Morphological Study of the Restoration of the Loire River

摘要

The lower part of the Loire River in France is trained with groynes and guide walls. Most of the river training structures were built in the first part of the previous century to improve navigation conditions on the river. The presence of the river training structures in combination with extensive sand mining have caused degradation of the riverbed, and the associated decrease in low flow water levels has reached 2 ～ 3 meters over an extended length of the river, with severe impact on the ecology of the river and the adjacent flood plain areas as well as on the water quality. The importance of the navigation on the river upstream of the city of Nantes (located 80 km from the Atlantic Ocean) has decreased in recent years, and it has been decided to sacrifice year - round navigation on the river in order to restore the river environment. The paper describes a mathematical modelling study of how the hydraulic conditions at low flow can be improved by reducing height and length of the groynes or by completely removing these. The objective of the study was to design a monitoring programme that would document the effect of a "full scale experiment" in the river, in which groynes would be modified over a length of 10 km. The modelling study was carried out using the two -dimensional curvilinear morphological model MIKE 21C. For the most favorable scenario examined an increase of 0.35 meters of the low flow water level was obtained. The modelling study was carried out with DHI France for Agence de l' Eau Loire -Bretagne and completed in 2004.