Two-Channel System Dynamics of the Outer Weser Estuary—A Modeling Study

In this paper, we unravel the mechanisms responsible for the development of the two-channel system in the Outer Weser Estuary. A process-based morphodynamic model is built based on a flat-bed approach using simplified boundary conditions and accelerated morphological development. The results are analyzed in two steps: first, by checking for morphodynamic equilibrium in the simulations and second, by applying a newly developed method that interprets simulations based on categorization of the two-channel system and cross-sectional correlation analysis. All simulations reach a morphodynamic equilibrium and develop two channels that vary considerably over time and between the simulations. Variations can be found in the location and depth of the two channels, the development of the dominant channel over time and the alteration in the dominance pattern. The conclusions are that the development of the two-channel system is mainly caused by the tides and the basin geometry. Furthermore, it is shown that the alternation pattern and period are dependent on the dominance of the tides compared to the influence of river discharge.

Supplemental Material: Morphodynamic equilibrium of lowland river systems during autoretreat

Additional details on the numerical model, code availability, and data from natural examples.<br>

Supplemental Material: Morphodynamic equilibrium of lowland river systems during autoretreat

Additional details on the numerical model, code availability, and data from natural examples.<br>

Supplemental Material: Morphodynamic equilibrium of lowland river systems during autoretreat

Additional details on the numerical model, code availability, and data from natural examples.<br>

A Numerical Investigation on Tidally Induced Sediment Transport and Morphological Changes with Changing Sea Level in South-East England

The impact of tide-induced morphological changes and water level variations on the sediment transport in a tidally dominated system has been investigated using the numerical model Delft3D and South-East England as a test case. The goal of this manuscript is to explore the long-term changes in morphology due to sea level rise and the large-scale morphodynamic equilibrium of the South-East England. Our results suggest that the long term (century scale) tidally-induced morphological evolution of the seabed slows down in time and promotes a vanishing net transport across the large scale system. Century-scale morphologically updated simulations show that both morphological changes and net transport values tend to decrease in time as the system attains a dynamic equilibrium configuration. Results further suggest that the presence of a gradual increase in mean sea level accelerates the initial morphological evolution of the system whose morphological rate of change gradually attains, however, same plateau values as in the absence of sea level rise. Given the same base morphology, increasing water levels enhance residual currents and the net transport near the coastline; and vice-versa, decreasing sea levels minimize both residuals and net transport near the coastline. The areas that are more affected by, water level and morphological changes, are the ones where the net transport is the highest. This manuscript explores and allows extending the idea of morphodynamic equilibrium at a regional scale, larger than the one for which this concept has been generally explored i.e., estuarine scale.

Numerical groyne layout optimisation for restoration projects in large rivers: An adaptive approach towards a desired morphodynamic equilibrium

Integrative restoration measures at large rivers target the improvement of morphological and ecological conditions, under consideration of economic demands, specifically navigational ones. Alternative groyne layouts with e.g. reduced groyne spacing and lowered crest elevation reduce ecological deficits and have the potential to cease frequently encountered river bed incision of heavily modified rivers. On the other hand, the induced change in the morphodynamic equilibrium may interfere with navigation by reducing the water depth in the fairway. In 2009, a pilot project was realised on the Austrian Danube, including an alternative groyne layout. As a consequence the desired aggradations in the fairway became too large, leading to an increased dredging effort. In 2014, a numerical groyne optimisation, specifically a 3D numerical model in combination with a sediment transport model, was applied. In 2015, after implementing the optimised groyne layout in the field, morphodynamic equilibrium was reached reducing the need of extensive dredging. This equilibrium could be shown by analysing subsequently observed bathymetries until 2017. Moreover, the morphodynamic changes due to the groyne optimisation in 2015 were reproduced successfully with the numerical models. Thus they represent a cost effective tool for planning and optimising future restoration measures in large and heavily modified rivers.