Evaluation of an Offshore Disposal Site in the Loire Estuary Through Field Monitoring and 3d Numerical Modeling

Abstract

This communication presents the assessment of an offshore disposal site, named La Lambarde, located in open water offshore of the mouth of the Loire estuary in France. The site is used since 1986 by Port Authority of Nantes-SaintNazaire (GPMNSN) to relocate fresh sediment dredged for maintenance purposes in the access channel and port installations located along the estuary. A six-year study, commissioned by GPMNSN authorities to ARTELIA, started in 2007 to optimize the location of the disposal site and improve the knowledge of the suspended sediment regional dynamics.Keywords: 3D, hydro-sedimentary, sand mud mixture, estuary, dredging, dumping INTRODUCTIONThe Loire estuary is one of the three major French estuaries. It is a macro-tidal estuary with a mean spring tidal range of about 5 m allowing the tide to propagate up to Ancenis, 90 km upstream from Saint-Nazaire. The long-term mean discharge of the Loire river is 825 m 3 /s with considerable variations ranging from 60 and 6,000m 3 /s. A dredged navigation channel serves Saint-Nazaire Harbor at the mouth of the estuary and Nantes Harbor 55 km upstream. Between 2004 and 2010, 36.6 million m 3 was dumped in the disposal area of Lambarde ( Assessments performed in 2001 and 2007 have shown that the disposal site is strongly dispersive with about 80% of the 5 million m 3 of very fine sediments dumped annually (mixture of 88% of silt and clay and 12% of sand; in situ density of 1.4) were remobilized by waves and currents and dispersed regionally. This conclusion raised questions among the administration and the stakeholders about the impact of this source of fine sediments on the neighboring coast and activities and about the ways to reduce it. A six-year study, commissioned by GPMNSN authorities to ARTELIA, started in 2007 to solve these questions, optimize the location of the disposal site and improve the knowledge of the suspended sediment regional dynamics. The study was divided into three parts:1. Preliminary study (2009( ) 2. Field campaign (2010( -2011( ) 3. 3D hydro-sedimentary model (2011( -2012 The third part is the main object of this paper. COASTAL ENGINEERING 2014 2 PRELIMINARY STUDYA classical 3D hydrodynamic model was first set-up and trajectories of particles with a critical shear stress from Lambarde site was calculated on the hydrological year 1999. 10 representative sequences of oceano-meteorological conditions were selected. Fig.3 is an example of results for two typical sequences. From these results and statistical weighting, the first annual short-term dispersion pattern was established (Fig.4). The study of patterns of dispersion and time cards exceeded several critical shear stresses allowed to propose three alternative disposal sites. The location of these three alternative disposal sites (Fig.5) is a compromise between different parameters as stability, dispersion, economic, biological, fishing, shipping and others… Field campaign at fixed pointThree fixed stations measurements are implemented, near the actual dumping area, near ...