ISSN:
1435-9537
Keywords:
Mots clés Sédiment
;
Vase
;
Aménagement
;
Erosion
;
Estuaire de la Loire
;
France
;
Keywords Sediment
;
Mud
;
Planning works
;
Erosion
;
Loire estuary
;
France
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Description / Table of Contents:
Abstract The planning works realized in the Loire estuary (France) during the course of the twentieth century have brought about an important inflow of sea water to this site. The consequences can be observed through important changes in the location of the extreme limits of both the dynamic tide and the salinity front at 0.5‰; the dynamic tide has progressed from 63 km from the river mouth in 1903 to 100 km in 1992 and the salinity front from 38 km in 1948 to 70 km in 1991. Modifications of the hydraulic conditions have also affected the dynamics of the cohesive sediments present in the estuary. The purpose of this study is to determine the impact of the past and future planning works in the Loire estuary on the erosion and deposition of cohesive sediments in relation to hydraulic conditions. The principal sedimentary processes (transportation, deposition, settlement, erosion) are simulated by simplified models from measurements carried out both in situ and in the laboratory. The dynamics of the cohesive sediment in the Loire estuary are linked with the fortnightly cycle of the tide as follows: when the tidal coefficient falls below 70, almost all the suspended matter is quickly deposited and collects near the estuary bed to form a lens-shaped body of fluid mud which consolidates during 6 days of neap tides. Finally, as soon as the tide coefficient rises above 75, this body of fluid mud is taken again into suspension and only the fraction that was highly concentrated in the course of the settlement remains on the bottom. The sediments remaining on the estuary bed in the course of a fortnightly cycle of the tide represent the residual sedimentation rate over this period. Knowledge of the maximum tide current velocities in the estuary permits the determination of the maximum hydrodynamic stress on the bottom and the critical concentration of the mud susceptible to being eroded by this current, taking into account the yield stress of the mud. In other respects, the quantity of mud that is greater than this critical concentration per surface unit and time unit can be calculated according to a theoretical model developed by Sanchez and Grovel. This methodology allows the evaluation of the residual sedimentation rate representing a layer of 0.08 m per year in front of the harbour of Montoir for the hydraulic conditions of 1976, with a maximum current velocity on the surface of 2.40 m s–1. Following the dredging works realized between St-Nazaire and Donges after 1976, the level of the bottom has changed from –10.00 to –13.25 m (with respect to the hydrographic datum). These works have brought about a reduction in maximum velocities of 15% and an increase in the residual sedimentation rate of about 100%. This increase is in agreement with the theoretical estimation of this study. This study also describes the high sedimentation rate observed in a locally dredged berth and located in the zone of the estuary where a lens-shaped body of fluid mud is formed during slack tides. The sedimentation rate depends then on the frequency of the dredging works because the dredged zone can be filled up in a few days. Finally, a methodology is developed in order to estimate the erosion rate induced by an increase in current velocities. This methodology takes into account the vertical concentration gradient in the bed resulting from consolidation, the hydrodynamic stress linked to current velocity, the yield stress of sediment which varies with concentration and the critical hydrodynamic stress for erosion estimated from experimental measurements in laboratory. All the methodologies presented in this paper have been applied only to the estuary of the river Loire. Application to any other similar site is possible after laboratory experiments on local sediments in order to adjust the behavioural laws used to evaluate the permeability coefficient, the effective stress and the yield stress. This study shows that knowledge of hydraulic conditions and local sedimentary processes is very important for predicting the impact of planning works in an estuary.
Notes:
Résumé Une conséquence des travaux d'aménagement réalisés dans les estuaires est la modification de l'hydraulique, ce qui affecte la dynamique sédimentaire des vases en créant des zones qui s'engraissent et d'autres qui s'érodent. La modélisation numérique permet de prévoir toute modification hydraulique liée aux travaux d'aménagement avec une assez grande précision. En revanche, du fait de la complexité du problème, les modèles hydrosédimentaires, même les plus performants, donnent des résultats plus qualitatifs que quantitatifs et ne permettent pas de répondre à un certain nombre des questions pratiques que peut se poser un ingénieur. Le but de cet article est d'étudier le lien entre les travaux d'aménagement de l'estuaire de la Loire, l'hydraulique, et les problèmes d'envasement et d'érosion observés dans différents secteurs de l'estuaire. Ceci est possible par l'utilisation de méthodes simples, grâce à la modélisation des principaux processus sédimentaires en jeu (transport, dépôt, tassement, érosion).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s100640000047
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