Formation and structure of the turbidity maximum in the macrotidal Charente estuary (France): Influence of fluvial and tidal forcing

Abstract

a b s t r a c tUnderstanding estuarine sediment dynamics and particularly turbidity maximum dynamics is crucial for the management of these coastal systems. Various processes impact the formation, movement and structure of the turbidity maximum. Several studies have shown that tidal asymmetry and density gradients are responsible for the presence of this suspended sedimentary mass.The Charente estuary is a highly turbid system (with suspended sediment concentrations often in excess of 5 g/L) that remains poorly understood despite its strong impact on local activities. In this study, a three-dimensional hydrosedimentary model is developed to represent the sediment dynamics of this estuary. Model validation demonstrates good accuracy, especially on reproducing semi-diurnal and spring-neap variability. Several simulations are performed to evaluate the influence of tides and river discharge on the turbidity maximum. Mean and maximum suspended sediment concentrations (SSC) and sediment stratification are calculated. SSC transects are also used to visualize the suspended sediment distribution along the estuary.The turbidity maximum generally oscillates between the river mouth and the Rochefort area (20 e30 km upstream). The model shows strong variations at different time scales, and demonstrates that SSC is mainly driven by deposition/resuspension processes. Spring-neap comparisons show that the turbidity maximum is not well-defined during neap tides for low and mean runoff conditions. Simulations of spring tides and/or high runoff conditions all result in a compact suspended sedimentary mass.Performing simulations without taking density gradients into account demonstrates that tidal asymmetry is the main mechanism leading to the formation of the turbidity maximum. However, density gradients contribute to maintaining the stability of the turbidity maximum. Vertical stratification traps sediments at the bottom. Longitudinal stratification ensures a sharper edge at the downstream limit of the suspended sedimentary mass, preventing a massive export of sediments.

Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season

“…This trend of shorter length at high tide is consistent with the analysis of the Charente ETM by Toublanc et al [19]. It is likely explained by the salinity distribution.…”

“…The convergence at the saltfront of landward sediment flux by the gravitational circulation and seaward sediment flux by the river flow is considered as a fundamental mechanism contributing to ETM formation [17][18][19]. In most estuaries, tidal resuspension has been recognized as a key factor in maintaining high SPM concentration [13,[20][21][22].…”

Suspended Sediment Dynamics in the Macrotidal Seine Estuary (France): 1. Numerical Modeling of Turbidity Maximum Dynamics

“…Turbulent mixing also decays from the upper estuary down to the stratified saline intrusion zone, so that sediment settling (and trapping) is enhanced in the latter area (Geyer, ). Recently, Toublanc et al () concluded that, in the highly turbid Charente Estuary (with SSCs often exceeding 5 g/L), the tidal asymmetry was the main mechanism leading to the ETM formation, whereas the density gradients contributed to maintaining the ETM stability. Vertical stratification trapped sediment at the bottom and the longitudinal density gradients ensured a sharper concentration gradient at the downstream limit of the ETM, preventing a massive export of sediments.…”

Influence des conditions hydrométéorologiques sur les flux et bilans sédimentaires en estuaire de Seine

“…YU et al, 2014 ;MCSWEENEY et al, 2016 ;JALON-ROJAS et al, 2016 ;TOUBLANC et al, 2016). De plus, la remise en suspension des sédiments sur les bancs à l'embouchure et les opérations de dragage le long des chenaux de navigation jouent un rôle très important (AVOINE et al, 1981 ;LE HIR et al, 2001 ;MARMIN et al, 2014).…”