Numerical modeling of bedload and suspended load contributions to morphological evolution of the Seine Estuary (France)

Abstract

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

Maintenance dredging in a macrotidal estuary: Modelling and assessment of its variability with hydro-meteorological forcing

“…Sediment is dredged and dumped according to specific rules given by port authorities. Main model characteristics are shortly reminded in sections 3.1 (hydrodynamics) and 3.2 (sediment dynamics), while detailed can be found in Mengual et al (2020). The methodology for computing sediment dredging and release is detailed in section 3.3.…”

“…The hydrodynamic model is coupled to the process-based sediment model for sand and mud mixtures MUSTANG (MUd Sand TrANsport modellinG) (Grasso, 2015;Le Hir et al, 2011;Mengual et al, 2017;Mengual et al, 2020). This multi-class sediment model computes both As detailed in Mengual et al (2020) the model simulates bedload according to Wu and Lin (2014) and distinguishes the three erosion regimes described in Le Hir et al (2011): noncohesive, transition from non-cohesive to cohesive, and cohesive, depending on the surficial mud fraction. The threshold mud fraction between non-cohesive and transition increases with the averaged non-cohesive sediment diameter to represent the decreasing ability of the mud to impede sand or gravel movement.…”

“…Hydro-sedimentary validation of the model in term of water level, wave propagation, SPM dynamics and morphological evolutions is provided in Grasso et al (2018) and Mengual et al (2020). However, the few adjustments operated on bottom friction coupling (section 3.1) have slightly modified some validation scores.…”

“…Suspended sediment concentrations had been thoroughly validated by Grasso et al (2018), but a slight change of parameterization had been introduced by Mengual et al (2020) in order to get a better agreement on morphological changes at the time scale of several years (see also Grasso et al, 2020). As a result, the agreement between the model and observed bathymetric evolution is quite reasonable: when splitting the 150 km 2 of the mouth into 7 boxes, model results and observations can be compared in terms of global sediment accretion or erosion: only one box shows opposite results.…”

“…The present work deals with the mouth of the Seine Estuary (France) which is submitted to the maintenance dredging of two major ports, Le Havre Port close to the mouth, and Rouen Port localized more upstream. Within the so-called Seine-Aval Scientific Program, a processbased model for hydrodynamics, sediment transport and morphological evolution has been developed for many years (Grasso et al, 2018;Grasso and Le Hir, 2019;Lemoine et al, 2020) and recently upgraded (Mengual et al, 2020). The interest of the modeling approach lies in the possibility to explore many physical processes likely to impact dredging quantities.…”