Mixed sediment transport in a stratified estuary: first insights from a field study

<p>Many estuaries are characterized by a mixture of clay, silt and sand. The erosion, (re-)suspension and transport of these sediments determine the bathymetry and stability of an estuary. Net estuarine sediment transport is the result of multiple processes. In stratified estuaries, gravitational circulation may lead to an inland near-bed sediment transport, which is directed opposite to the net sediment transport higher in the water column. Considering that coarse material is often transported near the bed, while suspended sediment usually consists of finer particles, gravitational circulation may cause a seaward flux of fine sediment and a landward flux of coarse sediment. The New Waterway in the Rotterdam Port area (The Netherlands) is such a stratified channel. Repeated channel deepening has intensified stratification, resulting in a strong salt-wedge type of flow. The channel is continuously dredged for navigation purposes, while the channel would naturally be gaining sediment (Cox et al., 2020). The amount of sediment entering the channel from sea and upstream, and the contribution of different sediment fractions however remain unclear. In this research, we combine  data analysis with numerical modelling to better understand and quantify sediment transport in stratified estuarine channels.</p><p>As a first step, we set up a field campaign which combines flow measurements with determination of suspended sediment characteristics. A measurement frame is equipped with a Sequoia LISST-200x and an YSI EXO Turbidity meter. Suspended sediment characteristics are determined every hour at three depths, next to water temperature, salinity and turbidity. Water samples are taken simultaneously to determine suspended sediment concentration, and flow is monitored continuously using a vessel-mounted ADCP. The full campaign includes two 13-hour measurements and covers two locations in the New Waterway.</p><p>The flow in the upper layer of the water column shows to be decoupled from the saline layer below. Before the flood acceleration phase, the upper and lower layer show an opposite flow direction, corresponding to the findings of De Nijs et al. (2010). The LISST-measurements confirm that suspended sediment in the upper water layer contains a high amount of clay and silt, while the material close to the bed is predominantly sand. This suggests a correlation between grain size and net transport direction. It should be noted that a major part of suspended sediment seems to be transported in the saline bottom layer, and that near-bed processes and local sediment availability could play an important role in the net sediment transport. Continued measurements and the modelling study will further reveal the sensitivity of the net sediment transport to sediment type, and provide insight in the effect of channel deepening.</p><p> </p><p>Cox, J.R., Y. Huismans, J.F.R.W. Leuven, N.E. Vellinga, M. Van der Vegt, A.J.F. Hoitink, and M.G. Kleinhans (2020). “Anthropogenic effects on the Contemporary Sediment Budget of the Lower Rhine-Meuse Delta Channel Network.” Manuscript submitted to Earths Future.</p><p>Nijs, Michel A. J. de, Johan C. Winterwerp, and Julie D. Pietrzak (2010). “The Effects of the Internal Flow Structure on SPM Entrapment in the Rotterdam Waterway.” Journal of Physical Oceanography 40, no. 11: 2357–80.</p>

The influence of the gravitational circulation on estuarine sand dune migration: an idealized modelling approach

<p>Estuarine sand dunes are – similar to river dunes and marine sand waves – large-scale rhythmic bed patterns. Their characteristics differ from their riverine and marine counterparts, owing to the complex and dynamic estuarine environment. Using an idealized process-based modelling approach, we investigate the effect of the gravitational circulation on estuarine sand dunes.</p><p>The gravitational circulation is a residual current typical to estuaries, as it results from a longitudinal salinity gradient. It constitutes a tide-averaged residual flow with an upstream-directed (landward) component at the bed and a downstream-directed (seaward) component at the water surface (Geyer & MacCready, 2014). Sediment transport primarily depends on the bed shear stress (and thus on the flow near the bed), and therefore this residual flow may well be responsible for upstream migration of these bedforms. Observations of sand dunes in the Gironde estuary, France, suggest that this may indeed be relevant to the migration direction of estuarine sand dunes (Berné et al., 1993).  </p><p>We incorporated the hydrodynamic features of the gravitational circulation in a morphodynamic model, which is similar to the one of Hulscher (1996). We then perform a so-called linear stability analysis, which shows that bedforms develop as free instabilities of the flat bed.</p><p>Results show that a longitudinal salinity gradient may cause upstream migration, provided that the river flow velocity is sufficiently small. During high discharge in the Gironde estuary, the salinity front is pushed outward (van Maanen & Sottolichio, 2018), thus increasing the salinity gradient at the position in the Gironde where the sand wave field is situated. Including this in the model shows that the strengthened gravitational circulation can overpower the increased river flow velocities during high discharge, and thus confirms the observation by Berné et al. (1993). We note that this mechanism is probably limited to estuaries which share similar characteristics as the Gironde estuary, i.e. symmetric tide, well-mixed, little wind and wave influence, and a small residual river flow velocity due to a significant increase in cross-sectional area. Future research will elaborate on the effects of (tidally varying) stratification through implementation of a time- and space dependent eddy viscosity.</p><p><strong>References</strong></p><p>Berné, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d</p><p>Geyer, W. R., & MacCready, P. (2014). The Estuarine Circulation. Annual Review of Fluid Mechanics, 46, 175–197. https://doi.org/10.1146/annurev-fluid-010313-141302</p><p>Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662</p><p>van Maanen, B., & Sottolichio, A. (2018). Hydro- and sediment dynamics in the Gironde estuary (France): Sensitivity to seasonal variations in river inflow and sea level rise. Continental Shelf Research, 165(May), 37–50. https://doi.org/10.1016/j.csr.2018.06.001</p>

A Modeling Study on the Influence of Sea-Level Rise and Channel Deepening on Estuarine Circulation and Dissolved Oxygen Levels in the Tidal James River, Virginia, USA

The impact of channel deepening and sea-level rise on the environmental integrity of an estuary is investigated using a three-dimensional hydrodynamic-eutrophication model. The model results show that dissolved oxygen (DO) only experienced minor changes, even when the deep channel was deepened by 3 m in the mesohaline and polyhaline regions of the James River. We found that vertical stratification decreased DO aeration while the estuarine gravitational circulation increased bottom DO exchange. The interactions between these two processes play an important role in modulating DO. The minor change in DO due to channel deepening indicates that the James River is unique as compared with other estuaries. To understand the impact of the hydrodynamic changes on DO, both vertical and horizontal transport timescales represented by water age were used to quantify the changes in hydrodynamic conditions and DO variation, in addition to traditional measures of stratification and circulation. The model results showed that channel deepening led to an increase in both gravitational circulation strength and vertical stratification. Saltwater age decreased and vertical exchange time increased with increases in channel depth. However, these two physical processes can compensate each other, resulting in minor changes in DO. A comparison of the impact of a sea-level rise of 1.0 m with channel deepening scenarios was conducted. As the sea level rises, the vertical transport time decreases slightly while the strength of gravitational circulation weakens due to an increase in mean water depth. Consequently, DO in the estuary experiences a moderate decrease.

Modeling the influence of the gravitational circulation on estuarine sand dunes

<p>Estuaries are hydrodynamically complex regions where a river meets saline water. In many estuaries, sand dunes can be found; these are large-scale rhythmic bedforms. Observational studies have revealed several estuarine processes that affect sand dune dimensions and dynamics. These are for instance sand-mud interactions and tidal amplification. Here, we build upon an observational study in the Gironde Estuary, France, which indicated that the gravitational circulation – present in many estuaries due to the interaction between (heavy) seawater and (light) freshwater – is significant enough to affect sand dunes (Berne et al., 1993). Our aim is to understand the effect of this circulation on bedform dimensions and dynamics, and to explain the underlying mechanisms.</p><p>To this end, we develop an idealized process-based model which contains descriptions for the motion of water and non-cohesive sediment transport within a local section of a generic estuary. On this geometry, we impose a steady river discharge, superimposed on an oscillatory tidal flow. Furthermore, we include the effect of salinity-induced density differences by following the model as presented by MacCready (2004). In here, we adopt a diagnostic approach, meaning that the along-estuarine salinity gradient is imposed on the domain instead of being an unknown which interacts with the flow. The alternative, a so-called prognostic approach, is also explored.</p><p>This model is analyzed using a so-called linear stability analysis, as applied earlier to e.g. marine sand waves (Hulscher, 1996) but not yet to estuarine dunes. Within this analysis, the reference state with a flat bed is slightly perturbed, and the model shows whether these perturbations decay (the flat bed is stable) or grow (it is unstable). The model results provide a generic insight into the role of the gravitational circulation on bedform dimensions and dynamics, particularly growth and migration; the latter possibly directed opposite to the river discharge. To test our model, it is then applied to the specific settings of the Gironde. Furthermore, a systematic sensitivity analysis shows the effect of environmental parameters on bedform development when subject to the gravitational circulation. Including this estuarine-specific process is a novel and first step in obtaining a solid understanding of the behavior of estuarine sand dunes.</p><p> </p><p><strong>References</strong></p><p>Berne, S., Castaing, P., le Drezen, E., & Lericolais, G. (1993). Morphology, Internal Structure, and Reversal of Asymmetry of Large Subtidal Dunes in the Entrance to Gironde Estuary (France). Journal of Sedimentary Petrology, 63(5), 780–793. https://doi.org/10.1306/d4267c03-2b26-11d7-8648000102c1865d</p><p>Hulscher, S. J. M. H. (1996). Tidal-induced large-scale regular bed form patterns in a three-dimensional shallow water model. Journal of Geophysical Research, 101(C9), 727–744. https://doi.org/10.1029/96JC01662</p><p>MacCready, P. (2004). Toward a unified theory of tidally-averaged estuarine salinity structure. Estuaries, 27(4), 561–570. https://doi.org/10.1007/BF02907644</p><p> </p>

Maximum power of saline and fresh water mixing in estuaries

According to Kleidon (2016), natural systems evolve towards a state of maximum power, leading to higher levels of entropy production by different mechanisms, including gravitational circulation in alluvial estuaries. Gravitational circulation is driven by the potential energy of fresh water. Due to the density difference between seawater and river water, the water level on the riverside is higher. The hydrostatic forces on both sides are equal but have different lines of action. This triggers an angular moment, providing rotational kinetic energy to the system, part of which drives mixing by gravitational circulation, lifting up heavier saline water from the bottom and pushing down relatively fresh water from the surface against gravity; the remainder is dissipated by friction while mixing. With a constant freshwater discharge over a tidal cycle, it is assumed that the gravitational circulation in the estuarine system performs work at maximum power. This rotational flow causes the spread of salinity inland, which is mathematically represented by the dispersion coefficient. In this paper, a new equation is derived for the dispersion coefficient related to density-driven mixing, also called gravitational circulation. Together with the steady-state advection–dispersion equation, this results in a new analytical model for density-driven salinity intrusion. The simulated longitudinal salinity profiles have been confronted with observations in a myriad of estuaries worldwide. It shows that the performance is promising in 18 out of 23 estuaries that have relatively large convergence length. Finally, a predictive equation is presented to estimate the dispersion coefficient at the downstream boundary. Overall, the maximum power concept has provided a new physically based alternative for existing empirical descriptions of the dispersion coefficient for gravitational circulation in alluvial estuaries.

Maximum power of saline and fresh water mixing in estuaries

Natural systems evolve towards a state of maximum power (Kleidon, 2016), leading to higher levels of entropy production by different mechanisms, including the gravitational circulation in alluvial estuaries. Gravitational circulation is driven by the potential energy of the fresh water. Due to the density difference between seawater and riverwater, the water level on the river side is higher. The hydrostatic forces on both sides are equal, but have different working lines. This triggers an (accelerating) angular moment, providing rotational kinetic energy into the system, part of which drives mixing by gravitational circulation mixing; the remainder is transferred into dissipated energy by friction while mixing. With a constant discharge over a tidal cycle, the density-driven gravitational circulation in the estuarine system performs work at maximum power, lifting up saline water and bringing down fresh water against gravity. The rotational flow causes the spread of salinity, which is mathematically represented by the dispersion coefficient. Accordingly, a new equation for the dispersion coefficient due to the density-driven mechanism has been derived. Together with the steady state advection-dispersion equation, this resulted in a new analytical model for gravitational salinity intrusion. The simulated longitudinal salinity profiles have been confronted with observations in a myriad of estuaries worldwide. It shows that the performance is promising in eighteen out of twenty-three estuaries, with relatively large convergence length. Finally, a predictive equation is presented for the dispersion coefficient at the boundary. Overall, the maximum power concept has provided an alternative for describing the dispersion coefficient due to gravitational circulation in alluvial estuaries.