. Creating resilient and liveable cities with Nature Based Solutions..............................22
Using rainwater as a resource to create green urban spaces with added benefits
10. The true value of water..................................................................................................27
A Danish perspective on how we can shape our water future
6 1. From climate adaptation to green urban development
1. FROM CLIMATE ADAPTATION TO
GREEN URBAN DEVELOPMENT
Using rainwater as a resource
"Copenhagen is preparing for the changing climate. We want to make sure that investments in flood
prevention also benefit the Copenhageners and their everyday life, therefore citizen involvement
is key. In the Cloudburst Plan, we aim to create spaces that increase urban nature, biodiversity and
create places for citizens to meet, when not being used for water management”
Ninna Hedeager, Mayor of Technical and Environmental Affairs at the City of Copenhagen
Many factors such as population growth, sustainable places to live and thrive in by and developing attractive urban spaces
increasingly polluted drinking water, flood- reducing heat islands, increasing resiliency for people to enjoy become two sides of
ing and more frequent periods of drought to climate change and making water more the same coin. As illustrated by the case
necessitate optimised rainwater usage. sustainable as a whole, by reusing rainwater examples, there are many synergies and
According to World Economic Forum, the for different purposes. cost-efficiencies to be achieved if rainwater
most likely and most impactful global risks management is integrated into the overall
as of 2021 is (amongst others) extreme In this white paper, you will be presented urban planning. The key is to have the right
weather, biodiversity loss and natural with the various possibilities of using tools and models to ensure a correct prior-
resource crises (see illustration below). As rainwater as a resource, as opposed to itisation of efforts and suitably designed
urban environments are expanding and considering it as something that should be rainwater management solutions as well as
more people are living in cities, there is a directed to the sewers. The aim of using involving relevant stakeholders at the right
need to green our cities and make them more rainwater as a resource is partly to reduce time throughout the process.
sustainable, as well as ensure a sustainable the risk of flooding by optimising rainwater
water balance in the city. Appropriate rain- management and partly to contribute to Several benefits can be achieved by using
water management e.g., through Nature creating more green and liveable cities. In rainwater to help cities maintain a sustain-
Based Solutions, can make cities healthy, this sense, adapting to a changing climate able water balance via various treatment
technologies. These ensure the proper
treatment of rainwater, where it infiltrates
into the groundwater aquifers or is dis-
charged into local water environments such
as lakes or streams. Finally, in areas suf-
fering from water scarcity, local rainwater
harvesting and recycling can be a valuable
source of non-potable water, which can be
used for watering plants, flushing toilets or
washing clothes instead of using precious
drinking water.
3.40
Price instability
Impact →
Source: World Economic Forum Global Risks
Perception Survey 2021
Likelihood → 3.28
1. From climate adaptation to green urban development 7
Photo: COAST Studio, Rasmus Hjortshøj
Creating a forest and low ravine for rainwater storage, particular focus on climate and biodiversity. Elevated footbridges
Copenhagen, Denmark and plateaus invite park visitors to take a walk or sit and linger. These
Remiseparken was a hidden, slightly neglected, green gem in constructions also make it possible to explore the forest even after
Copenhagen, located in the middle of a social housing complex it has been flooded by stormwater following a heavy rainfall event.
known as 'Urbanplanen'. The renewal of Remiseparken therefore Rainwater is also collected in a wadi-like trench that runs along the
plays an important role in making Urbanplanen a safer and more at- edge of the park. Combined, these solutions form a large delay basin
tractive area as well as functioning as a green, dual-purpose, climate with a total capacity of 2,000 m3. The different plant species in the
adapted area. forest and the wadi contribute to a high degree of biodiversity and
resilience, and also attract a rich selection of animals, insects and
A small forest of 20 different species of alder trees (Elleskoven) in birds.
Remiseparken delay water during periods of heavy rainfall. The Courtesy: BOGL , Rambøll
forest is a new, natural space in the park, and it is designed with a
Copenhagen Cloudburst Management Plan, Denmark The plan outlines the methods, priorities and measures recom
Copenhagen has experienced a number of extreme rainfall events mended to address climate adaptation, including extreme rainfall
since 2010 and the frequency of these types of events are predict- events. It designates seven water catchment areas and has resulted
ed to increase in the future. As extreme rainfall events present in a catalogue of approximately 300 surface projects that will be im-
enormous challenges (which vary from area to area), they cannot plemented over the next 20 years. When prioritising which projects
be solved by a single initiative such as upgrading the sewerage to initiate, the city considers factors such as the hydraulic aspects in
system. For this reason, the City of Copenhagen decided to co- establishing the sequence of projects, where the risk of flooding is
ordinate and consolidate actions into a Cloudburst Management greatest, where it is easy to start, where other construction work is
Plan that was launched in 2012, and which has been continuously already planned and where synergies with urban development are
updated since. possible.
Courtesy: City of Copenhagen
8 2. Managing rainwater with Nature Based Solutions
2. MANAGING RAINWATER WITH
NATURE BASED SOLUTIONS
NBS a tool for climate change adaptation and increasing urban biodiversity
Nature Based Solutions (NBS) are measures the sewerage system. Runoff from catch- implementation. The product might be
that encapsulate the notion of water as a ments will arrive at the usual bottlenecks subject to a European Standard and re-
resource. NBS support a triple bottom line at different times. By delaying and reducing quired to adhere to certain specifications.
of planet, profit and people, where the aim the maximum runoff, it is possible to reduce Or the producer might need to document
is to heighten urban resiliency. At times, the risk of flooding. the water balance of a new NBS element
NBS solutions are rainwater management, or the permeability of a specific pavement.
inspired by nature’s methods such as perme- Designing NBS to handle different Denmark has more than 30 years of experi-
able pavement and underground storage; at types of rain events ence with testing traditional components in
other times the solution is nature-based el- In Denmark, there is no standard definition sewerage systems and today it is also possi-
ements that support biodiversity. Examples of how to design NBS. In practice, the rec- ble to test new climate adaptation products
of typical NBS elements are defined on page ommendation is that every time you design in a certified lab, where tests are run in a full
9. an NBS, you need to consider everyday rain, scale setup, using up to 30 l/s. Companies
design storms and cloudbursts (as illustrat- from other countries can also use the lab.
Taking the pressure off the traditional ed in the 3-point approach method below). Learn more about the testing facilities at
sewerage system Often professionals and stakeholders tend www.climateadaptationlab.dk.
Due to the large amount of impermeable to focus on one type of rain event. However,
surfaces present in cities, rainwater runoff there can be numerous problems in an area A tool for adapting to a changing
in a city differs from the runoff pattern that related to the different rain events. It is climate
occurs prior to the urbanisation process. therefore highly important to focus on NBS, when strategically planned, represent
The hyetograph below reveals that urban- all types of rain events when designing a sustainable alternative of a storage/drain-
isation has an impact. The runoff from a solutions. NBS are particularly efficient for age facility compared to traditional rain and
city covered with impermeable paving will solving everyday rain challenges but when stormwater infrastructure. It is expected
result in quick and high runoff. As runoff used carefully, NBS can contribute substan- that more natural structures will drastically
from several catchments arrive at the same tially to solving some of the problems asso- decrease the use of concrete structures and
time to the same places in the sewers, it ciated with heavy rainfall and stormwater. energy demanding technologies. Hence,
creates bottlenecks, heightening the risk of integrating NBS into urban areas not only
flooding. Testing NBS elements to meet has the potential to solve climate change
international standards challenges, but also meet CO2 emission
By viewing Nature Based Solutions as an When developing new or using existing reduction needs, mitigate heat islands and
extension to the traditional sewerage climate adaptation products, there is often increase biodiversity and public health via
system, the aim of NBS is to smooth the a need for full-scale testing, optimisation the creation of additional green areas in the
runoff hyetograph and reduce pressure on and documentation of the product before city.
Event Volume
Depth Handling
Cloudburst events
(mm) (per cent)
50-85 C 100
Urban drainage
design events
Everyday
rain events Pluvial flood
B mitigation 99
30-50
Urban
Rainwater storm
resource drainage
A 80
10-20 utilisation
Return
Period
0.2 10 100 (year)
The 3-point approach
Pinpointing three main domains where decisions related to stormwater management take place.
Examples of typical nbs elements 9
EXAMPLES OF TYPICAL NBS ELEMENTS
Clima ponds
To increase biodiversity, ensuring a permanent Climate roads
water table, climaponds handling stormwater Permeable asphalt is getting more common. The
directly from the downspouts can be estab- stormwater infiltrates through the surface and
lished. The trench in the inlet can be designed, the bearing layers underneath, ensuring the
so the inlet flow is smooth, even throughout water transport through all roadbox.
cloudbursts.
Green roof and walls
Green roofs/walls are roofs covered with a multi-
layer system consisting of: growth medium,
Ditches drainage layer and water-proof membrane that
A ditch is a narrow channel dug in the ground, delay runoff. The degree of delay and volume
typically used for drainage alongside a road or reduction increases with the thickness of the
the edge of a field. growth medium. Green roofs/walls insulate
structures from heat and can provide a habitat
for certain insects and birds. Retained water
evaporates.
Irish crossings
Infiltration from surface
Using Irish crossings, the stormwater is able
Infiltration from surface occurs when discon-
to cross a road, whenever this is needed. The
necting the downspouts and discharging the
trench is shaped as a pre-immersion in the
rainwater on the permeable surface.
asphalt itself.
Permeable pavement
Linear drainage systems
Permeable pavement provides a horizontal
Using linear drainage systems makes it possible
surface suitable for walking or driving with
to transport stormwater visibly just underneath
(heavy) traffic load but also allows rainwater
the road. The linear drainage systems can carry
to infiltrate. The infiltration capacity of the
heavy traffic. It can also be in the form of steel
permeable pavement depends on the design
drain grates in a driveway or conrete drain
and on the hydraulic capacity of the bearing
grates alongside a motorway.
layers underneath.
Soakaway or infiltration trench
Rain gardens A soakaway (dry well, infiltration well) is a pit
A rain garden is a depression in the terrain in the ground, stabilised with a porous material
designed to receive, store and filter runoff wrapped in geotextile and covered with topsoil
from roofs or surfaces and is also designed as a and vegetation. An infiltration trench is a
specially planted area with selected plants that soakaway shaped geometrically like a trench,
can cope with dry and with wet conditions. for example, 60 cm wide, 1 m deep and several
metres long.
Trenches
Swales
Trenches are used for transporting water above
A swale is a rain garden placed in the side of a
ground in places where open trenches do not
road, with a soakaway underneath. Typically,
inconvenience road users. Trenches can be a
the swale also serves as a traffic harassment.
recreational element in an urban landscape.
Underdrains Mix of NBS
An overlooked NBS is underdrains. Combined All the above mentioned NBS can be combined
with all other NBS elements, underdrains in many different ways. Permeable pavements,
contribute to distribute the stormwater into - or linear drainage systems, raingardens,
out from - the NBS, optimising infiltration rate underdrains are all pieces of a larger puzzle,
from the NBS or securing a far bigger infiltration all contributing to the water infrastructure of
area. climate adaptation.
10 3. Cross-disciplinary collaboration in climate adaptation
3. CROSS-DISCIPLINARY
COLLABORATION IN
CLIMATE ADAPTATION
Creating synergies and saving costs through collaboration between
different stakeholders
Water crosses both administrative and and financed through water tariffs. Due to early on in the project’s development phase,
geographical borders. Danish experience a specific co-financing regulation, water NBS projects are more likely to become
shows that collaboration across disciplines utilities can help municipalities and private successful. It is thus crucial to identify all
and institutions can create valuable syner- owners finance climate adaptation projects. relevant stakeholders and invite them to
gies; resulting in greener and more liveable The financing can be given to projects that start a dialogue as early on in the process as
cities with a higher degree of recreational are based on above-surface solutions, such possible.
value for the local community that also as roads, water courses and recreational
increase biodiversity and lift environmental areas. As Danish water utilities have a Sustainability and digitalisation as a
standards. The collaborative approach monopoly on managing water and sewage, platform for collaboration
is, however, challenged by the fact that their activities and investments are regu- To create the necessary momentum and
the different parts of the water cycle, i.e. lated by the Water Utility Secretary under ownership of climate adaptation projects,
sea, sewage, rainwater, rivers, lakes and the auspices of the Danish Competition and urban planners, environmental managers
groundwater, are often regulated by differ- Consumer Authority, which secures effi- and engineers from the municipal depart-
ent legislative bodies. ciency and high standards on water quality, ments often join forces with engineers
security of supply and general performance. and planners from water utilities at an
Legislation and financing mechanisms early stage. To ensure buy-in from the local
in Denmark Early stakeholder involvement is community, local citizens, property owners,
The responsibility for climate adaptation crucial local businesses and environmental NGOs
is often divided between municipalities, Using a cross-disciplinary, cross-institution are also involved in the process. A strong
water utility companies and private proper- and watershed-based multiple stakeholder use of digital tools and platforms make the
ty owners. This is also the case in Denmark. approach can increase the sense of own- project more accessible for all stakeholders,
Danish municipalities are required to include ership by parties involved, of solutions including the ability to follow and comment
climate adaptation plans in their local such as NBS. Furthermore, by using this on the project as it takes form. Using the
development plans. In order to assist the holistic approach, solutions often have a UN Sustainable Development Goals as key
municipalities, the Danish Nature Agency higher degree of synergy with other activ- performance project indicators ensure both
has developed a set of guidelines and tools ities taking place in the municipalities. The the right project design and increased local
for climate adaptation plans, providing in- results are often more liveable cities with a community engagement. This could be a
spiration on how municipalities can manage higher degree of recreational value for the local partnership using climate adaption
climate adaptation as part of their overall local community and higher environmental projects to flood drained low-lying farmland
planning process to create green-blue live- standards. and wetlands, reducing greenhouse gas
able cities of the future. emissions and urban flooding and at the
Experience from Danish NBS projects shows same time creating more biodiversity,
In correlation with the Danish Water Sector that when city planning, construction, en- nature and areas for recreation, contribut-
Act, expansions of the sewerage system vironmental issues, financing mechanisms ing to SDG 6, 11, 13, 15 and 17.
are managed by water utility companies and stakeholder relations are all addressed
Cross-disciplinary collaboration in Denmark
DNNK, Danish Network for Climate Adaptation (established in 2020) is an innovation
network consisting of more than 150 knowledge institutions, government agencies,
municipalities, water utilities and private companies (tetrahelix structure). The purpose of
DNNK is to develop, document and present climate adaptation technologies and associated
planning tools for the transformation of existing urban areas in Denmark. All information
and experience with NBS in Denmark have been compiled on the website:
https://en.klimatilpasning.dk3. Cross-disciplinary collaboration in climate adaptation 11
Photo credit: LYTT Architecture
Engaging and mobilising the local community in ideas. The Climate City project, which focuses on climate adaptation
creating a Climate City in Middelfart, Denmark through urban design of public areas, also functions as a driving
Covering an area of 450,000 m2, “The Climate City” project in force, which inspires local home owners to manage rainwater within
Middelfart demonstrates how to integrate climate adaptation their own gardens. Therefore, The Climate City is to a great degree
with urban development. Through a dialogue based co-creational about mobilising the local citizens to become an integrated part
process, the municipality and wastewater utility have worked of the project. The Climate City is a partnership project between
closely together with landscape architects, engineers, local citizens Middelfart Municipality, Middelfart Wastewater Utility and the
and other stakeholders in developing the project. From the pro- philanthropic organisation, Realdania.
gramming phase, through the architectural competition process and
subsequent project development and detailed project design, citi- Courtesy: Middelfart Municipality, Middelfart Wastewater Utility,
zens have actively contributed with local knowledge, comments and Realdania, Bascon, WSP, LYTT Architecture and ADEPT
Separating rainwater from wastewater The citizens can choose either to handle rainwater on their
on the surface, Aarhus, Denmark own plots, or to lead the rainwater to a nearby public area via
The Municipality of Aarhus has a vision of making the city an even the surface, where Aarhus Vand handles it. Getting citizens
more attractive place to live through its climate adaptation projects, on board on a project such as this is a crucial task. Thanks to a
and the local water utility, Aarhus Vand, works closely together with successful citizen involvement process with a combination
the municipality to fulfill this vision. of workshops, extensive FAQ’s and even individual advisory
sessions on private plots, the house owners embrace the new
Together, they make rainwater a visible element in the form of rain- recreational solutions.
water lakes, rainwater beds and rainwater drains. They establish
hollows and dams, create entirely new urban spaces and find new Courtesy: Aarhus Vand, EnviDan, Aarhus Municipality
ways to lay out green areas. In this way, rainwater is handled in a
combination of pipes and surface solutions, using various Nature
Based Solution (NBS) elements.12 4. Nature based solutions in simulation models
4. NATURE BASED SOLUTIONS
IN SIMULATION MODELS
Assessing the urban water cycle and the impact of NBS
As many cities around the world are experi- Hydraulic model of NBS elements as systems. The model can simulate typical
encing more frequent and heavier rainfalls, part of sewerage systems NBS features such as green roofs, rainwater
simulation modelling tools to help predict When using simulation models, it is tanks, swales, infiltration devices, and
and understand the urban water cycle are possible to quantify the hydraulic effect overflow from one NBS feature to another.
proving ever more pertinent. Modelling of water-sensitive urban designs versus Other significant processes in the urban
different scenarios of terrain flooding from expansions in grey infrastructure such water cycle are simulated, i.e. runoff from
sewer overflow or the impact of implement- as larger pipes or traditional basins in a semi impervious and impervious surfaces,
ing NBS to the entire urban water cycle sewerage system. The models simulate infiltration through the unsaturated zone to
can aid decision makers in prioritising the the impact of rain events; calculating water the groundwater, groundwater flow within
projects needed to handle future incidents depth and flow in the sewers and manholes, hydrogeological layers and groundwa-
of rain. In Denmark, the very first simulation and rivers related to different rain events ter-related interactions with wells, rivers,
models of the sewerage system were – and also calculate the extent of flooding channels as wells as suburban infrastruc-
produced in 1985. Since then, the need to at the terrain. The models can simulate tures such as leaky sewer pipes, drainage
be able to simulate the whole water cycle in where overflows and flooding will occur pipes and infiltration trenches.
detail has only become more pressing, what and which effects the different solutions
with the heavier rainfalls the country have will have on the water level, the overflows A key model characterisation is the detailed
experienced over the last decade and new and on the risk of flooding, as well as the simulation of NBS on allotment scale and the
water-sensitive urban designs to handle extent of the damage to buildings and infra- upscaling to neighbourhood and city level
them. Climate change scenarios have structures. In addition, the effect of NBS on scale. Thus, the effect on the groundwater
become an integral part of urban water water quality issues can be simulated. The table from several infiltration devices can
cycle management. simulation model, MIKE+, makes it possible be simulated and used to analyse the risks
to create an overall hydraulic assessment of a shallow water table causing damage to
Future climate change scenarios for Danish of implementing different green solutions building foundations, basements, road in-
cities predict an increased frequency of in a catchment area, such as rain gardens, frastructure etc. Similarly, the model can be
extreme rainfall events that cause floods swales, infiltration trenches, permeable used to design and simulate drainage fea-
and massive damage to buildings and pavements, barrels and green roofs as an tures – possibly in combination with NBS – to
infrastructure. Similarly, rising sea levels, extension of the existing sewerage system prevent a rising urban groundwater table.
increased river flows and higher ground- as well as modelling bioretention cells and Catchment runoff (i.e. stormwater runoff,
water levels are expected. To meet these other green solutions hydraulically. The drainage and overflow from NBS structures)
challenges, massive investments in flood MIKE+ is an integrated modelling platform can be given as input to sewer pipe models
protection and climate change adaption for modelling collection systems, rivers and to simulate the hydraulic consequences
measures are required. By using urban flooding. from NBS on sewer overflows and flooding
water simulation models, decision makers events. The model can therefore be used in
are able to decide upon and prioritise their Urban hydrological models and NBS the planning of sustainable Nature Based
efforts, as the models help quantify the Another type of simulation model Solutions and drainage strategies for neigh-
effects of investments. Thereby it is possi- ‘MODFLOW-URBAN’ focuses on the entire bourhoods, and can reduce the risk of poor
ble to minimise the risk of poor investment urban water cycle and the consequences of investment strategies.
strategies. applying NBS instead of sewer-based pipeFigure 1:
Simulation of a NBS-strategy 13
on allotment scale:
A Drawing of NBS elements and the
direction of stormwater;
B Simulated accumulated runoff to the
sewer system in a 10 Yr design storm;
C simulated average water balance.
Figure 2:
Simulation of sustainable stormwater infiltration groundwater table underneath buildings and delaying stormwater
strategies in Odense, Denmark runoff to the existing sewer system. As the illustration in figure
The developed urban hydrology model ‘MODFLOW-LID’ was 2 shows, the model was used to simulate different stormwater
demonstrated for case-areas in the city of Odense. Different NBS- infiltration strategies and the associated depth to the groundwater
strategies were simulated on detailed scale (Figure 1) and upscaled table in a residential area: (a): actual situation (baseline scenario)
by the model to neighbourhood scale (Figure 2). NBS-strategies without stormwater infiltration; (b): massive stormwater infiltration
included the use of rainwater tanks, green roofs, raingardens and leading to a critical shallow groundwater table in low-laying parcels;
soakaways on private parcels as well as swale-trench systems with (c): the use of green roofs to reduce stormwater infiltration volume
water brakes and overflow to the existing sewer system underneath leading to less critical depth to the groundwater table.
roads. Special attention was given to the simulation of rainwater
harvesting (rain tanks) and green roofs in combination with infiltra- Courtesy: The Foundation for Development of Technology in the
tion devices to manage sustainable infiltration strategies in terms Danish Water Sector - VTU, ALECTIA, VCS Denmark, Municipality of
of both maximising the infiltration, minimising the risk of a shallow Odense, Aarhus University, GEUS
Evaluation of flood management in Malmö, Sweden the system was set up in the simulation software MIKE FLOOD. This
The area of Augustenborg in Malmö, was retrofitted with a green included the rain on grid and infiltration, to enable correct quanti-
open stormwater system in the late 1990s. When a major rainstorm fication of stormwater infiltration in green areas. A scenario of the
hit the city of Malmö in August 2014, Augustenborg was less affect- current stormwater system and one for the old pipe-based system
ed by flood damage than nearby areas. Since the retrofitting of the was created. This methodology allowed for evaluation on compar-
stormwater system, it was unknown if the solution was efficient for ison of the two. Results showed that the retrofitted green storm-
extreme rainfall events with a 100-year or higher return period. water system would result in an approximately 80 percent lower
discharge and hence substantially decrease the risk of flooding.
The nature based stormwater solution in Augustenborg, consists
of open canals, swales, ponds and green roofs as well as adapted Courtesy: Division of Water and Wastewater Engineering Lund
levelling of green areas to ensure controlled flooding. A model of University, DHI, VA Syd (Water utility in the city of Malmo)14 5. Breaking through the surface
5. BREAKING THROUGH
THE SURFACE
Managing rainwater in densely populated areas
with impermeable paving
Due to the increasing amounts of large, Permeable paving allows transportation, layer are permeable. When using permeable
impermeable surfaces in cities, the rain- storage and delay but in general, there is no asphalt to store and delay rainwater, several
water runoff from a city differs from the evaporation. issues need to be taken into account. Firstly,
natural runoff pattern that occurs prior to it requires a mental leap to allow water
urbanisation (as explained in chapter 2). Considering infiltration from areas with per- into the construction of the road – a major
When impermeable surfaces are the main meable paving, experience from Denmark shift from traditional designs that ensure
types of surface present in an area, the shows that it is possible to construct no water is entering the road construction.
runoff from several catchments will arrive permeable paving both with – and without – When allowing rainwater in the road con-
simultaneously in certain sections of the infiltration. For instance, if the municipality struction, the lifetime of the asphalt has to
sewers, where the capacity of the pipes is is worried about the water quality of runoff be considered as well as the lifetime of the
lower than the accumulated flow. The lack of from a large parking lot, it is possible to con- permeability layer. What is the risk of the
capacity creates bottlenecks in the sewers, struct permeable paving with a membrane asphalt clogging, which will prevent infiltra-
increasing the risk of flooding. When es- underneath and lead the runoff through tion of the rainwater into the construction
tablishing NBS, peak runoff is delayed and pipes into a sampling well. Sampling can be of the road? How can this risk be minimised?
reduced, which, in a majority of cases, also carried out at a specified frequency that In addition, traditional requirements need
minimises the risk of flooding. the municipality determines. This makes to be considered, such as noise reduction,
it possible to document the quality of the rolling resistance (see figure 3) and not
Replacing impermeable pavements runoff from the parking lot and based on least, safety.
but preserving the carrying capacity this, assess whether it will be possible to
When establishing NBS in a densely populat- allow runoff to be infiltrated in the long run. Although this is still relatively new terri-
ed city, it is important that the functionality tory, Denmark has accumulated enough
of the base surface - including the carrying Permeable asphalt: the principle of the experience to present the first answers to
capacity - is preserved. This is possible climate adapted road these questions, thanks to a new method
when paving roads or streets with perme- In general, the principle of the permeable of maintenance and operations, which is a
able asphalt, or when using permeable road is to allow water in the road construc- focal point when using permeable asphalt
tiles in the pavement or at the parking lot. tion, as both the wear layer and the base to ensure a long lifetime of the permeability.
Particles and
air quality
Permeability Mobility Noise
Safety
Carrying Capacity
Durability
Usability
Rolling
Resistance Sustainability
CO2
Figure 3: Functional requirements of the road. Permeability is one of the requirements, which
is ensured by periodical and thorough maintenance and operations.5. Breaking through the surface 15
Retention and infiltration of rainwater an infiltration tank, it was possible to allow for the right amount of
in a schoolyard in Horsens, Denmark water storage to be accomodated. The AquaCell cassettes are made
Increasing amounts of rainfall heighten urban planning challenges. of 100 per cent recycled PP and can hold up to 96 per cent of their
This also applies to municipal institutions, such as schools. The volume. Due to the stackable design, the contractor could easily
Danish primary school, Østerhåbskolen, which is located in the store the 648 cassettes in the schoolyard while the installation took
southern part of Jutland experienced flooding in the schoolyard place. With the retention and infiltration solution in place, the risk of
during periods of heavy rainfall. The challenge was to find a solution flooding the schoolyard has been reduced considerably.
for managing rainwater in the schoolyard, which consists of an area
of 187 m2 and has a water capacity of 179 m3. By using a new gen- Courtesy: Wavin
eration of AquaCell rainwater cassettes for rainwater retention and
Obtaining climate adaption in grocery from the roof of the store. The roof runoff is led under the car park’s
store car park, Hedensted, Denmark asphalt, enabling the runoff to be handled together with the
The City of Hedensted has experienced of the costly aftermath of rainwater that falls on the car park. The parking area consists of a
heavy rainfalls that are occurring with increasing frequency in many permeable asphalt coating and a unique base layer, with the same
countries. As such, implementing climate adaptation measures has drainage capability of a traditional fascine. This ensures that the
become a priority in the area. Therefore, when the supermarket water does not flood or form puddles in the car park and that the
chain Lidl wanted to open a new store in the area, they included water drains slowly into the ground, rather than straining the
rainwater management in their planning from the outset. sewage system.
Courtesy: NCC, Lidl
The store had a car park installed that deals not only with the
rainwater that falls on the parking area, but also with the rainwater16 6. Increasing urban biodiversity
6. INCREASING URBAN BIODIVERSITY
Creating green corridors and resilient cities
by integrating nature into urban life
Globally, biodiversity is under pressure NBS and the water cycle are becoming the cities and creating wild, green and blooming
and the majority of the dying species are focal point when creating green corridors lushness everywhere via rain gardens,
threatened as a result of human activities. and resilient cities by integrating nature swales and green roofs.
Vital ecosystems are weakened and food into urban life.
chains are destroyed. In Denmark, consec- In addition to improving green corridors,
utive governments have worked to reverse It is possible to choose a strategy for the NBS also offer a number of other benefits
the decline in the diversity of the country’s chosen plants in NBS elements to support for urban life. For example, the urban heat
nature by creating contiguous and resilient certain insects (e.g. bees or butterflies) and island effect, i.e. higher temperatures in
nature areas with improved living condi- thereby birdlife, amphibians and/or native cities compared to the surrounding coun-
tions for native animals and plants. Another plants. Denmark has experience with strat- tryside, is reduced locally when the number
benefit of such areas is the creation of egies that support native plants which serve of green areas is increased. There is also a
enhanced outdoor experiences for the local as habitats for certain species – for example growing overall trend among architects and
community. salt marshes or meadows. The concept is city planners to find inspiration in nature
called ‘Urban Green’ where the plants are and to consider both nature and wildlife
Bringing nature back into the cities selected to ensure that the composition when planning and designing new urban
The occurrence of NBS solutions such as of the plants support each other, forming areas.
green roofs, rain gardens and swales can a symbiosis between the different plants.
contribute to increasing local biodiversity. The concept is bringing nature back into the
Using rainwater to combat urban heat-island be stored and redistributed to the surrounding areas. Vegetated
effect with Green Bus Stops, Poland ground boxes around the bus stop can also retain rainwater and
Many cities today suffer from Urban Heat Island effect (UHI) and create stepping stone habitats for the local fauna (e.g. insects,
excess of rainwater runoff in the streets, creating challenges for birds). During intense rain events, the excess storm water is drained
liveability and causing costly damages to infrastructures. Four cities with patented internal vents, into the vegetated boxes containing
in Poland have chosen to use the Green Bus Stop as a Nature Based optimised drainage layers. Any excess rainwater from the boxes is
Solution to reuse rainwater as a resource and contribute to the led to sewers or nearby green areas. Besides adding ecological con-
reduction of UHI effect. nectivity and biodiversity to urban areas, the Green Bus Stop emits
less heat than the traditional counterpart does, at times as much as
The green roof of the Green Bus Stop can retain up to 90 per cent 10°C less.
of the rainwater falling on its surface. The excess of rainwater falls Courtesy: Amphi Consult & FPP Enviro
into an underground rainwater container, where the rainwater can6. Increasing urban biodiversity 17
The Lakeside Garden in Singapore habitat for dragonflies. All fallen trees have been repurposed into
Once a mangrove swamp, the Jurong region in the South-Western site furnishing and landscape features such as bird platforms,
Singapore is being developed into a new business and leisure des- habitat logs, pathway curbs or nature trail features - all in order to
tination called the Jurong Lake District. The Lakeside Garden is the support and improve biodiversity.
first phase of the Jurong Lake Gardens, which is the recreational area
of the new district. There is a nature-inspired play area, which is the largest of its kind
in the country. It offers a variety of experiences for children, such as
One of the most visible features of the garden is the ‘Rasau Walk’, the opportunity to crawl through a ‘squirrel’s nest’ and glide through
which is a winding, barrier-free, waterfront boardwalk along the a tree canopy. Jurong Lake Gardens is a park where people, animals
Jurong Lake shoreline. Other features include grasslands for bird and plants can co-exist and mutually benefit.
hides, islands for herons and a stream forest, which is a suitable Courtesy: Ramboll
It’s raining frogs: stormwater solution improves a reduction of the risk of combined sewer overflow to the stream
biodiversity in Alleroed, Denmark as well as an increase in wet and semi-wet habitats. Exposure of
Heavy rain events have caused multiple sewer overflows in Lynge, a mineral soil in combination with the addition of stormwater and
district of Alleroed Municipality that is located north of Copenhagen. planting of selected species is expected to increase the diversity of
The area was previously a marsh area, which could store large plant species found in the area, and in turn, increase the number of
volumes of stormwater, but due to drainage, Lynge had lost parts food sources for insects. Furthermore, a new pond was established
of its natural character over time. With a combination of land- to attract amphibians. The pond only receives stormwater that has
scape-based stormwater solutions and nature improving initiatives, passed through a dual porosity filter implemented centrally in the
Alleroed Municipality decided to improve stormwater management, area.
increase biodiversity in Lynge and improve conditions for the
recipient, Lynge Stream. The developed stormwater solution holds Courtesy: University of Copenhagen, COWI, HedeDanmark, LiAn
back stormwater in dry and wet basins, and slowly discharges it into Landscape Design, Alleroed Municipality, City of Copenhagen,
Lynge Stream. The result is a more stable waterflow in the stream, Albertslund Municipality and Aarhus Municipality18 7. Nature based solutions and treatment of runoff
7. NATURE BASED SOLUTIONS
AND TREATMENT OF RUNOFF
Improving the water cycle by ensuring an
appropriate quality of rainwater runoff
Local management of storm water runoff in municipalities must determine whether for a maximum of 24 hours due to concerns
Nature Based Solutions includes the runoff treatment of the runoff is required for each about pathogens. If the treated runoff is to
to be collected, delayed and/or stored and case that utilises NBS. This requirement be sprayed or pumped for play, e.g. at play-
hereafter used as a recreational asset in depends on the pollution level of the runoff grounds, UV disinfection is often necessary.
channels or ponds in the city or discharged as well as the sensitivity of the recipient.
slowly to a receiving water body, such Thus, when choosing a treatment solution, Several solutions for treatment of runoff
as a groundwater aquifer, the ocean, it is important to ensure the water can be are available and they are often already an
streams or wetlands. However, runoff can treated sufficiently to meet the quality integrated part of the NBS. These include
contain a variety of pollutants such as oil requirements related to the particular basic treatment mechanisms such as sedi-
components, heavy metals, endocrine dis- recipient. mentation, filtration, absorption, biological
rupters, pesticides, road salt and nutrients. degradation and flocculation – often in
Therefore, runoff must often undergo If the water is infiltrated, it must not con- combination. Proper documentation of the
treatment before being discharged to the taminate the groundwater. If the water is treatment efficiency of the solutions is par-
receiving water body. discharged to a lake or a stream, it must not amount to ensure that the receiving water
deteriorate the biological and ecological bodies are protected. Establishing a sound,
In accordance with the European Water state of the receiving body of water. And if pragmatic and useful documentation proce-
Framework Directive and the Danish it is used for recreational purposes, the rule dure is still an ongoing process in Denmark,
Environmental Protection Law, Danish of thumb is that rainwater can be stored as well as in many other countries.
Maximum permissible values:
PH: 6-9
Suspended Solids:7. Nature based solutions and treatment of runoff 19 Treating rainwater through curb The curb extensions on Gyvelvej are constructed by using a two-sec- extensions, Hørsholm, Denmark tion system in which the first section collects fallen leaves and sand Sewer capacity is often maxed out during periods of heavy rainfalls. particles. The second section of the curb extension infiltrates the Therefore, an unconventional management approach to rainwater rainwater through a special type of soil which binds and delays from roads has been applied to Gyvelvej in the city of Hørsholm. organic and nonorganic nutrients. Clean water then infiltrates into A technology which is becoming more widespread in relation to the groundwater aquifers. In the case of extreme rain events, the treating dirty runoff water from roads is Nature Based Solutions. By water is bypassed to underground infiltration trenches in order to using alternative methods for handling road water, a double profit utilise the full capacity of the system and subsequently discharge is gained in the form of a decreased load on the sewer system and a the water to the sewer. facility with recreational value. Courtesy: WSP
20 8. Urban recirculation of rainwater
8. URBAN RECIRCULATION
OF RAINWATER
Using rainwater harvesting to create a city in water balance
In many areas of the world, increasing water Basin Connected Cities and Water-Wise design) and contractors (in charge of estab-
scarcity and drought is a highly critical Communities. lishing the system for collecting rainwater
problem, forcing cities and countries to and performing leakage detection).
focus on how to harvest and optimise the The Danish innovation consortium ‘Cities in
reuse of rainwater. In Denmark, water Water Balance’ aims to provide urban climate Safety issues when reusing rainwater
scarcity has not yet been a serious issue, adaptation options that address both the The legislation in Denmark for reusing
but climate change predictions indicate that increased risk of rainwater flooding and the rainwater is very strict, forcing companies
prolonged drought periods will occur more increased risk of drought by linking rainwa- to focus on optimal design and safety stand-
frequently in the future. ter management systems either directly or ards. It must be documented that there is
indirectly to water supply systems and in no contact between the rainwater system
The water balanced city this way progressing towards a closed urban and the drinking water system. Thus, there
A recurring objective for cities that invest water cycle. Based on the overall concept are two compulsory technical solutions
in rainwater harvesting and recycling is of the water balanced city, it is possible to to prevent the drinking water from being
often to ensure that the city is able to rely customise concepts for areas where there is polluted; a physical gap between the two
on the water resources available within a heightened risk of water scarcity. systems and a safety valve that prevents
the city limit. Many cities are faced with the drinking water from coming into physi-
rising population growth due to increased The focal point of the water balanced city cal contact with the rainwater.
urbanisation. Rainwater harvesting and concept is how to increase infiltration,
recycling rainwater can assist the city in evaporation and rainwater reuse. However, The design and safety regulations are de-
maintaining its overall water balance in the issue of leakages in the drinking water scribed in detail in a manual which includes
spite of a growing population, thereby man- system should also be considered in order guidelines for designing all parts in the
aging urban water in a sustainable manner. to reduce water loss. Finally, educating system; including fittings, filters, manholes,
The International Water Association has citizens on how to conserve drinking water storage tanks, pipes and the back-security
developed a comprehensive framework and reuse rainwater should also be part of valve. For instance, the size of the storage
for sustainable urban water management, the concept. A team is put together for each tank is designed by calculating water con-
resilience and liveability. Its approach, case under the concept. Each team typically sumption, the amount of runoff from the
which is termed ‘Water-Wise Cities’, consists consists of a consortium of knowledge roof and considerations of the residence
of 17 principles supporting four highly in- institutions (in charge of development of time to reduce the growth of bacteria in the
terconnected actions; Regenerative Water concept, documentation and teaching), con- tank.
Services, Water Sensitive Urban Design, sulting engineering companies (in charge of
WATER SOURCES DIRECT REUSE:
Stormwater Toilet flushing, laundry, recreational purposes
Greywater Watering of green infrastructure
TREATMENT
Dual porosity,
UV filtration
STORAGE
Tanks
Cassettes
Infiltration
Trenched
GROUNDWATER RECHARGING
A city in water balance relies on water resources available within the city limit. Rainwater harvesting and wastewater recycling allows for
population growth.You can also read
