Tackling floods and droughts with NBS

Brookwetterung (drainage system). Source: D. Ackermann, LSBG

Authors: Dr.-Ing. Natasa Manojlovic, Senior Researcher at Hamburg University of Technology, member of the scientific advisory team for the German Demonstrator; Christian Ebel, The Ministry for Environment, Agriculture and Energy (BUKEA) of the City of Hamburg; Dieter Ackermann, Agency for Roads, Bridges and Waterways (LSBG) of the City of Hamburg.

The river system of Dove and Gose Elbe is located in the south-eastern part of the City of Hamburg, Germany, formerly part of the inland delta of the Elbe river with marsh characteristics. It has been intensively developed and modified by man over decades to improve its flood protection from the tidally-influenced Elbe estuary, but also to meet the irrigation needs of the adjacent agricultural areas, so as to protect the water extraction area Curslack (see Figure 1). In this process, the river system of Dove and Gose Elbe has been hydraulically decoupled from Elbe River by dikes and sluices and is nowadays a part of the drainage system Vier- und Marschlande, which also includes the river Bille, the surface waters of Schleusengraben and the Old and New Brookwetterung (drainage system), see Figure 1. This drainage system evolved in a complex set of ditches, drainage channels, tranches and their interconnections. In order to regulate the water levels within this drainage area, highly complex monitoring and control system has been developed over time and serve as a basis for the RECONECT activities.

Figure 1: Geographical location of the Demonstrator

Water-related hazards affecting the area

The area is mainly exposed to floods (affecting mainly settlement areas and agriculture) due to the combination of:

  • High water levels in the Elbe River over a long period (several days) due to wind from western directions.
  • High discharges within the river system Dove/Gose Elbe and its tributaries (Bille) due to continuous rainfall over a long period (several days).

However, at the latest after the dry summers of 2018 and 2019, it became clear that this area should also develop strategies to cope with droughts, despite traditionally having safe water supply. Droughts (affecting mainly agriculture, water extraction and biodiversity) are a result of the combination of:

  • Low water levels in the Elbe River over a long period (several weeks) due to low inland runoff and wind from eastern directions.
  • Low discharges within the river system Dove/Gose Elbe and its tributaries (Bille) due to long droughts (several weeks).

NBS implementations in the area and benefits expected

Nature-based Solutions in the area focus on the reactivation and the distribution of the storage capacity of the rivers Bille, Dove and Gose Elbe, their tributaries and trenches and their flood plains in the area of 175 km², to create more retention volume for water during flood events. At the same time, necessary stable water levels in the rivers can be provided during droughts.

The distribution of the storage volume during flood events is managed by the RECONECT NBS operation and controlling system (Figure 2). It has made use of the previous studies and mathematical models (rainfall-runoff and hydrodynamic) as well as of the conceptual design of the operational system mainly developed in the STUCK Project. Within RECONECT, the system has been further developed to be ‘ready-to-use’ by the operators and asset owners.

Figure 2: the envisaged NBS in the German demonstrator

In addition, the potential retention areas along the rivers Dove and Gose Elbe are being identified and assessed to which extend they can be used and/or redesigned as a NBS.

One important aspect of the NBS implementation in the area is the alignment between RECONECT activities, the EU Water Framework Directive (WFD) and the ongoing nature-based projects in the area such as Natürlich Hamburg. In this way, the RECONECT NBS approach can enhance its impact and outreach, but also this enables embedding the RECONECT activities in the current and future NBS strategies of the State of Hamburg.

The expected measures in this demonstration area will generate benefits and co-benefits into the main NBS categories Water, Nature and People as follows:

Water:

  • Reducing flood hazard and lowering of water level
  • Economical savings from operating the pumping stations
  • Reduction of the total amount of water
  • Creating storage for the dry periods (to be addressed in the next project phase)

Nature:

  • Improved ecological condition of the area due to optimised use of the retentions and floodplains
  • Increase in the biodiversity of flora and fauna

People:

  • Improved liveability and social value of the area also contributing to the tourism, that is gaining relevance in the area
  • Reduced number of assets affected by floods

In addition, RECONECT is expected to have two overarching benefits by the end of the project:

  • The grey infrastructure is being converted into hybrid solutions by combining the existing grey assets (such as sluice gates and pumping stations) and NBS. The project is thus contributing to the mindset change and as such sets a cornerstone in the future management of large scale NBS in Hamburg and beyond.
  • We are expanding the existing linear and disciplinary approach (a piecemeal approach to floods and drought management) into a holistic, which will be taken up by the governance structures in Hamburg, after the project lifetime.

Status of work

The project is currently in the implementation phase. All necessary preparatory actions have been accomplished, including the pre-processing of data and models for the RECONECT NBS operation and controlling system, the purchase of equipment and the preliminary screening of the potential retention areas.

The key activities currently being implemented are summarised in Figure 3. The focus of works has been to optimise the operation of the Tatenberger Schleuse sluice gate, in a way to achieve maximal water level reduction at the reference gauge (here Allermöher Deich). Different options have been studied and tested. An important implementation aspect is the redesign of the sluice gate to enable fish migration in the sense of the Water Framework Directive. In the next step, the pre-processed calculations will be used for the flood early warning system for the area implemented within the FEWS platform (step 4 in Figure 3). The potential retention areas are being analysed. However, the major challenge is in the widely spread areas, which have been used as landfills in former times and are still contaminated or exposed to pollution. Their decontamination is beyond the scope of the project. In the forthcoming period, we will take a closer look at the areas and elements that are envisaged for the WFD implementation and develop a plan of action for the successive consideration of areas based on their availability and suitability to serve as retentions.

Figure 3: Implementation of the NBS for the Dove and Gose Elbe river system- the current status (1)-(3) and some further steps (4) and (5)

What has been the main lesson in working on RECONECT?

NBS as an idea is appealing and usually well received by the administration and decision-makers. However, the mechanisms for its implementation are still underdeveloped and their development and consideration will require time. Moreover, although it appears reasonable to consider both benefits and co-benefits for the implementation of measures, in practice it turns to be far more challenging than presumed. It lies to a large extent on the different responsibilities and expertise required to address different benefits e.g. flood peak reduction and improvement of the liability of the area.

Have you identified other areas where to use the same approach?

The presented approach has a replication potential in those areas where drainage capacity is limited due to the tidal regime and influence. This can be found at tributaries along the Elbe Estuary (Stör, Krückau, Este, Oste). Apart from similar challenges, those areas share similar geographical features, but also governance, social and cultural settings. Other rivers in Northern Germany discharging into the North Sea such as Weser represent areas with further replication potential and should be further explored.

What is a relevant achievement you are most proud of?

The RECONECT activities in the German demonstrator brought change and new developments in two main aspects: technical and social/institutional. The first one is related to the deployment of the innovative technology for smart management of hydro-meteorological events, mainly floods, demonstrating the potential to upgrade the grey infrastructure with the NBS approach i.e. optimising the operation of the existing sluice gates and creating necessary retentions, rather than implementing another mere technical solution (for example by constructing another sluice gate).

The social/institutional dimension describes the achievement to gather all relevant institutional and thematic players in the area joining forces to develop and implement such an ambitious project. It also implies firm support from the local authority (Borough of Bergedorf) so as the required logistics and support from the Hamburg Water Utility (Hamburg Wasser).

The German RECONECT Team presented the project approach and results to the local political representatives (Regionalausschuss Bergedorf) receiving good acceptance and recognition, which has also been reported in the local media.

In the following steps, the communication activities will be intensified to create a dialogue and exchange on the demonstration activities, which will also be coordinated with the co-creation activities envisaged within RECONECT.

For more information, please contact Ms Ana Julia Kuschmierz, Hamburg Senate’s Chancellery, Coordinator of the German Demonstration Project

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The RECONECT project demonstrates the effectiveness of Nature-Based Solutions for hydro-meteorological risk reduction in rural and natural areas www.reconect.eu