Anthropogenic effects on the hydro-morphological development of turbid estuaries

Abstract

Estuarine sediment dynamics involves estuarine hydrodynamics, sediment transport, and morphology, and strongly influence ecosystem dynamics and sustainability. In the current geological epoch, referred to as the Anthropocene, human activities are exerting increasing impacts on the environment on all scales, changing hydrodynamics, sediment transport, and morphology in many ways outcompeting natural processes. For sustainable use of these impacted estuaries, human activities must be part of, or compensated by, estuarine sediment management strategies. Such management strategies must be based on detailed physical knowledge of natural sediment dynamics, and the effect of human activities thereon. The Yangtze Estuary is a system where estuarine sediment dynamics is influenced by intensive human interventions in the upstream river basin and within the estuary. This study aims to explore the changes in morphological development and tidal evolution in the Yangtze Estuary on a decadal scale. More specifically, this dissertation differentiates the effects of local engineering works and of the decline of fluvial sediment supply due to human interventions in the upstream river basin, and understand the role of sediments on hydro- and sediment dynamics in highly turbid estuaries.Using a series of bathymetric maps from 1953 to 2016, we investigate the morphological changes in the mouth zone of the Yangtze Estuary. Both shoals in the mouth zone, i.e., the Hengsha flat and Jiuduan shoal, exhibited accretion at different rates until ∼2010, followed by a period of erosion since then. The Hengsha flat accreted slower than the Jiuduan shoal. Moreover, the accretion in the Hengsha flat is mainly ascribed to land reclamation whereas salt marsh introduction strongly contributes to the accretion in the Jiuduan shoal. Erosion in the two shoals after 2010 may be caused by the reduction in silt supply due to dam constructions in the river basin. Erosion of the entire mouth zone that occurred between 1997 and 2010 is mainly the result of local engineering works whose impact masks the impacts of the fluvial sediment decline. Our results further indicate that dredging volumes should be included in the analysis of bed level changes accounting for their large contribution (∼50%) on the erosion after 2010. A response time lag of ∼30 years is suggested to occur in the mouth zone to the riverine sediment decline.The changes in tides are evaluated with the water levels observed at Xuliujing and Yanglin in 1990-1991, 2009-2010 and 2019-2020 and the yearly-averaged high and low water levels between 1996 and 2011 in seven downstream stations. Data reveal a strong reduction in tidal damping from 1990 to 2010, followed by a slightly enhanced tidal damping from 2010 to 2020 in the South Branch. The reduced tidal damping in the South Branch from 1990 to 2010 is controlled by sediment decline which induces an increase in water depth (erosion). In the mouth zone, tidal damping is enhanced from 1997 to 2010 and weakened after 2010. The change in tidal damping in the mouth zone is not as pronounced as that in the South Branch, and the effect of morphological changes is limited. We applied a two-dimensional (2D) barotropic numerical model to explore the effect of bed friction. The model suggests a 60% increase in the effective bottom roughness from 1990 to 2010 in the South Branch, which is probably caused by the observed 80% decrease in suspended sediment concentration (SSC). This effect enhances tidal damping, which counteracts the contribution of water depth increase on amplifying tides between 1990 and 2010 and may dominate the stronger tidal damping from 2010 to 2020. In the mouth zone, the effective bottom roughness mainly becomes larger due to engineering works but may be counterbalanced by the opposite role of fluid mud formation. The influence of fluid mud may become progressively larger, leading to a decrease in friction after 2010. Overall, we have identified that the strong effects of SSC influence tidal dynamics through its impact on bed level and effective bottom roughness. The changes in tides in the South Branch are controlled by the sediment decline whereas the changes in tides in the mouth zone are still dominated by the local engineering works.To obtain insight into the density-induced effects of SSC on hydro- and sediment dynamics in highly turbid estuaries, we set up and calibrated a three-dimensional (3D) baroclinic sediment transport model for the Yangtze Estuary. In this model, sediment transport is supply-limited implying that sediment is prescribed at the model boundaries and not as an initial condition. The computed estuarine turbidity maximum (ETM) therefore results from converging sediment transport pathways and not from local bed erosion, representing equilibrium conditions. Model results suggest that the horizontal and vertical sediment-induced density gradients have an opposite effect: the horizontal sediment-induced density gradients lead to the dispersion of the ETM and the vertical sediment-induced density gradients promote sediment trapping. Vertical sediment-induced density gradients influence trapping directly by reducing vertical mixing but also by indirectly through its effect on water levels, velocities and salinities. Furthermore, comparisons between the dry and wet seasons indicate that horizontal and vertical SSC density gradients are relatively more important under weaker and stronger salinity stratification conditions, respectively.The effect of sediment-induced density gradients is also influenced by tidal asymmetries. To evaluate this effect, we set up a schematized model and carried out simulations with a symmetric tide and asymmetric tide prescribed at the open sea boundary. Depending on the type of tidal asymmetry (represented by the relative phase lag between semi-diurnal and quarter-diurnal tides), sediment-induced density effects strengthen or weaken the ETM due to enhanced or weakened landward tidal pumping, respectively. Higher near-bed sediment concentrations as a result of water-bed exchange processes strengthen the effect of estuarine circulation and therefore promote ETM formation, but simultaneously strengthen the divergence of sediment by tidal pumping.We finally explore the importance of sediment-induced density effects on saltwater intrusion in the Yangtze Estuary for conditions representing climate change and human interventions (changes in river discharge, sediment supply and sea-level rise (SLR)). Changes in river discharge and SLR affect sediment trapping efficiency and ETM location, thereby influencing saltwater intrusion. Therefore, the impact of future changes is influenced by the turbidity of the estuary. For realistic future scenarios, the period in which sufficient freshwater is available at a major freshwater intake (Qingcaosha reservoir), decreases by several days for sediment concentrations below∼2 kg/m3 but may exceed a month for sediment concentrations exceeding 10-30 kg/m3. A 70% decline in the Yangtze sediment load leads to reduced sediment concentrations in the estuary, which leads to a seaward migration of the salt wedge of ∼3 km and an extension of freshwater supply period for over 2 months. A reduction in the sediment load of the Yangtze therefore mitigates saltwater intrusion and water shortage issues.Concluding, the morphology and tides have been regulated by various human interventions in the Yangtze Estuary. The Yangtze Estuary response to human interventions varies spatially: the changes in tides, SSC, and morphology in the South Branch and mouth zone are controlled by the riverine sediment decline and local engineering works, respectively. Temporally, the short-term effects of local engineering works mask the long-term effects of riverine sediment decline (time lag effects). The response of turbid estuaries to interventions is influenced by sediment-induced density effects, introducing feedback mechanisms coupling changes in the hydrodynamics (saltwater intrusion) and sediment dynamics (ETM formation). A better understanding of the estuarine sediment dynamics in response to riverine sediment decline requires detailed monitoring and integrated studies relating the observed changes to sediment-induced feedback mechanisms controlling hydrodynamics and sediment transport.