Sct Annae Square as Flash Flood Street Approche adaptative holistique dans le square Sct Annae à Copenhague

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1 NOVATECH 2013 Sct Annae Square as Flash Flood Street Approche adaptative holistique dans le square Sct Annae à Copenhague Christian Nyerup Nielsen*, Sanne Slot Hansen**, Anita Lindholm Krak, Sanne Mai Andersen***, Jes Clausson-Kaas****, Ole Bach***** *Ramboll Denmark, Hannemanns Allé 53, DK2300 Copenhagen **Schonherr, Esplanaden 8C, DK1263 Copenhagen Lundgaard & Tranberg Arkitekter A/S, Pilestrædet 10,3. DK1112 Copenhagen ***Municipality of Copenhagen, DK1505 Copenhagen ****Copenhagen Energy, Ørestads Boulevard 35, DK2300 Copenhagen *****Kvæsthusselskabet, Jarmers Plads 2, DK1551 Copenhagen RÉSUMÉ L une des rues les plus typiques de Copenhague sera reconstruite avec pour objectif de créer un square de haute qualité en harmonie avec la nouvelle façade portuaire au niveau du nouveau Théâtre Royal Danois (Royal Danish Playhouse). Le site est sujet aux inondations mais détient parallèlement une solution éventuelle aux problèmes d inondation dans une zone plus vaste de hectares du centre-ville. Le profil de la route sera reconstruit et constituera une section abaissée de type espace vert central. Cette section verte est en mesure de rejeter de grandes quantités d eau pendant des crues torrentielles, tout en transformant la route existante en un espace vert urbain. Il s agit d un projet pilote dans le cadre du financement de la protection contre les inondations et de l adaptation au changement climatique au Danemark. De plus, le projet comprend la mise en œuvre de solutions d eau visibles ainsi que l utilisation des eaux pluviales à des fins d irrigation et de récréation dans les aires de jeux, etc. Le ruissellement sur la route est traité localement avant d être rejeté dans le port de Copenhague. Le processus de conception est toujours en cours. ABSTRACT One of the most characteristic streets in Copenhagen will be reconstructed with the purpose of creating a high-quality square matching the new harbor front at the new Royal Danish Playhouse. The site is susceptible to flooding and at the same time it holds a possible solution to the flood problems in a larger area of hectares of the city centre. The road profile will be reconstructed, as a lowered central green park-like section. This green section is able to discharge large amounts of water during flash floods, while at the same time transforming the existing road into a green urban space. The project is a pilot project with respect to the financing of flood protection and climate change adaptation in Denmark. At the same time the project involves visible water solutions as well as use of rainwater for irrigation and recreational purposes in playgrounds etc. Road runoff is treated locally before discharge into the harbor of Copenhagen. The design process is still ongoing. KEYWORDS Advanced flooding calculations, Architectural design, Financing, Flash flood street, Water reuse 1

2 A6 - QUARTIERS RÉAMÉNAGEMENT / DISTRICT REDEVELOPMENT 1 INTRODUCTION In 2012 the Municipality of Copenhagen issued a draft version of a cloudburst plan /1/ based on the Climate Adaptation Pof 2011 /2/. The Cloudburst plan divides the city into topographic catchment areas and the overall methods for reducing the risk of flooding are pointed out for each catchment area. In the catchment area between Nyhavn and the Royal Palace, Amalienborg the solution to flooding from extreme rainfalls will be a flash flood street in Sct Annae Square. Overview of the area between Nyhavn and Amalienborg with overlying results of flooding calculations Sct Annae Square is revitalized with the purpose to create a new high-quality square connecting to the new area, Kvæsthusmolen. The square is designed with a lowered central area collecting rainwater during extreme rain events, and discharging the water to the harbor. Furthermore, the everyday rain events as well as rain events with a return period of up to 10 years are separated from the combined sewer. The water is treated and used for irrigation and recreational use before discharge into the harbor. 2 METHODS The primary focus of the new square is to create and support the urban life and recreational use of the area. Based on this overall target the challenge is to find a balanced solution, taking into account the reduction of flood risk as well as traffic issues, architectural design, nature, environment etc. The design method is based on the collection of all requirements for the project including input from an intensive public involvement. The project is a pilot project regarding technical design and especially concerning financing. The project is financed by the municipality, the water utility company and private investors, and it is thereby one of the primary test areas in Denmark covered by the regulation on financing of flood protection and climate adaptation measures. 2.1 Hydraulic design extreme rainevent The topographic catchment of Sct. Annae Square is defined using a digital elevation model and the catchment is adjusted in order to fit the adaptation projects of the surrounding catchments. Following the definition of the catchment borders, advanced hydraulic simulations are carried out using a hydraulic model, which combines the pipe flow and the flow on terrain through the digital elevation model. The total catchment area is 18 hectares. During extreme rain events the sewers will overflow and rainwater will flow from the streets into the 2

3 NOVATECH 2013 lowered part of Sct Annae Square before finally being discharged into the harbor. The hydraulic model simulates this process, and the simulation is used when designing the profile of the street enabling it to handle a 100-year rain event in the year Based on Danish standards this implies an increase of 40 % compared to the present day 100-year rain event. The design criteria for this 100-year rain event is that the water level in the road must not exceed 10 cm, meaning that the profile should be able to discharge 3,5 m 3 /s into the harbor, taking into account the retention capacity in the lowered part. The surplus amount of water is discharged into the new rainwater system, see 2.2. Catchment prior to the adjustment to surrounding catchments During the design phase, three different methods of hydraulic calculations have been used: Time-area method, using the catchment and an estimated time of concentration (excluding the sewer system) Standard canal simulation using the profile of Sct Annae Square loaded with flow corresponding to the overload from all manholes in the catchment Fully timestep-coupled hydraulic calculation of pipeflow model (1D) and surface flow model (2D) The coupled model is also used for detailed surface design in order to secure that rain water from the surface is led to the lowered parts of the street and not into the basements. 2.2 Hydraulic design every rain events Most of the rainwater in the area is decoupled from the existing combined sewer system. The new system for rainwater consists of different methods varying for overland flow to flow in pipes. A part of the roof runoff is used for irrigation and other parts are intended for recreational use in playgrounds. Road runoff is being treated prior to discharge to the harbor. Treatment facilities are designed with the purpose to treat more than 95 % of the annual precipitation which will ensure that the entire first flush is treated. The treatment process is designed with the combination of sedimentation and/or adsorption to filter materials such as limestone. 2.3 Hydraulic design meets traffic solutions and landscape architecture Sct Annæ Square is located in the inner City of Copenhagen, in the 16th century planned city area of 3

4 A6 - QUARTIERS RÉAMÉNAGEMENT / DISTRICT REDEVELOPMENT Frederiksstaden next to the Royal Palace of Amalienborg. The square is divided into two parts. The western part meets the city and is characterised by its classical, central fenced green area with two parallel rows of lime trees, situated between two three-laned roads. The eastern part is today a large, open trafficked space that meets the harbour and the newly opened Theatre House and surroundings at the Kvæsthus pier. Sct Annæ Square is 400 meters long and 41 meters wide. Lowered central part of Sct Annae Square The primary focus of the new square is to create and support peaceful local urban life, a general recreational use of the area, and in addition to this, to create a new connection from city to harbor. Based on this overall target, the challenge is to find a balanced solution, taking into account the reduction of flood risk as well as traffic issues and classical landscape architectural design. To create a calmer and safer urban space - especially for pedestrians and bicyclists - the traffic is regulated. The two one way streets are narrowed to slow down traffic, and to create a wide promenade that can accommodate pedestrians, local restaurants etc. The green elements, grass and trees, of the western part of the square, are continued on the new eastern part of the square to span from one end of the square to the other, from city to harbour. In addition to this, the central green area is widened and opened towards the streets, to invite people to use the area to relax and to promenade. The flood-protection project is made visible to the public by lowering an area in the central section in the green area. To do this, the critical area around the surface-roots of the existing characteristic trees has been thoroughly investigated. The solution protects the roots, and at the same time, contributes to a modern interpretation of the classical design of the park, and offers a new long promenade with natural elements for pausing in the middle of the green space connecting the city to the harbour. Clean water from the roof-tops is used as a recreational element in the new urban environment. Where the park is accessed, specific water elements are designed. One element could be a traditional water fountain, based on rainwater, the other is a lowered playground with a water feature as the main attraction. On the widened pavements, water-channels with water from roof-tops visualise the story of the running water. 2.4 Financing The total cost of the project is approx. 15 m. Euro, of which 5 m. Euro cover the flood protection and separation of rainwater. These 5 m Euro will provide the main flood protection infrastructure of an 18 hectares area in the centre of Copenhagen and further fund the separation of rainwater from the combined sewer in % of this area. 4

5 NOVATECH 2013 The project is one of the first to be financed under a new Danish regulation regarding investments in climate adaptation measures. Thus the project is financed by the municipality, the water utility company and private investors. The new regulation allows the water utility companies to fund constructions or other measures carried out under private or municipal projects, with the purpose to reduce flooding risk and meet the required service level of public areas. In 2013 and 2014 the utility company is allowed to pay 100 % of the costs related to flood risk reduction measures, and from %. The scope of the new regulation is to ensure that the investments in public infrastructure are made with the highest socio-economic benefit possible. This will be documented through economic calculations of two alternatives for flood protection, one of them being a traditional pipe solution. 3 RESULTS AND DISCUSSION The profile of the square is designed to combine all the aspects mentioned above, The water channel constitutes a connecting element of the square, and can be used for promenading and pausing, yet still allowing passage across the square. At crossing roads and other access points, smaller squares will be shaped as ramps that allows for "level-free" access for the disabled and baby carriages etc. to the park and also serves as rainwater inlets from the streets during extreme rain events. It is not socio-economically beneficial to design the square to handle even more critical rain events such as the event that hit Copenhagen 2 nd of July 2011 with a theoretical return period exceeding years. However, it is important to stress that this design will offer a considerably better protection than the current situation. In order to handle these extreme events it will be necessary to construct a quite deep lowering all the way to the harbour which will severely damage the possibilities of an interesting urban life or to establish a very expensive tunnelled pipe to the harbour. 4 CONCLUSION It has proved possible and appropriate to design the new Sct Annae Square to meet the many different requirements regarding flood protection, revitalization as well as traffic and parking planning etc. The result is not a poor compromise but a sustainable square which supports urban life in the best way thinkable. Further the it has been possible to cut the total constructions cost for the flood protection part by more than 30 % compared to traditional pipe solutions even though the planning and design phase is more complicated. The design of Sct Annae Square will meet the flood protection service level and the intentions regarding decoupling of impervious areas to a separate system. The project is one of the first to be financed under the new Danish regulation regarding investments in climate adaptation measures. The scope of the new regulation is to ensure that the investments in public infrastructure are made with the highest socio-economic benefit possible. The design process is ongoing. LIST OF REFERENCES Municipality of Copenhagen 2011 by COWI, Copenhagen Climate Adaptation Plan Municipality of Copenhagen 2012 by COWI, Copenhagen Cloudburst Plan 5

6 A6 - QUARTIERS RÉAMÉNAGEMENT / DISTRICT REDEVELOPMENT Principle solution 6