The role of Greater Copenhagen Utility in implementing the city's Cloudburst Management Plan

Utility companies and municipalities increasingly face hydraulic and hydrologic challenges due to climate change. The recommended design criterion for combined sewer systems in Denmark is a rain event with a 10-year return period. Such an event is a combination of rainfall intensity and duration that is expected to occur statistically once every 10 years based on historic rain gauge data. Combined sewers may thus overflow to the ground once every 10 years as specified in the municipal wastewater plans. The precipitation from events with return periods of less than 10 years will be transported to wastewater treatment plants through the sewerage system. Events exceeding the 10-year return period event are not expected to be contained within the sewerage system, which will overflow. Terrain elevations and urban landscape will determine the hydrologic flow paths of the flood runoff to local blue spot areas where floodwaters will accumulate.

Climate change is expected to increase the 10-year design rain event by 30% in Denmark over the next century, based on IPCC's climate scenario A2 (The Water Pollution Committee of The Society of Danish Engineers 2008). The existing sewerage network has insufficient capacity to handle such an increase. In addition, increasingly extreme weather events such as cloudbursts are expected to occur.

Copenhagen experienced cloudbursts in 2010, 2011, 2014 and 2016, and recorded its worst rainfall on record (1933 to 2016) on 2 July 2011, when more than 50 mm fell in 30 minutes. The Danish Meteorological Institute (DMI) defines cloudbursts as rain events with an intensity of at least 15 mm precipitation within 30 minutes (DMI 2015). The rain gauge in the Botanical Gardens in the city center measured 135 mm of rainfall in less than two hours (DMI 2011), equal to twice the average precipitation in July for the Greater Copenhagen region (DMI 2016).

Damage costs from cloudbursts experienced since 2010 combined with the expectation that such extreme rain events will gradually increase in intensity in the future calls for urgent action. This requires developing a robust hydrologic infrastructure in addition to the hydraulic infrastructure throughout the capital. Following the cloudbursts in 2010 and 2011, which caused damage valued at EUR 1 billion, the Municipality of Copenhagen in cooperation with Greater Copenhagen Utility, HOFOR, issued a Climate Change Adaptation Plan (Copenhagen Municipality 2011). This sets out actions to minimize the risks and damage arising from cloudbursts.

One year later, Copenhagen Municipality issued a Cloudburst Management Plan (2012) defining an optimal service level for cloudbursts based on a socio-economic cost-benefit analysis of flood damage costs in relation to investments in cloudburst solutions. A 100-year rainfall event was identified as the optimum. The cost-benefit analysis was performed on data specific to Copenhagen and the objective was to ensure that flooding during a 100-year event does not exceed depths of 10 cm.

The Cloudburst Management Plan also outlined the necessity of managing the expected increase in design rain events, by reducing runoff connections to the combined sewerage system and disconnecting runoff from 30% of the impervious catchment area, to avoid sewers overflowing too frequently.

The outline visions of the Climate Change Adaptation and Cloudburst Management plans have been elaborated in an ambitious cloudburst management strategy developed by HOFOR and Copenhagen Municipality in 2013 (Copenhagen Municipality 2013). The aim was to develop conceptual solutions to manage both climate change challenges in integrated solutions, handling runoff from both cloudbursts and increasing design rainfall events.

Special emphasis is given in the strategy to the blue and green fingerprints of the city, and implementing local rainwater management measures, thereby mitigating urban heat island effects, increasing the city's biodiversity, and enhancing recreational use of the urban landscape. An entirely new blue and green infrastructure will be implemented in Copenhagen.

The concept includes infiltration, delay, storage and transportation of urban floodwaters. The methods employed will involve both existing and newly created green and blue elements in the city, managing runoff locally in water sensitive urban design (WSUDs) elements, installing new and traditional stormwater components (tunnels, grates, line drains, outlets, pipes, basins, etc.) and manipulating terrain elevation (sloping roads, elevated sidewalks and bicycle paths).

Flood management will be implemented on urban surfaces where possible, because construction costs are lower than for the traditional expansion of existing pipe capacity below ground. The Danish Government issued new legislation early in 2013 under which water utilities can finance surface measures through consumer tariffs that facilitate transportation of daily rainfall or cloudburst run-off. Water utilities can now assume responsibility for the hydraulic and hydrologic overview of coherent water systems, above and below ground.

Cloudburst solutions will form one coherent cloudburst infrastructure that can enable infiltration, delay, store and transport surface runoff from the most upstream sub-catchment to final discharge in the harbor. Like the branches of a tree, the infrastructure will consist of an intricate matrix of interdependent solutions. Moving downstream along one branch of the infrastructure, cloudburst solutions will increasingly be handling accumulated runoff from upstream cloudburst solutions. The ambition is to handle as much of the cloudburst runoff in blue-green solutions as possible. As recipient proximity increases, cloudburst solutions will increasingly be characterized by surface runoff transport. Green-blue solutions need supplementary pipes and cloudburst tunnels to handle the large quantities of water during a flood.

HOFOR is responsible for ensuring the hydraulic and hydrologic overview of the entire portfolio of cloudburst solutions, including assessing the hydraulic effect of individual solutions. As the solutions are interdependent, HOFOR also evaluates the consequences of changing the hydraulic capacity or performance of intermediate solutions to ensure that the cloudburst infrastructure's total service level is upheld. In order to achieve this, HOFOR maintains a specialized team of hydraulic engineers.

HOFOR, as a utility company, also comments on all local development plans to ensure that the overall hydraulic solution for a catchment area is considered in every proposal for change in surface area. With the current flood management strategy, its visibility in the urban landscape increases dramatically.

Since HOFOR has experience in planning and implementing large-scale water infrastructure, and in-house expertise in storm water modelling, it was natural for it to become responsible for guidance regarding the hydraulic part of the cloudburst strategy. It was thus realized early that proper hydraulic modelling of the combination of sewers and surface flows is the basis of designing the necessary cloudburst interventions, and keeping a total overview of the hydraulic and environmental consequences of the combined interventions for a larger catchment area.

Decentralized blue-green solutions will reduce the capacity required in closed sub-surface systems. With the high rainfall intensities experienced during a 100-year cloudburst, however, open channels, pipes and tunnels remain necessary.

The cloudburst management strategy prepared by HOFOR and the Municipality of Copenhagen is aimed at managing daily rainfall and cloudburst runoff in the city's seven catchment areas. The strategy incorporates 350 specific cloudburst projects. Its implementation started in 2013 and the budget is EUR 1.3 billion. It should be complete by 2035.

Many solutions are designed with recreation in mind, thus coordinating climate change adaptation with city planning and large infrastructure changes, which brings significant benefits in a dynamic city like Copenhagen.