The impact of storms and stratification on sediment transport in the Rhine region of freshwater influence

Abstract

We present measurements of along and across‐shore sediment transport in a region of the Dutch coast 10 km north of the Rhine River mouth. This section of the coast is characterized by strong vertical density stratification because it is within the midfield region of the Rhine region of freshwater influence, where processes typical of the far‐field, such as tidal straining, are modified by the passage of distinct freshwater lenses at the surface. The experiment captured two storms, and a wide range of wind, wave, tidal and stratification conditions. We focus primarily on the mechanisms leading to cross‐shore sediment flux at a mooring location in 12 m of water, which are responsible for the exchange of sediment between the nearshore and the inner shelf. Net transport during storms was directed offshore and influenced by cross‐shelf winds, while net transport during spring tides was determined by the mean state of stratification. Tidal straining dominated during neap tides; however, cross‐shore transport was negligible due to small sediment concentrations. The passage of freshwater lenses manifested as strong pulses of offshore transport primarily during spring tides. We observe that both barotropic and baroclinic processes are relevant for cross‐shore transport at depth and, since transport rates due to these competing processes were similar, the net transport direction will be determined by the frequency and sequencing of these modes of transport. Based on our observations, we find that wind and wave‐driven transport during storms tends move fine sediment offshore, while calmer, more stratified conditions move it back onshore.

Wave Generation of Gravity‐Driven Sediment Flows on a Predominantly Sandy Seabed

“…The high concentrations observed during the WSGF resulted in elevated cross‐shore sediment transport (Figure d). The net cross‐shore sediment flux due to the WSGF event was an order of magnitude greater than the suspended load transport in the overlying water column (0.5 mab to the surface) measured nearby at the 12 m site (Flores et al, ) or the bedload transport based on the formula of Madsen () for the same time period (Figure d). In fact, the offshore transport during this 1.5‐day WSGF event was much higher than the suspended sediment transport observed during a 30‐day period at the nearby 12 m site.…”

“…While the emplacement of the Sand Engine undoubtedly provides a large source of coastal sediment, monitoring has shown that the particle size distribution around it has coarsened since its construction (Huisman et al, 2016). Moreover, observed suspended sediment concentrations are in the same range as those observed prior to its construction (Flores et al, 2017;Horner-Devine et al, 2017;van der Hout et al, 2015). This suggests that the Sand Engine does not provide a large source of fine sediments.…”

“…The measurements presented here document a WSGF event during the first major storm of the 2014 fall season (Figure c), which coincided with a period of low river outflow; discharge from the Rhine River was persistently below its annual average of 2,500 m 3 /s during the 30 days prior to our deployment (Figure d). High waves and strong tides occur in this region and are mostly responsible for the observed sediment transport patterns (Flores et al, ; Pietrzak et al, ). Depth‐averaged tidal currents are on the order of 1 m/s and severe storms during the fall and winter generate wind waves that can easily reach 4–5 m. Nonetheless, the conditions and setting are unique relative to previous observations of WSGF.…”

The Influence of Tide and Wind on the Propagation of Fronts in a Shallow River Plume

“…The orange circle represents a meteorological measurement station, and the star represents the location for discharge data. (c) Detailed schematic of the measurement setup (Figure after Flores et al, ). A third frame is placed at 12 m depth.…”

Flow Regimes and Adjustment to Wind‐Driven Motions in Lake Pontchartrain Estuary: A Modeling Experiment Using FVCOM

“…Journal of Geophysical Research: Oceans, 123, 8460-8488. https://doi.org/10. 1029 in offshore or open ocean waters (e.g., Flores et al, 2017;Horner-Devine et al, 2015), estuaries are semienclosed (Cameron & Pritchard, 1963;Hansen & Rattray, 1965, 1966 and lagoons can be more limited in exchange with the coastal ocean (Kjerfve, 1986;Kjerfve & Magill, 1989) and may even have multiple openings (Duran-Matute et al, 2016;Herrling & Winter, 2015;Jia & Li, 2012;Li, 2013). The constrictions of such system at entrance can act as low-pass filters that diminish tidal influence into the basin (Fernandes et al, 2004) or change the fortnightly tidal phases (Hill, 1994).…”