| Popis: |
The lower part of the Loire River has groynes and guide walls along the banks. The presence of these structures, in combination with extensive sand mining is responsible for the degradation of the riverbed and the associated decrease in low flow water levels of 23m eters over an extended length of the rive r, primarily due to sand mining. This paper describes a study of how the hydraulic conditions at low flow can be improved by a reduction in height and the extension of the groynes, i.e. how sediment can be forced to resettle in the main channel and increase low flow water levels. The modelling is carried out using the two-dimensional curvilinear morphological model MIKE 21C. For the most favourable scenario examined, where the groynes are modified over a reach of 10 km, an increase of 0.35 meters is obtained for the low flow water levels. At the beginning of the previous century groynes were constructed along the banks of the lower part of the Loire River in order to improve the navigational conditions and to reduce the meandering tendency of the river. Today, 100 years later, the construction of the groyne fields and the narrowing of the main channel together with extensive sand mining have resulted in the degradation of the river bed and the associated decrease in low flow water levels of 2-3 meters over an extended length of the river. As a result, almost all secondary branches of the river have dried out destroying most breeding areas for fish. The stability of docks in the city of Nantes is also jeopardized, and salinity and turbidity have moved several kilometres upstream causing problems for water intakes. Various methods for the restoration of the river have been considered, including the partial removal of the groynes. It is envisaged that the reduction of the groynes will initiate a redistribution of sediment, whereby sediment will be transported from the areas between the groynes into the main channel. The redistribution of the sediment is expected to cause a change of the channel cross section, which will decrease the conveyance capacity of the main channel and hence steepen the water surface profile, i.e. increase low flow water levels. The purpose of this ongoing study is to investigate and quantify the effects on the low flow water levels, which can be obtained by a height or extension reduction of the groynes. |
