The sediment capture efficiency of a managed realignment polder

The sediment capture efficiency of a managed realignment polder

Hilgersom, Tijmen (TU Delft Civil Engineering & Geosciences)

Aarninkhof, S.G.J. (mentor)
van Maren, D.S. (graduation committee)
van Weerdenburg, R.J.A. (graduation committee)
van Prooijen, Bram (graduation committee)

Delft University of Technology

Civil Engineering | Environmental Engineering

RestCoast

2023-03-08

The Ems Dollart estuary is facing high levels of turbidity, which hinders the growth of algae and impacts the entire ecosystem. In recent years, turbidity has increased here with 0.5 to 3% per year between 1990 and 2010, corresponding to a doubling of suspended sediment concentration (SSC) within 24 years for the upper bound of 3%. This trend is mainly attributed to a decrease in the amount of sediment sinks. The Ems-Dollart 2050 program aims to address this issue by exploring various solutions, aiming to extract one million tons of dry matter from the estuary per year. This study investigates the potential of creating additional sediment sinks by scaling up managed realignment polders. The aim of the research is:

To investigate the important factors of influence for sediment capturing efficiency in a managed realignment polder, to make the mud extraction from a turbid estuary as efficient as possible while creating new natural and agricultural value.

Using the Delft3D FM numerical model, the study focuses on two key factors that influence sediment capturing, namely the effects of the surface area and the effect of the tidal prism (i.e. the volume of water that flows into an area during a tidal cycle), with the goal of making sediment extraction from the turbid estuary as efficient as possible. The study finds that the general concept of using a managed realignment polder to extract sediment is viable. However, the calibration process using Polder Breebaart revealed that the model struggled to accurately replicate some of the accumulation patterns in the low-dynamic system in this polder. While the model seems more appropriate for the project area, significant uncertainty remains due to the sensitivity of the results to sediment properties and calibration parameters.

The study's results suggest that a smaller tidal range can result in a more uniform sediment distribution across the area. This can be achieved by implementing a submerged weir behind the open entrance to the area. An even distribution of sediment could enhance the natural values in the intertidal area, which is a key goal of the Ems-Dollart 2050 program. In addition, wind-generated waves appear to have a major influence on the distribution of sediment over the area. Due to the extra shear stresses, waves provide more resuspension, which transports the sediment further.

Furthermore, the study observes a significant accumulation of sediment in a side-basin near the entrance. The high sediment accumulation in this area, compared to adjacent areas without accumulation, may be attributed to local low energetic conditions. This finding has potential implications for raising agricultural land using transitional polders.

In conclusion, this study provides insights into the effectiveness of using managed realignment as a method for extracting sediment from a turbid estuary. While it highlights some important aspects and influences, it also underscores the need for further research to confirm the findings and reduce the uncertainty in the results. Nevertheless, the potential benefits for both natural and agricultural values make this an intriguing avenue for future exploration.

Managed realignment
Polder Breebaart
Eems Dollard
Morphodynamics
Sediment transport
Morphodynamic modelling
Delft3D FM
Suspended Sediment Concentration

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Student theses

master thesis

© 2023 Tijmen Hilgersom