Modelling and experimentation

As a first step, we defined a series of ecosystem hydrodynamic and geomorphological boundary conditions to facilitate and enhance tidal wetland creation and development. Numerical modelling provided insight into potential (critical) riverbank erosion effects of structures being designed. Simulated (turbulent) flow impacts and erosive forces were taken into account in the design, engineering and installation procedures of the riverbank configuration. Some additional laboratory experiments were conducted to evaluate the proposed stepped river emba nkment configurations of mud flats and/or marshlands. These experiments were performed at the Mesodrome facility, a hydraulic laboratory test flume located at the “Drie Eiken” campus of the University of Antwerp. The Mesodrome facility and the adopted flow measurement techniques were crucial in assessing the performance of different schematised geometries inspiring both technical and ecological design ideas.

Bankbusters site

The Bankbusters pilot site is located at the “Ketenisse” wetland spot along the left bank of the Scheldt estuary, next to the Waterbus stop at Kallo (Belgium). This area, just downstream from the Kallo sluice entrance, is known to be highly erosive and susceptible to both strong local tidal flow patterns and explicit ship induced wave impacts. Local riverbank profiles are currently protected from this excessive erosion by means of rock deposit on both the tidal water shoreline and higher at the toe of the dyke. The pilot site currently shows very little vegetation due to the high erosive impacts. As a result, explicit vertical erosive cliffs are observed at the high water line. The soil of the pilot site is characterised partly by sandy sediments with areas of grey alluvial heavy clay. Further downstream of this erosive spot – just around the estuary corner – the Ketenisse natural wetland is used as a nearby natural reference riverbank profile site.

Beneficial reuse of sediments

The Bankbusters tidal marsh restoration project emphasises the beneficial reuse of sediments; both to promote a circular economy of building materials and to optimally enhance the ecosystem services as delivered by stable and healthy wetland riverbank systems along the estuary.

The choice of backfill material for the upper marshland soil massive is a critical determinant for the success of the wetland restoration scheme, as it directly impacts morphodynamic status, fine sediment retention, the groundwater dynamics, the gully system, organic content, pioneering vegetation and marsh habitat vegetation growth development. The key objectives for the fill material selection included:

• Ease of handling and positioning of the soil in the pilot test setup:
• Acceptable geotechnical stability of the upper marshland as part of the adapted river cross profile:
• Granulometry reflecting natural marsh conditions with fine sediments typical for tidal wetlands:
• Appropriate organic matter levels supporting vegetation growth and nutrient cycling: and
• Reusing locally available (dredged) materials to minimise environmental impact and promote circular economy principles.

Limited availability and complex environmental permitting, in the end, became the determining factors for fill material selection. Given strict boundary conditions and the existing operational constraints, available and fully compliant silty sandy material was chosen for the pilot setup of Ketenisse.

DRECO units

Based on the initial assessment of the boundary conditions and the actual, updated intertidal observations and experiences along the river Scheldt, we prioritised the marshland part of the intertidal cross profile restoration due to its technical feasibility and ecological value.

Structural integrity analysis of the Dredged Ecological Compartments (DRECO units) led to engineering optimisations for soil stabilisation, hydrodynamic loads, vegetation development and performance under extreme tides. Conceptual designs included erosion control, sediment f luxes and vegetation conditions. Collaborating with Van Aalsburg BV, we developed a final containment bund configuration, addressing challenges like scour protection, sediment retainment and modular willow matrices for tidal conditions. Further detailed design revealed additional technical opportunities and challenges.

Biosafe materials

Bankbusters materials are entirely natural and can withstand harsh environmental conditions to form tidal marshlands in a riverbank setup. They need to be biosafe, enhancing structural integrity, natural resilience and biodiversity. Meaning only materials that do not harm the environment through weathering or leaching should be used. Furthermore, targeting a more sustainable riverbank ecosystem, judicious material selection reduces the carbon footprint as well as the overall production impact.

Legislation

The first Bankbusters field pilot project finally received an integrated building permit in th e autumn of 2023. Taking into account the complex applicable legal provisions, an extensive time consuming application has been elaborated for the pilot setup. Following assessments were made as part of an integrated environmental and social impact evaluation.

As the project location is situated within the habitat directive area Scheldt and Durme estuary from the Dutch border to Ghent (BE2300006) and the bird directive area “Salt marshes and polders of the Lower Scheldt” (BE2301336), the permit required a specific legal status and falls under the provision of an intertidal site with “wet and dry infrastructure”, nature conservation and the natural environment, and waterways and non-navigable watercourses within a certain category.

Dewatering: key to enhance marshland development

Natural drainage of the soil massive is crucial for tidal marsh restoration and creation. Effective drainage is essential to assure overall stability at the start of the marshland development, to facilitate the settlement of the marshland soils, to grow pioneer vegetation and to maintaining the overall ecological integrity of the system towards a robust riverbank ecosystem. Proper drainage helps regulate both groundwater and surface water level, prevents waterlogging and allows for the natural movement of tidal and groundwater flows. Additionally, managing drainage in a way that mimics the natural processes of tidal marshes (for example, through dedicated gully systems) can promote vegetation growth, reduce erosion and support biodiversity.

Several drainage solutions can be implemented in tidal marsh construction to mimic natural tidal flow (Ndongo, 2024). The selected silty sand material aids efficient marshland development. Our DRECO units act as containment bunds but with the specific aim to allow optimal groundwater drainage and rewetting of marshland. The Bankbusters structure ensures marsh sediments are kept in place and dynamically dewatered, supporting tidal marsh restoration and enhancing riverbank ecosystem services and biological value.

Monitoring

The marshland at the pilot site has a dynamic character. Short-term field monitoring records the pilot setup’s performance during kick-off and initiation phase of marshland development, while long-term monitoring aids further operational management of the (nature-based solution) riverbank flood protection schemes. The actual integrated monitoring approach includes a field measurement protocol to assess – in the initial development phase – the overall ecosystem status of the marshland and the level of flood risk protection as a riverbank system. Other ecosystem services will be observed as part of the further wetland riverbank environment development.

Key developments in the actual Bankbusters monitoring framework include a real- time groundwater flux sensor, modular telemetry data infrastructure and a monitoring protocol roadmap. Drone observations and bathymetrical recordings provided insights into the marshland’s morphodynamics, highlighting erosion, sedimentation and soil compaction. The permanent operational camera system adds value by visualising tidal processes and aiding flood frequency analysis. The monitoring programme aims to develop a scientifically underpinned strategy and detail the marsh’s ecological and morphological evolution, ensuring the long-term success of tidal marsh restoration efforts.

Ecosystem services business model

Tidal wetlands provide essential ecosystem services, acting as natural buffers against storm surges and flooding, improving water quality by filtering pollutants and supporting biodiversity. They also sequester carbon, helping mitigate climate change. These services are crucial for balancing economic activities, ecological values and flood protection. The Bankbusters brand, allied with Coastbusters, focuses on Nature-based Solutions (NbS) for more environmentally friendly riverbank systems. These projects aim to restore disturbed river cross-profiles, offering a new business model for estuarine river management.

Future potential

Bankbusters' nature-based solutions have global potential, integrating digitalisation and experimental approaches with smart engineering to protect eroded riverbanks. The innovative modular concept, using biosafe building materials, adapts traditional natural elements for marshland restoration in estuarine habitats.

Emphasising circular economy and resource reuse, particularly in dredging and navigation, Bankbusters uses cutting-edge monitoring techniques to optimise marshland recovery. The Bankbusters projects connect to the growing need for more resilient, sustainable and healthy estuarine ecosystems which are a valuable source of ecosystem services. The market for this kind of ecosystem restoration is burgeoning, with future scenario projections indicating a high potential NbS growth driven by the increasing recognition of the critical role biodiversity and ecosystem services can play in our adapted socio-economic system.