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Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme

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Morphological management

Morphological management of an estuary aims at pursuing an estuarine morphology sustaining all its different functions. This should be based on a continuous analysis of the estuarine morphology. The big challenge is situated in the dynamics of the estuarine environment, which should be maintained. Therefore it is necessary to define a long term vision of the estuary, against which the observed morphological evolutions can be evaluated.

In morphological management steering the morphology means working with nature as much as possible. If negative evolutions – impeding the long term vision of the estuary – are observed, one must attempt to curb this unwanted evolution. The engineering measures available to manage the estuarine morphology include morphological dredging and disposal strategy, modification of existing hard bordering and construction of soft structures.

1 Morphological management

The main goal of morphological management of an estuary is to ensure a morphological functioning sustaining the different functions of the estuary. This management should aim at influencing and even steering when needed the changes in channels and sandbars in order to conserve the morphological diversity, complexity and mobility of the bar and channel system. Besides offering a safe navigation route, the resulting morphology should also have a positive effect on the dissipation of the tidal energy reducing the propagation of the tidal wave, should increase as much as possible the self-erosive action of the currents at crossings and maintain – and when possible even improve – the diversity of the ecosystem. Achieving these goals requires first an identification of unwanted situations and their causes. Morphological analysis must be a continuous activity, based on knowledge of past changes and their causes, on the observations of ongoing changes and on monitoring of the impacts of actions.

The need to preserve the dynamic behaviour of an estuary, including the mobility of its channels and sandbars, must be recognised. The approach to follow should not be a conservative one aiming at “keeping what we have”, but should be more progressive with a calculated risk allowing “giving up something locally, to reach benefits on other locations”. It is not realistic to define one single “ideal” shape, pattern and depth of estuary elements and then to “steer” the evolution towards it. The big challenge is situated in the dynamics of the estuarine environment and how to coop with this. Moreover the social and economic requirements change with time, while the nature does its own work. However, there is a need to have a long term vision, to define what is desirable. Based on this long term vision, it should be possible to define situations which should be achieved through management. Natural changes must be evaluated and checked against the long term vision; some assessed as positive, others as negative or unwanted.
In order to favour natural positive changes and to reduce or halt unwanted changes, one must understand the morphological functioning of the estuary, especially which elements are controlling it. As such special attention should be given to the effect of the hard bordering (including erosion-resistant geological layers, bank protection works, groynes and levees) on the morphology, more specifically on the location of channels, sandbars, slikke and schorre. In-depth studies are needed to achieve such morphological understanding. The causes must be understood, so that appropriate actions can be designed to modify them if needed.

In morphological management, “steering” the morphology should mean working with nature, not against it, except if unwanted evolutions are to take place, in which case one must attempt to curb this unwanted evolution. Although the concept of working with nature is not new, only in recent years the idea has received more attention (e.g. PIANC working group Working with Nature) and has been used in several projects. When one wants to start to manage the morphology of an estuary in an appropriate way, the goal must be to improve the morphology in the present channel layout with the measures mentioned in the paragraph 4.3. But before implementing measures, extensive research on which measures are most appropriate should be performed. The tools available to do such research are described in the next paragraph.

2 The research tools

River and coastal morphology remains an experimental science, in which theories are not yet well established and bring little help for solving engineering problems. If flow and sediment movements are key elements to the morphological behaviour of an estuary, they are certainly not the only ones. The way the flow and sediment transport patterns are influenced by controls like harder river bed material and by hydraulic structures is also essential.
A sound morphological management of an estuary starts in understanding its past morphology. This morphological analysis must be a continuous activity, based on analysis of topo-bathymetric maps from the past and the present situation. As mentioned before, information about hard geological controls is essential for understanding these processes. Monitoring of different parameters such as flow velocities, discharges, sediment transport, … is necessary to increase the knowledge about the local hydrodynamic and sediment transport processes. Analysis of such monitoring data will help in interpreting the observed evolutions on topo-bathymetric surveys.

Where monitoring data reveals information about local processes, models – numerical as well as physical – can be used to understand the processes on a larger scale. It is important that such models are accurately calibrated and validated using the monitoring data in order to be sure that the model is reproducing well the real physical processes. Where it is generally accepted that the physics of hydraulic models is very well understood, this is not the case for morphological models. Therefore hydraulic models are to be preferred, where the morphological predictions should be made based on expert judgement. From point of view of sediment transport, physical scale models can be used. However, full morphological modeling still has its limitations: where a lot of improvement has been made in recent years, the lack of knowledge on sediment transport processes causes rather poor results for this kind of models, both on short as well as on the longer term [18].

Where one is confronted with the fact that all research tools have disadvantages, it is believed that a multi-tool approach as suggested here (combining analysis of historical topo-bathymetric maps, field measurements, numerical and/or physical models and last but not least expert judgement) combines the advantages of all research tools, minimizing the uncertainties of the final result. Besides the research tools mentioned above, an in situ test – including an extensive monitoring program to detect all possible effects – can be used to give absolute certainty. Especially for projects with a possible large negative effect this option can be preferred: where such a test will give 100% certainty about the results, the negative effects that might occur will be small and most of the time reversible due the scale of the in situ test.

3 The engineering measures

Engineering works in estuaries are traditionally structural (such as groynes and spur dykes, jetties, bank revetments, guiding banks or guiding levees) and/or dredging. Obviously, engineers consider their works as influencing the morphology. However, this is more in the sense of controlling, sometimes even “taming” nature. For example the common technique of disposal of dredged material in the side channels of a multiple channel system – aimed at getting rid for some time of the sediment before it comes back to the navigation channel – is obviously affecting morphology but this is not what we understand by morphological disposal. Morphological management must be flexible, responsive and holistic. Local interventions or too rigid approaches have failed. The possible measures for managing the morphology of an estuary include [2]:

1) Modification of hard bordering
In the past hard bordering along estuaries (including hard structures such as groynes, spur dykes, …) was often built without taking into account the morphology of the estuary. And if this was taken into account at the time of construction, the hard bordering might not have remained appropriate anymore in the present situation since the morphology – and as a consequence also the hydrodynamics – is changing all the time. Therefore, the hard bordering of estuaries may influence its current morphology negatively, for example by orientating water flow in an unfavourable direction. A clear example of how hard bordering influences the morphology in a negative way can be found around the bend of Hansweert in the Western Scheldt (see paragraph 4.4.1).

2) Construction of new (soft) structures
In order to guide the flow or to affect the erosion-transport-deposition process, one could build flow guiding structures. Since the morphology of estuaries is changing all the time, the efficiency of a structure may decrease over time, possibly even causing negative effects after a while. Therefore preference should be given to soft structures, which can be removed or adapted easily if necessary. Structures built with concrete or riprap should be avoided if possible (see negative review of scenarios including such measures in chapter 5).

3) Morphological dredging
Dredging is used to create new channels of deepen and/or widen existing channels (capital dredging) or to maintain existing navigation channels (maintenance dredging). However, dredging can also be used as a tool to influence the morphological behaviour in the estuary. Morphological dredging is meant to trigger evolutions, so that the channels and sandbars become more adapted to the needs of navigation and ecology. This may include dredging to rectify channel borders or to initiate new channels, either a main channel or a secondary channel.

4) Morphological disposal of dredged material
Morphological disposal is not just getting rid of the dredged material. In the vision of morphological management, the disposal of the dredged material is seen as an opportunity to initiate certain desired evolutions. As an example disposal can be used to provoke wanted evolutions of sandbars, or to influence the lateral movement of channels.

5) Combination of the above works
In order to implement morphological management of an estuary, one will most likely need a combination of all engineering measures mentioned above. However, the goal of these works is not to work against the natural evolution, but rather with the natural evolution. Extensive research is necessary in order to determine the most optimal morphology of the estuary, sustaining and if possible even improving all the functions of the estuary.