Restoring flow capacity in the Loire River bed

Abstract

During the 19th century oblique groynes were built in the bed of the Loire River, with the aim of channelling the river during low flow in order to maintain sufficient navigation conditions on a permanent basis. The long-term effect of these structures, in combination with massive sediment extraction over the past 50 years, has been to deepen the minor bed and aggrade old flood-prone areas. The goal of the present study was to restore the original flood conveyance properties of the river bed. The methodological approach combined refined flow modelling using the TELEMAC-2D program with traditional morphological expertise. Justifications for making this choice over alternative methods are given. Constraints and objectives for site correction were determined by multidisciplinary analysis. In particular, this included an environmental analysis aimed at preserving the area's fauna and flora and at restoring its former ecological diversity. The analysis also concerned the hydromorphological behaviour of the system. Proposals for site corrections are described. These essentially involved modifying the structures and dredging in order to recover flow conditions whereby the secondary branches of the river bed would be self-maintaining. The main objective of lowering flood levels was thus achieved, together with some restoration of the ecological diversity of the river system. The study was an opportunity to compare a now well-established method for 1-D fluvial modelling with specific 2-D modelling aspects. It is an example of the progress that can be made by combining advanced modelling results with more traditional approaches. Copyright © 2000 John Wiley & Sons, Ltd.