Initial managed realignment site configuration steer its long-term hydromorphological development: an idealized modelling study
de Vries, T. (2023). Initial managed realignment site configuration steer its long-term hydromorphological development: an idealized modelling study. MSc Thesis. University of Twente, Faculty of Engineering Technology, Department of Water Engineering : Twente. 62 pp.
Managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. The opening of a managed realignment site can help in restoring previously present nature and act as a Nature-based Solution for coastal safety. From a NbS perspective interfering with nature should be kept to a minimum after initial breaching. However, Many recently opened managed realignment sites tend to neglect the effect of initial design on the long-term objective and its effect on the estuary. This study aims to explore and quantify the hydromorphological interplay between the estuary, the inlet and the managed realignment site as a result of dike breaching and reintroducing tidal exchange. To this end, a idealized 2DH morphological model is set up, resembling a possible location for a managed realignment site behind the Hellegatpolder in the Western Scheldt. The influence of different physical forcings (tidal constituents, sediment concentrations) and initial design (inlet, basin) on the development of the managed realignment site, inlet area and estuary channel are evaluated over a time span of 25 years. The model is able to produce accretion rates over the first two years (4.1 & 3.1cm) comparable to observations from the Perkpolder (3.2 & 2.6), a similar recently opened managed realignment site in the Western Scheldt . To determine the equilibrium inlet area below mean sea level (MSL) the regime channel method is introduced. The method is a more physically based analytical approach, as opposed to the more conventional equilibrium relationships. The regime channel method is able to correctly predict the equilibrium inlet area based on observations from the Perkpolder. Additionally, the regime channel method is able to determine inlet area over the full time span (25 years) of morphological development based on model results. After opening, accretion in the managed realignment site is high. Accretion rates over time as the hydroperiod decreases. The accretion pattern in the managed realignment site is twofold. Firstly, cohesive sediment are able to reach the back of the basin promoting landward infilling. Secondly, non-cohesive sediment eroded from the inlet produce ’humps’ of sediment that remain locationally stable over 25 years, indicating the importance of initial system configuration on long-term development. Accretion rates are 34% higher in the top half of the basin as a result of flood dominant flow in the estuary channel. The estuary is pushed away from the managed realignment site due to sedimentation at the end of the inlet. The redirection of flow increases erosion downstream and deposition upstream of the inlet. When the initial inlet area is closer to equilibrium, the initial redistribution of non-cohesive sediment is almost completely depleted. As a result, the site is able to accrete 2.5cm of cohesive sediment more and the impact of the managed realignment site on the estuary diminishes over the full domain. Increasing the site area increases accretion rates and trapping efficiencies. However, it comes at the cost of an increased impact on the estuary morphology and careful consideration should be made about its impact. The initial design of the managed realignment site thus has a significant impact on its long-term development. For future realignment projects the long-term objective should be taken incorporated in the initial design to maximize the potential and minimize its impact the local system.
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