Naturally and anthropogenically forced sediment dynamics in navigational waters of the Jade-Weser-Estuary

verfasst von

Jannek Gundlach

betreut von

Torsten Schlurmann

Abstract

Sediment management in intensively used estuaries, is currently one of the greatest challenges for coastal engineers. In recent decades, both the volumes and the costs per cubic meter re- located have increased as a result of maintenance dredging. Considering the socio-economic developments of recent decades and the predicted effects of climate change, the need for sus- tainable and innovative sediment management strategies is increasing. As part of the coastal zone, estuaries are semi-enclosed, tidally influenced river mouths. They exhibit extremely complex sediment dynamics, as the sediments can be of marine and fluvial origin and are transported variably over time and space by tides, waves and river discharge. In most estuaries, a net upstream transport of sediments is observed, caused by barotropic, tide- averaged and baroclinic effects. Depending on the geometry of an estuary, its hydrodynamic condidtions and the impact of technical measures, such as channel deepening or hydraulic struc- tures varies, potentially causing undesirable effects, such as hyper-turbidity or loss of intertidal area. Therefore, numerical models are used to simulate the hydro-morphodynamic response of an estuary to technical measures. For an dedicated assessment, it is important to simulate both the natural response of an estuary as well as the anthropogenically influenced response. For the long-term prediction of the estuarine morphodynamic development, assumptions and simplifications are necessary (e.g. morphological scaling factor, model schematization or input reduction). A consideration of purely natural morphodynamics, requires a model-based gen- esis the estuary morphology, as pre-anthropogenic conditions and measurements are hardly available or not detailed enough, when being geologically derived. Simulations of the present estuarine morphodynamic activity, on the other hand, are limited by the ability of a large-scale model to include small-scale structure-induced effects (e.g. individual groynes) or processes (e.g. local sediment disposals). A model-based assessment of sediment management activities requires coupling with specialized models developed for simulating the disposal process which enables more precise predictions of large-scale effects. However, the number and scope of models available in this field is limited. This thesis, addresses these limits by three research objectives. The first research objective focuses on the driving processes of the long-term natural tidal channel evolution of the Jade- Weser Estuary. The second research objective is the mathematical description and development of a specialized near-field model for the dedicated simulation of sediment disposals. The third research objective is the validation and application of the developed model to evaluate the large- scale impact of sediment disposals. For the natural development of the Jade-Weser Estuary a process-based morphodynamic model was built based on a flat-bed approach using simplified boundary conditions and accelerated morphological development. The results were analyzed by checking for morphodynamic equi- librium and by applying a newly developed Correlation analysis of (cross-sectional) evolution (CASE) method. All simulations reached a morphodynamic equilibrium and developed two channels that vary considerably in the location and depth over time and between the simu- lations. Concluding, the development of the two-channel system was mainly caused by the interaction of the tides with the basin geometry, while the alternation pattern and alternation period depended on the relation between tides and river discharge. For the second research objective, to improve model-based representation of sediment dispos- als, the near-field model PROVER-M is presented that projects the active distribution of fine sediments after a disposal of dredged material. PROVER-M provides valuable input for far- field models, enabling a more accurate consideration of fine sediment disposals in larger scale models. Based on the input, PROVER-M calculates the dynamic plume behavior, including the convective descent of sediments and their dynamic collapse on the bottom. The result is a spa- tial distribution of disposed sediments through the water column and on the ground. The model is developed open source and comes with a self-installing executable graphical user interface. To address the third research objective, this model was tested and validated for three sets of disposal simulations. First, new small scale laboratory experiments of instantaneous disposals are presented, documenting the dynamic behavior of fine material disposed in shallow waters. Second, results of the PROVER-M model are shown for: (1) a field-scaled study complemen- tary to the laboratory set-up, (2) a parametric study of sequentially varied model input and (3) a far-field model coupling for estimation of the PROVER-M impact. By comparing results of the laboratory experiments to the PROVER-M model, the physical behavior of PROVER-M is successfully validated. The impact of the ambient setting and dredged material parameters show non-linear, complex interdependencies of these input parameters on disposal properties. Finally, a real set of disposals in the tidally influenced Weser estuary is simulated by coupling PROVER-M to a far-field model, illustrating the potential impact of PROVER-M including an increased maximum suspended sediment concentration (SSC) of up to 10%. By 1) realizing the natural development of an estuary through engineering modeling approaches and idealizations, 2) developing a new near-field model and 3) successfully demonstrating its validity and added value in three different applications the defined research objectives are met and a reconsideration of sediment management in its current form is enabled. The utilization of estuaries in conjunction with climate change-induced transformations of its morphology make sediment a valuable resource and its redistribution an opportunity for synergistic effects. For example, disposed sediments at the right place during the right time in tidal motion could potentially allow the intertidal area to remain intact although exposed to sea-level rises. Such a sediment management concept must aim for a the long-term ecological net added value as its primary maxim. However, this requires dedicated, interdisciplinary studies involving all stakeholders. From an engineering perspective, both the methodology (in terms of modeling and monitoring) and the conceptualization of this collaborative sediment management design is a challenge, neglecting legal or administrative obstacles. Since holistic sediment management is needed on our coasts, this thesis can be one step towards a feasibility study of such a concept.

Organisationseinheit(en)

Ludwig-Franzius-Institut für Wasserbau, Ästuar- und Küsteningenieurwesen

Typ

Dissertation

Anzahl der Seiten

126

Publikationsdatum

08.08.2025

Publikationsstatus

Veröffentlicht

Ziele für nachhaltige Entwicklung

SDG 13 – Klimaschutzmaßnahmen

Elektronische Version(en)

https://doi.org/10.15488/19356 (Zugang: Offen)