Longshore drift

Summary

Longshore drift from longshore current is a geological process that consists of the transportation of sediments (clay, silt, pebbles, sand, shingle) along a coast parallel to the shoreline, which is dependent on the angle of incoming wave direction. Oblique incoming wind squeezes water along the coast, and so generates a water current which moves parallel to the coast. Longshore drift is simply the sediment moved by the longshore current. This current and sediment movement occur within the surf zone. The process is also known as littoral drift. Beach sand is also moved on such oblique wind days, due to the swash and backwash of water on the beach. Breaking surf sends water up the beach (swash) at an oblique angle and gravity then drains the water straight downslope (backwash) perpendicular to the shoreline. Thus beach sand can move downbeach in a sawtooth fashion many tens of meters (yards) per day. This process is called "beach drift", but some workers regard it as simply part

Official source

https://en.wikipedia.org/wiki/Longshore_drift

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Development of overturning circulation in sloping waterbodies due to surface cooling

Damien Bouffard, Tomy Doda, Cintia Luz Ramon Casanas, Hugo Nicolás Ulloa Sánchez, Alfred Johny Wüest

Cooling the surface of freshwater bodies, whose temperatures are above the temperature of maximum density, can generate differential cooling between shallow and deep regions. When surface cooling occurs over a long enough period, the thermally induced cross-shore pressure gradient may drive an overturning circulation, a phenomenon called 'thermal siphon'. However, the conditions under which this process begins are not yet fully characterised. Here, we examine the development of thermal siphons driven by a uniform loss of heat at the air-water interface in sloping, stratified basins. For a two-dimensional framework, we derive theoretical time and velocity scales associated with the transition from Rayleigh-Benard type convection to a horizontal overturning circulation across the shallower sloping basin. This transition is characterised by a three-way horizontal momentum balance, in which the cross-shore pressure gradient balances the inertial terms before reaching a quasi-steady regime. We performed numerical and field experiments to test and show the robustness of the analytical scaling, describe the convective regimes and quantify the cross-shore transport induced by thermal siphons. Our results are relevant for understanding the nearshore fluid dynamics induced by nighttime or seasonal surface cooling in lakes and reservoirs.

CAMBRIDGE UNIV PRESS2021

Differential cooling and near-shore dynamics in a large lake (Lake Geneva)

Ulrich Lemmin, Rafael Sebastian Reiss

Density currents generated by differential cooling in lakes with shallow shore regions or side basins are associated with increased vertical mixing, deep water renewal and cross-shore transport of nutrients and other substances. Previous field experiments in Lake Geneva, the largest lake in Western Europe, showed that such cold-water density currents are a common phenomenon during winter, frequently reaching the thermocline at O(100 m) depth. As part of a project investigating large-scale interbasin exchange in Lake Geneva, we examined near-shore wintertime density currents in order to better understand cold-water formation. The study is based on field observations taken on the northern shore of Lake Geneva near Buchillon, 20 km west of Lausanne, in the winters of 2016-2018. Bottom temperatures are inferred from a fiber-optic Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1 m) and O(0.1°C), respectively. Temperature and current velocities in the water column were recorded by ADCP and thermistor chain moorings. The time-series data are complemented by several CTD profiles taken along transects normal to the shoreline near the moorings. Data from a nearby meteorological mast were used to select calm days favoring convective cooling. Earlier studies focused on the steady-state dynamics along one transect. Our goal is to investigate the generation of density currents and the effect of different shore geometries. During periods of surface heat loss, i.e., positive surface buoyancy flux B0, DTS bottom temperatures show cold fronts progressing downslope with the coldest temperatures in the near-shore part, illustrating differential cooling. However, our observations indicate that even during periods with low wind and strong cooling, currents on the shallow shelf have a dominant alongshore component. This suggests that local wind stress and possibly the large-scale circulation might have to be taken into account when investigating nearshore cold-water formation and cooling dynamics in Lake Geneva. The wide and narrow parts of the shelf show significantly different responses to surface cooling. Starting with similar temperatures the wide shelf cooled down by 0.5°C whereas only minor temperature fluctuations were observed at the narrow shelf. At the wide shelf a cold bottom layer of up to 10-m thickness was visible in the CTD transect until the thermocline at 100-m depth. To further elucidate the effect of the shallow shelf width on the generation of cold-water density currents as well as the contribution of both surface heat loss and wind stress a numerical modeling study is planned.

2018

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Numerical simulations were carried out to determine the residence (or flushing) time of water in Vidy Bay (north shore of Lake Geneva) for different meteorological conditions. A hydrodynamic model (Delft3DFLOW) was applied to simulate the flow field in the embayment during 2010 and January 2011. Using these results, particle tracking was applied to estimate transport of wastewater effluent discharged into the embayment. The model predictions compared well with published field measurements of dissolved species (as given by electrical conductivity profiles) within the wastewater. The pelagic boundary of the embayment was defined by the largest within-bay gyre. Based on this definition, particle tracking was used to quantify the residence time under dominant wind conditions. Similarly, particle tracking was used to determine the travel time (i.e., time to exit the embayment) for each of Vidy Bay’s three inflows (stream, stormwater and wastewater effluent). Although the wind field over the lake is variable, current patterns in the embayment can be simulated using the hydrodynamic model forced by a spatially uniform wind field. For a given wind speed, the main factor influencing residence and travel times is the wind angle. The presence of gyres leads to high mean residence times with large variability. As the wind direction becomes more aligned with the shoreline (i.e., with increasing westerly or easterly components), longshore currents dominate. These disrupt gyre formation and markedly reduce the mean and variability of embayment residence time. The numerical model was utilized to assess the potential for plume movement (in plan) from above the wastewater effluent outfall towards one of Lausanne’s drinking water intakes. In the most direct pathway, westward longshore currents can move water from the embayment to the water column above the intake location.

Springer Verlag2014

Differential cooling and near-shore dynamics in a large lake (Lake Geneva)

Ulrich Lemmin, Rafael Sebastian Reiss

2018