Report Summary
Intro
This report presents the outcomes of the TKI project – Meegroeidijk Lauwersoog, focusing on the development and testing of the Growing Dike Concept (GDC). This report describes the outcomes of the TKI project – Meegroeidijk Lauwersoog. The GDC, using thin layers of dredged material to reinforce dikes, had before this TKI project only been developed on paper, starting in 2019. This was done partly through a conceptual feasibility study, but never tested on small-scale experiments or in a field setting. Within this TKI project the first implementation of the GDC was tested and analysed in small-scale experiments at Deltares (Delft) and in a field pilot at the dike east of Lauwersoog.
The TKI project is an innovative collaborative project together with partners from the waterboard Noorderzijlvest, Wageningen Marine Research, HAN University of Applied Science, Klaei BV, Havenbedrijf Lauwersoog and Deltares (project activities coordinator).
The GDC involves the reinforcement of flood defences by periodically adding locally dredged material, aligning with the principles of ‘Slow Building’, ‘Building with Nature’ and ‘Beneficial Use of Sediment’. This approach has the potential to enhance long-term flood defence reinforcements by repurposing locally dredged sediment as a valuable building material instead of being removed from the system as undesirable material. In addition, this approach has potentially an impact on reducing Green House Gas (GHG) emissions compared to conventional reinforcements.
The GDC offers a promising integrated solution to major challenges like sea level rise and subsidence, improving water safety in coastal and river areas. In addition, the local re-use of raw material, such as sediment from ports and waterways dredging, could potentially reduce the GHG impact of dike construction and maintenance. The gradual application of sediment may expand the lifespan of flood defences, delaying or even preventing costly maintenance while mitigating flood risks. In addition, the direct use of local sediment eliminates the need for maturation in specific depots, streamlining the connection between dredging and dike maintenance.
In this TKI project, the GDC was implemented, tested, and evaluated for the first time, marking a significant milestone in transitioning to circular sediment reuse and promoting sustainable flood risk management.
The main challenges the GDC aims at addressing in an integrated way are:
- Dike maintenance, sea level rise and subsidence
- Reduction of GHG emissions
- Circular reuse of dredged sediment
The key research questions driving this project and addressing these challenges are:
- What are the most effective application methods for different test locations?
- How can we minimize environmental impacts during and after application (landscape and smell)?
- What is the most suitable material for different test locations? What is the final height increase of the dike after consolidation and drying of the applied sediment, and how long does this process take?
- How quickly can dike vegetation recover, and how does this depend on sediment characteristics, layer thickness, initial vegetation composition and height, and seed availability within the sediment?
- Does the application of sediment mixed with fresh, and saltwater affect the chemical parameters of the topsoil layer of the dike?
Techniques and methods
To test the feasibility of the GDC and address the research questions, comprehensive research involving desk studies, laboratory experiments, field trials and numerical model testing was conducted.
The small-scale experiment on the Deltares campus focused on the effects of sediment layer thickness and initial vegetation length. Two scenarios with different sediment layer thicknesses and vegetation lengths were tested, monitoring key sediment properties such as bulk density, water content, and ion concentration during the first month. Additionally, the shear strength of the top layer was also measured.
In parallel, a qualitative application experiment explored the most efficient way for GDC implementation on a real scenario (i.e., the same dike of the pilot at Lauwersoog). This involved testing the stages of collection, mixing, transportation, and application of dredged sediment to develop a “Spraying protocol”. The latter delineates a set of instructions for achieving uniform layer thickness across the dike slope specific for the situation at the dike of Lauwersoog.
Within the pilot, the GDC was tested on a real-scale dike at Lauwersoog, following the outcomes of the previous small-scale and application experiment. Seven treatments were tested in duplicate, varying in sediment layer thickness, mixing with fresh or saltwater, and seed addition. Sediment layer thickness evolution was monitored using rulers, Sediment Erosion Bars (SEBs), and a drone. Vegetation regrowth after the application of the sediment layer was evaluated by assessing species cover and estimating growth within permanent quadrants (PQs). In addition, chemical analyses investigated the potential ion runoff and leaching, with subsequent variation of chemical composition of the dike topsoil, while GHG fluxes were measured before, immediately after, and three months post-application using a portable in situ gas monitoring system.
The modelling approach was conducted in parallel with and after the application experiment and the pilot. The results obtained during the pilot are used to get insights into the value of the modelling approach. Additionally, while trialling various application methods, considering fluid mechanics may help identify the most efficient application method.
Results
The combined results of the small-scale and application experiment, along with the pilot, provided an initial overview of the feasibility and applicability of the GDC.
The water content of the applied sediment decreased from approximately 65% to 25% in the first week, affecting the sediment layer thickness, which compacted by approximately 65% during the same period due to dewatering and consolidation. After three months, there was no statistically significant change in layer thickness.
Chemical analyses showed runoff and leaching of ions, such as bromide, chloride, sodium, ammonium, and nitrate, into the soil. A slight, non-significant increase in organic matter was observed in the soil, likely due to biomass growth through the sediment layer. Phosphate and sulphate concentrations increased in both soil and sediment, the former likely due to mineralization triggered by the change in chemical composition post-application of the sediment. Conversely, the increase in sulphate is probably the result of drying of the marine sediment.
As the sediment dried, it hardened and cracked, allowing vegetation to grow through. After three months, vegetation cover, which was mostly composed by grass, ranged between 40% and 75%, and exceeded 90% after nine months. The addition of seeds to the sediment mixture slightly accelerated vegetation recovery, depending on the plant’s species in the seed mix. Vegetation recovery was unaffected by sediment dilution and layer thickness, but it may be influenced by factors like initial vegetation composition, dike composition (i.e.,slope, angle, height), and weather conditions.
The characteristics of the material, the spraying method, and the dike’s geometry (excluding the vegetation) served as input parameters for the numerical model used to estimate the theoretical thickness. The calculation resulted in a layer thickness of approximately 2 cm.
GHG flux analyses from the dike surface showed that the sediment application initially halted vegetation uptake processes, leading to CO2 and CH4 emissions. After three months, as vegetation partially recovered and sediment organic matter oxidized, GHG fluxes decreased.
This project, composed by the small-scale and application experiment, the pilot at Lauwersoog and the numerical modelling, enabled us to investigate the feasibility of implementing the GDC for flood defence reinforcement.
The application experiment provided a qualitative overview of all the GDC phases, from dredging and mixing the sediment at the harbour to its transportation and application on the dike. This experiment was crucial for developing a “Spraying Protocol”, which served as input for the pilot.
The small-scale experiment and the pilot, enabled to quantitively test and evaluate the GDC implementation at two different scales. During both experiments, the principal physical and chemical properties of the sediment were measured and monitored to study the evolution of the sediment layer over time. The impact on vegetation was also assessed, focusing on the differences between the two sediment treatments tested. The Lauwersoog pilot provided the first test of the GDC, highlighting key parameters and issues to address to effectively implement this approach.
By using the numerical models one can get insight in expected layer thickness development when the material is sprayed considering the specific inputs of dike characteristics and applied sediment properties.
This first feasibility and implementation tests of the GDC provided many practical insights and lessons. However, additional research is needed on several points: ensuring homogeneous sediment layers with specific thickness, evaluating the timescale and impact of ion infiltration into the soil with following variation of characteristics, assessing the resistance of the applied sediment layer and its adhesion to the dike.
