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Types of structure and methods of construction |
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Channels can be widened on one side or both. In principle it should be assumed that channels that are widened on both banks will show more development dynamic than single-sided widening. Channels that are widened on only one side may also take longer before the space provided is taken up by the watercourse. |
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The constructions are differentiated according to the degree of human intervention: |
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Self-dynamic development |
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Only the bank reinforcement is removed. Then, the widening is left to be formed by the natural fluvial dynamic. The development of the widening depends on events. According to the flood hydrology, the widening of the channel can take several years. In addition, for rivers with a tendency to erosion, a channel form that remains stable over years has been observed, i.e. the space made available was not taken up and the channel underwent practically no morphological changes.
At the start of the development process, the banks are not laterally reinforced, so the development of the widening should be constantly monitored. If necessary, corrective interventions should be carried out using river engineering. Here, the principle of the line of argument and intervention have proved themselves (see box). The return of the flow at the lower end of the widening must be ensured in every case (structural measures). |
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Guiding structures
at the lower end of the widening |
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Fig.1: Self-dynamic development |
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Self-dynamic development with initial measures |
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As in the self-dynamic development, in this structure the main work is left to the river itself. The formation of the widening is however accelerated by initial measures (e.g. lowering the banks, depositing islands) so that the morphological alteration of the channel and the new creation of riparian habitats take effect relatively quickly. From the viewpoint of "ecology" this is therefore the favoured measure.
Even for this measure, the principle of the line of argument and intervention should be applied, in order to intervene correctively where necessary. |
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Guiding structures
at the lower end of the widening |
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Fig.2: Self-dynamic development with initial measures |
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Mechanical widening |
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In mechanical widening, the entire work of widening the streambed is carried out using construction equipment. In order to prevent further lateral erosion, which is usually not desired, shoreline protection measures are necessary.
In mechanical widening, too, the gravelly material should not be removed, but left in the system, as removing the material intensifies the existing bedload deficit of most rivers, and adversely affects the stabilisation or raising of the streambed that is an objective of the widening. In addition, if there is an erosion tendency, strange partial channels and islands will form. Observations have shown that, even if the width is adequate, if there is lack of bedload is usually stays at a single channel, which does not divide. |
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Fig.3: Mechanical widening |
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Citation:
Rohde, S., Cosandey, A.-C., Hunzinger, L., Junker, B., Marti, C., Requena, P., Vogel U., 2005: Integrales Gewässermanagement. Erkenntnisse aus dem Rhône-Thur Projekt. Synthesebericht Gerinneaufweitungen. Eawag, WSL, LCH-EPFL, VAW-ETHZ. 69 pp.
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