Sediment Advection and Influencing Factors

1.1 Particle Sources and Sedimentation

1.1.3 Sediment Advection and Influencing Factors

Sediment advection can incorporate large and small amounts of sediment, and can occur at both fast and slow rates. The type and amount of sediment advected is a factor of the energy input to the system. Advection involving mass sediment movements can be rapid and energetic, as in the case of sediment avalanches induced by earthquakes (Hilbe et al., 2011; Schnellmann et al., 2004) and rock falls (Kremer et al., 2012), and in the case of

Setting the Stage

Figure 1.2 Bernoulli’s Principle and its influence on Entrainment. Adapted from Sloss et al.

(2012)

Sediment Characteristics

Resuspension depends not only on the energy of the system, but also on sediment character-istics including the surface charge, particle density, and particle diameter. Should a particle be too large, too dense, or be cohesive in nature (electrostatic attraction between particles), the shear stress applied by sediment surface currents needs to be much greater to surpass the critical stress required to achieve sufficient lift on the particle for it to be entrained. It has previously been shown that clay and silt can be resuspended with current velocities as low as 2 to 3 cm s⁻¹, while sand could be resuspended at velocities around 20 cm s⁻¹(Lam et al., 1976).

A better understanding of entrainment can be found in the Bernoulli Principle. A particle on the lakebed experiences a drag force that is perpendicular to the gravitational force.

According to Bernoulli’s principle, higher fluid velocities on one side of an object lowers the pressure on this side and subsequently induces a lift component on the object. This is quite the same principle for the lift component experienced by the wings of an aircraft.

Depending on the density of the particle and the velocity of the water flow above it, the particle at rest can be resuspended and transported should the lift component be sufficiently large. A depiction of Bernoulli’s Principle and the forces acting upon a particle are depicted in Fig. 1.2.

Current velocity, particle size, and the water content of the sediment surface govern the processes of erosion, transport, and deposition. Velocity curves, or Hjulström curves, are presented in Figure 1.3 and show the relationship between the particle size and water content as it relates to threshold velocities. The upper curves show the critical erosion

1.1 Particle Sources and Sedimentation

Figure 1.3 Velocity curves showing the relationship between current speed, sediment poros-ity, and particle size on the entrainment of a particle of a given density. Taken from Chapman (1996).

velocity (cm s⁻¹) for different sediment surface water contents while the lowest curve shows the critical deposition velocity. The plots show several concepts regarding the relationships between erosion, transportation, and deposition. For larger diameter particles, friction is the dominant force preventing erosion or entrainment; both the erosion and deposition curves follow each other closely with increases in velocity with increasing particle size. However, for finer, more cohesive sediments (i.e. clay and silt) the erosion velocity is seen to increase with decreasing grain size. This relationship stems from electrostatic forces being of greater significance between these smaller grain sizes (Gargani, 2004).

An example of how increasing current speeds influence entrainment and erosion can be found from the Flix reservoir in Tarragona, Spain. Here, a mass release of pollutants, most notably mercury, affected the local drinking water supply and killed thousands of fish (Her-rero et al., 2013). It was determined that the most probable cause for this acute contaminant

∼500 m³

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organisms may not actually be the suspension of sediments itself, they do alter the sediment micro-surface structure and allow for facilitated entrainment by water currents.

Bottom dwelling fish are known to be one of the main sources of sediment entrainment, where they are found. Ground-dwelling fish in a fjord in British Columbia, Canada, were found responsible for the resuspension of surface sediments from the coastal shelf. Resus-pension by these fish was deemed to be the reason for a 1.7 factor increase in sediment accumulation rate (SAR) with depth between 50 m and 115 m depth (Katz et al., 2012).

These fish,∼10 cm long) were estimated to resuspend approximately 1.3 L m⁻²d⁻¹of bulk sediment (Yahel et al., 2008).

The degradation of organic matter in sediments, both oxic and anoxic, produces various gases, which from time to time escape the sediment and travel to the water surface. This gas ebullition reworks the surface sediments and incites suspension and entrainment of finer particles. The production of these gases is well known to Vidy Bay and is more pronounced upon the extraction of sediments from this area of the lake. Gas ebullition has also been shown to rework sediments in other lakes. One study investigated pockmarks centralized near the Rhine tributary of Lake Constance (Germany). Biogenic gases were found to be released from the edges of sand waves or the sides of old channels carved into the Rhine delta region. Most pockmarks were between 5 m to 8 m in diameter with 0.5 m to 1.5 m depth and were responsible for the movement of large quantities of sediment (Wessels et al., 2010).

Hydrodynamics

The hydrodynamics of a body of water has a significant impact on sediment entrainment and advection. Wind is the main force driving circulation within a lake (Wüest et al., 2003).

Should the lake be large enough, the Coriolis force influences the lake’s circulation. The Coriolis force results in the rightward deflection of a water mass in the northern hemisphere, with leftward deflection in the southern hemisphere.

Internal waves form at density gradients in the water column (typically the thermocline) and are incited by the input of wind energy to the lake. Internal waves take two forms in Lake Geneva, Kelvin-type, and Poincaré. Poincaré waves occur in open water of larger lakes with their reflections generating Kelvin waves when encountering shore boundaries.

The important difference between Poincaré and Kelvin waves is that Poincaré waves extend, with undiminished amplitude, across a basin, whereas Kelvin waves decrease in amplitude away from shore and are thereby “trapped” or constrained to travel along the shore (Wüest et al., 2003).

1.1 Particle Sources and Sedimentation In the case of strong, sustained winds on a stratified system an internal seiche can be produced. This occurs through the epilimnetic build-up at the lee end of a lake due to wind forcing. While the epilimnion is increased in depth, the thermocline is tilted in response.

As the wind subsides, the wind stress is removed and the tilted water mass attempts to achieve a state of equilibrium. However, due to its momentum, the equilibrium state is surpassed and results in a rocking about one or more nodal points (Lemmin et al., 2005).

Addition of the geostrophic effect to the water circulation results in a circular oscillation pattern (or long internal wave), as depicted in Fig. 1.4. The combination of thermocline tilting and the geostrophic effect has previously been demonstrated in Lake Geneva (Bohle-Carbonell, 1986). The importance of these long waves becomes apparent when considering their interaction with water masses and the shoreline boundary.

Internal waves on the thermocline are roughly an order of magnitude or more larger in amplitude than waves found on the surface of large lakes. They propagate and break much the same as surface waves are seen to. The turbulence associated with internal waves is analogous to that at the surface; however, it occurs on a much larger scale. Since these large internal waves break at the sides of the basin, their effects coupled with the vertical movement of the seiche on which they move, are particularly significant. Their vertical displacement has been recorded to attain velocities of 20 cm s⁻¹, which is sufficient for the resuspension and movement of surface sediments (Thorpe et al., 1996).

Ekman transport, in the form of up- and downwelling, can have significant implica-tions for water behaviour near shorelines, particle transport, and sediment resuspension.

Upwelling or downwelling can produce current speeds, which can be sufficient to incite sediment entrainment and transport in the water column (Fer et al., 2002). With this type of transport, should the wind be blowing along a shoreline, water can either be pushed to-wards or away from the shoreline, depending on the direction of the wind. In the case of upwelling, Ekman transport pushes the water body away from the shoreline and hence water from deeper parts of the lake is brought up towards the shore. This typically brings with it nutrient-rich waters and is typical of fishing banks along continental oceanic coastlines. In

Setting the Stage

Figure 1.4 Internal seiche dynamics about the thermocline (Wetzel, 2001). A rotating two-layered internal seiche lake model. Key diagram shows, (i) the oscillating lake surface (heavy line); (ii) the equilibrium lake surface position (thin line); (iii) the equilibrium inter-face position (broken line); and (iv) the oscillating interinter-face (shaded surinter-face).

The two diagrams in the upper left show a hypothetical layer distribution during wind stress, setting a seiche in motion. B and A respectively indicate the wind-driven surface and return currents in the upper layer and C indicates the lower layer.

Eight phases of one oscillation cycle of the first mode internal seiche are shown. Directions of flow in the upper and lower layers are shown by heavy arrows. P is the nodal point, the point of zero elevation change.

1.1 Particle Sources and Sedimentation mann et al., 1995; Palanques et al., 2002) to the development of simple to complex models (Merritt et al., 2003). Resuspension is of great concern in oceanic settings (Baker et al., 1978; Baskaran et al., 2002; Giffin et al., 2003), as well as in large (Eadie, 1997; Lou et al., 2000) and small lakes (Hongve et al., 1995; Kelderman et al., 2011), and in rivers (Herrero et al., 2013; Navarro et al., 2010; Terrado et al., 2006). Resuspended and advected sedi-ments have been shown to account for a significant amount of the total SAR. For example, resuspended sediments were estimated to account for ∼34 % of the SAR in Lake Honda, The Netherlands (Kelderman et al., 2011), and ranged from 10 % to 30 % in offshore traps in Lake Superior (Urban et al., 2004).

Sediment advection has been well studied, albeit predominantly along oceanic coast-lines and in the great lakes (Buesseler et al., 2009; Dibb et al., 1989; Mercier et al., 2007;

Rao et al., 2012; Schmidt, 2002; Urban et al., 2004; Wilson et al., 2007). These studies typi-cally monitor the movement of suspended sediments from shallow coastal zones into deeper waters. Although the study settings differ, their findings are fairly consistent whereby sus-pended sediments are typically found to be transferred in a long-shore direction, as opposed to a cross-shore direction. These findings evidence an an increased sediment accumulation rate in near-shore zones, as opposed to deeper, main basin or profundal zones.

Few studies have investigated water-movement or contaminant transport in bays. Those which have, typically investigate the influence of tidal pumping on contaminant spread since these systems are influenced by oceanic tides (Wilson et al., 2007). This tidal flow occurs in and out of the bay, perpendicular to the mouth of the bay. One interesting publication investigated the formation of a gyre in Blackpool Sands embayment (Devon, U.K.) (Elwell, 2004). This gyre forms from the detachment and recirculation of a main coastal current which runs parallel to the mouth of the bay; much the same case as that found in Vidy Bay. In essence, it has been shown that the formation of a gyre under these hydrodynamic conditions segregates the water packet of the main coastal current from that of the gyre in the bay where the two are seperated by a shear-layer. This shear layer does not readily allow for the trasfer of waters from the bay into the main basin, nor vice versa (Elwell, 2004).

Setting the Stage

Nepheloid, or bottom boundary layers, are well known in oceanic settings (Bell et al., 1983;

Bourgault et al., 2014; Gardner et al., 1985) and also in large lakes (Bell et al., 1983; Gard-ner et al., 1985; Urban et al., 2004). This bottom boundary layer has also been detected in mid-sized lakes and has been estimated to have a mean thickness of∼11 m in Lake Geneva (Bouffard et al., 2013) and to reach up to 15 m in thickness in Lake Lugano (Hofmann et al., 1999) and 10 m in Lake Hallwill (Bloesch et al., 1986). A summary of resuspension and the methods used to measure it can be found in Bloesch (1994).

The identification of resuspended sediments has been attempted through various means.

Sediment characteristics, such as grain size, have been used to identify resuspended sedi-ments, zones of transport and zones of deposition in lakes (Blais et al., 1995; Håkanson, 1977). The composition of trapped sediments has also been used to distinguish between the vertical and lateral (or resuspended) sediment components (Wieland et al., 2001). Natural and anthropogenic radionuclides are commonly used as tracers for sediment and particle transport. The use of long-lived and short-lived radionuclides, most typically¹³⁷Cs, ⁷Be,

234Th, and²¹⁰Pb, have proven useful in the estimation of resuspended sediments and particle pathways (Amiel et al., 2002; Cornett et al., 1994).

The use of single radionuclides can provide good estimates of parameters such as settling velocity and residence times; however, they contain inherent errors based on assumptions of their use. The first being that, with respect to fallout radionuclides (⁷Be and²¹⁰Pb), their in-put into a given aquatic system is mainly driven by wet precipitation events and also on their production in the atmosphere. This introduces seasonal variation which if not accounted for would alter calculation-derived conclusions. The second error is that the use of a single ra-dionuclide cannot necessarily help differentiate between particles descending from the lake surface (vertical component) and those that are resuspended (lateral component) (Dominik et al., 1989; Wieland et al., 1991). However, in using the ratio of a short-lived to a long-lived radionuclide, seasonal production and input variations can be minimized and the influence of lateral sedimentation to vertical sedimentation pathways can be estimated. In order for this to work, the two radionuclides need to be introduced to the aquatic system in the same manner, and they need to behave in a similar manner (i.e. adsorption onto, and transport with, the particles) within the aquatic system.

1.2 Environmental Settings

1.2.1 Lake Geneva

Lake Geneva, also referred to as Le Léman, is the largest freshwater basin in Western Europe in terms of volume and the second largest lake in terms of surface area (surpassed only by Lake Constance, Germany/Switzerland/Austria). Lake Geneva is considered a mid-sized lake and is classified in the warm monomictic category. Due to its size, it is large enough to include all the physical-limnological processes known to lakes (i.e. Coriolis Force, Poincaré waves). Lake Geneva is greatly influenced by the local wind regime. The predominant wind is northerly and is funnelled by the central Alps on the eastern side of the lake and the Jura mountains on the western side. This northerly wind (regionally known as “la Bise”) is the driving source of metalimnetic breakdown and oxygenation of deeper waters. Complete overturn in the lake is typically seen on a six to seven year time scale (Lazzarotto et al., 2006, 2013). Table 1.1 provides some of the characteristics of the lake (CIPEL, 2010).

Table 1.1 Characteristics of Lake Geneva Average Volume 89 billion m³or 89 km³

Surface Area 580.1 km²

Maximum Length 72.3 km

Maximum Depth 309.7 m

Average Depth 152.7 m

Residence Time 11.3 year

The lake is fed from its alpine watershed in Switzerland running into the main tributary, the Rhône River. The Rhône tributary feeding Lake Geneva has an average input flow of 182 m³s⁻¹, while the outflow from the lake has an average flow of 250 m³s⁻¹ (CIPEL, 2010). The Rhône River provides approximately 73% of the affluent waters to Lake Geneva (Loizeau et al., 2003). The particle load of the Rhône River varies between 20 mg L⁻¹ to

−1

Setting the Stage

the main basin and changes in wind direction (Bohle-Carbonell, 1986; Razmi et al., 2013).

The fact that the bay is the most contaminated part of the lake (Loizeau et al., 2004) results in social, economic and health implications for the City of Lausanne and the Canton of Vaud. Vidy Bay is a recreational area and its beach has been closed on occasion due to the health risk associated with high levels of bacteria in the water. These bacteria stem from the local wastewater treatment plant (WWTP), which dumps its treated effluents into the bay. In addition to the WWTP effluents are the effluents from combined sewer overflows, which relieve the wastewater load to the WWTP during periods of heavy precipitation. The overflow effluents bypass the WWTP and the untreated wastewater is diverted directly to the bay. Typical WWTP discharge rates vary between 1 m³s⁻¹to 3 m³s⁻¹, reaching as high as ∼7 m³s⁻¹ during heavy rain events (Razmi et al., 2013). The overflow discharge rate from the Lausanne catchment area varies based on the amount of precipitation received and was around 6 million m³in 2008 which corresponds to approximately 16 % of the collected sewer waters. The majority of this overflow (∼80 %) occurred at WWTPs in the Lausanne catchment area with ∼20 % occurring at combined sewer overflows in the network before arrival at the WWTPs (Assainissement Lausanne, 2009). The only other source of water to the bay is the Chamberonne River, and it has input rates of∼0.2 m³s⁻¹ and∼4.8 m³s⁻¹, for periods of low and high precipitation, respectively (Razmi et al., 2013).

The issue of effluent surfacing on the Vidy beaches was improved in 2001 when the end of the outlet pipe from the WWTP was moved from 300 m offshore (15 m depth), to 700 m offshore (35 m depth). The displacement of the outlet meant that the WWTP effluents were introduced to the bay below the thermocline during the stratified season and reduced the transport of effluents to the shoreline by surface currents.

The WWTP was originally constructed in 1964. It initially consisted of a two-stage treatment plant (mechanical and biological treatments), which was equipped with a chemical stage treatment in 1971. This stage adds ferric chloride (FeCl3) to precipitate dissolved phosphate. In 1976, the WWTP was expanded and the efficiency of wastewater treatment improved.

Due to the combined treated and untreated wastewaters, Vidy Bay is a significant source of metals, organic micropollutants, and faecal indicator bacteria (Haller et al., 2009; Loizeau et al., 2003; Poté et al., 2008). Vidy Bay has previously been studied for its circulation patterns (Goldscheider et al., 2007; Le Thi et al., 2012; Razmi et al., 2013), water-soluble contaminant and pathogen fate (Bonvin et al., 2011; Czekalski et al., 2012), and the spread of various contaminants in the surface sediments (Gascon Diez et al., 2013; Loizeau et al., 2004; Poté et al., 2008). Determining the pathways and dynamics of suspended sediments

1.2 Environmental Settings in Vidy Bay is of great importance to understanding the fate of contaminat-laden particles and is the basis for the following studies.

A

near Rhône-Alpes — France

1 of 1 0

0 55 10 km10 km

B

C

Figure 1.5 A topographic representation of Vidy Bay (map C). The red dot shows the loca-tion of the WWTP outlet, while the red circle denotes the localoca-tion of the outlet prior to 2001.

Maps A and B show the situation of Vidy Bay with respect to Lake Geneva and Switzerland as denoted by the blue arrows.

Setting the Stage

1.3 About This Thesis

The studies presented herein form part of an overall study which focuses on the under-standing of sedimentation processes, the influence of hydrodynamic conditions on sedi-ment transport, and colloid and particle stability as key issues for the question of long-term hydrophobic contaminant transport and water quality in mid-sized lakes, using Vidy Bay (Lake Geneva, Switzerland) as a case study. This manuscript is written as an amalgama-tion of manuscript and article-based formats. Each chapter is self-containing and represents the form of a published article or article in the process of being submitted. In such, minor repetition of common subject matter is inevitable. Each chapter addresses one of the main objectives of the overall study. The first study characterized suspended colloids and particles

Dans le document The fate of sediment-bound contaminants: a case study of Vidy Bay (Lake Geneva, Switzerland) (Page 32-0)