4.3 Environmental Setting, Materials, and Methods
4.4.2 RCM9 Current Meters
The Pearson correlation coefficients for both sites are summarised in Table 4.1. At both sites, p-values indicated that correlations were highly significant (p <0.001), with the ex-ception of temperature and turbidity at NG2 (r = 0.018, p<0.05). The correlation coeffi-cients indicate that all variables were weakly and positively correlated, with the exception of current velocity and turbidity at NG3 which is weakly and negatively correlated (r =
−0.170, p <0.001). The strongest correlations at NG2 lay between turbidity and current velocity (r = 0.328, p<0.001), while the strongest correlation at NG3 was found between current velocity and turbidity (r =−0.170, p<0.001). Regardless of the significance of the correlations based on the high number of observations, all values show small correlations between variables.
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
Table 4.1 Pearson correlation coefficients for RCM9 multimeter data at NG2 (n=15 362) and NG3 (n=9465). Coefficients are found in the top part of the table with the corresponding p-values in the lower portion of the table.
NG2 Current Velocity Temperature Turbidity
Current Velocity ***** 0.201 0.328
Temperature <0.001 ***** 0.018
Turbidity <0.001 0.024 *****
NG3
Current Velocity ***** 0.140 -0.170
Temperature <0.001 ***** 0.092
Turbidity <0.001 <0.001 *****
Table 4.2 Pearson correlation coefficients for seasonal decomposition residues for site NG2 (n=15 362) and NG3 (n=9465). Coefficients are found in the top part of the table with the corresponding p-values in the lower portion of the table.
NG2 Current Velocity Temperature Turbidity
Current Velocity ***** 0.113 0.017
Temperature <0.001 ***** 0.007
Turbidity 0.030 0.378 *****
NG3
Current Velocity ***** -0.007 -0.117
Temperature 0.515 ***** 0.033
Turbidity <0.001 0.002 *****
94
4.4 Results Hydrodynamic Parameter Analysis for the 2010 Lacustrine Cycle
Temperature The temperature at NG2 (138 m) had a mean of 5.59◦C, with a maximum value of 6.51◦C and a minimum of 5.47◦C (Fig. 4.2). Wavelet and spectral analysis of the time series (Fig. 4.3) indicated two broad peaks centred around 150 h (∼7 d) and 575 h (∼25 d) which is typical of Kelvin waves at this depth and time of year in Lake Geneva (Bouffard et al., 2013). Both oscillations are present primarily between October and Jan-uary when seasonal stratification is deep prior to its breakdown. NG3 (192 m) had a mean temperature of 5.50◦C, with a maximum of 5.89◦C and a minimum of 5.45◦C (Fig. 4.2).
Contrary to NG2, temperatures at NG3 do not show any significant oscillations. The tem-perature at NG2 and NG3 showed a significant positive correlation of r = 0.572 (p<0.001) over the 2010 annual cycle (table 4.3).
Current Velocity The current velocity at NG2 had an overall mean of 1.62 cm s−1, with a maximum of 9.48 cm s−1and a minimum of 0.59 cm s−1(Fig. 4.2). NG3 showed surpris-ingly greater velocities than at NG2. The mean current velocity at NG3 was 3.39 cm s−1, with a maximum of 8.05 cm s−1 and a minimum of 1.64 cm s−1 (Fig. 4.2). Rotary spectra analysis of the currents at NG2 and NG3 indicated a clear clockwise oscillation with a pe-riod around 14 h (Fig. 4.4). This signal is the signature of near inertial motion permanently weakly energizing the bottom boundary layer (Bouffard et al., 2013). A broad peak centred at 300 h (12 d) is observed at NG2 but not at NG3. The current velocities at both sites were not significantly correlated (r = 0.010, p<0.001, table 4.3).
Turbidity The turbidity at NG2 had a mean of 1.25 FTU, with a maximum of 3.21 FTU and a minimum of 0.67 FTU (Fig. 4.2). The turbidity at NG2 was markedly greater than at NG3. NG3 had an overall mean of 0.51 FTU, with a maximum of 1.04 FTU and a minimum of 0.23 FTU (Fig. 4.2). The turbidity was noted to have a significant, positive correlation between the two sites (r = 0.360, p<0.001, table 4.3).
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
Table 4.3 Correlation coefficients for circular-circular, circular-linear, and linear-linear 2010 annual cycle hydrodynamic data (n = 8070). Values in the top-right portion of the table are for site NG2, the bottom-left portion for NG3, with the like-variable coefficients between sites NG2 and NG3 running along the diagonal (boxed values). All values are significant at
a p<0.001 level with the exception of values marked with∗ which were found to have p
>0.5.
Current Velocity Current Direction Temperature Turbidity
Current Velocity 0.001∗ 0.004 0.207 0.122
Current Direction 0.018 0.007∗ 0.02 0.015
Temperature 0.173 0.137 0.572 0.147
Turbidity -0.244 0.01 -0.001∗ 0.360
To visually interpret the current direction and velocity data, progressive vector plots (PVP) were produced. PVP’s are a point velocity time series plotted as the vector sum of individual current vectors and can be used to approximate particle trajectories and estimate overall particle transport. The PVP’s for NG2 and NG3 are presented in figures 4.5 and 4.6.
The PVP for NG2 shows small-scale variability in direction and a tendency for the displace-ment to loop back upon itself with a slight overall displacedisplace-ment to the southwest. These small-scale variations reduce the overall displacement of particles in any given direction as is reflective of the smallρ value found at NG2. The tendency for the displacement to loop back upon itself supports the presence of the secondary gyre extending down to the sedi-ment surface. Contradictory to the seasonal decomposition, the current vector displacesedi-ment of particles at NG2 was not found to change with lacustrine seasons, as was the case at NG3.
The current vectors at NG3 showed seasonal patterns with the displacement being signifi-cantly less variable than that of NG2. Currents at NG3 displaced to the west until May 10, 2010, when they shifted to the northeast until September 29, 2010 during thermally stratified conditions. During the breakdown of thermal stratification, displacement was eastward until October 9, 2010, followed by a southward displacement until October 29, 2010. Now with a lack of thermal stratification in Lake Geneva, the vector displacement was northwest until February 1, 2011, when regional winds are typically found to mix the water column of Lake Geneva. From this point until the end of the annual cycle, displacement was to the south-west. For the periods typical of thermal stratification and no stratification, displacements ranged between 13° and 24°, and 308° to 319°, respectively.
96
4.4 Results
Figure 4.2 RCM9 current meter data for sites NG2 and NG3. Data presented has been smoothed using a two-pass moving average.
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
Figure 4.3 Wavelet and power spectral analysis for the time series at site NG2. (a) Tem-perature spectra in the BBL. (b) Fourier transform wavelet spectra. Two peaks, one at 102.2
∼150 h and another at 102.7∼500 h, are shown by the colour bright green. A peak signifies a wave of a given period. (c) The power spectra (mean of the periods) as compared to the significance threshold (dashed line) verifying the presence of the waves.
98
4.4 Results
Figure 4.4 Fourier transform rotational spectra for sites NG2 and NG3. The power rota-tory spectra differentiate between inertial (rotational) oscillations and other wave forms. If both clockwise (red) and counter-clockwise (grey) spectra peak, the oscillation is not rota-tional. If only one spectra peaks, then the oscillation rotates in the direction indicated by the spectra colour. NG2 shows a non-rotational peak at frequency∼10−2.5h−1∼300 h (up ar-row) while both NG2 and NG3 show a clockwise oscillation at frequency∼10−1h−1∼12 h (downwards arrows).
Bottom boundary layer hydrodynamics and sediment focusing: implications for a contaminated bay
Figure 4.6 Progressive vector plot for site NG3 from the period April 2010 to the end of March 2011. The PVP shows distinct directional patterns relating to the seasonal changes in the main basin currents.