Vertical current profile

An investigation of the vertical variation in current speed and current direction over the water column was performed using the measurements at the Hesselø F-LiDAR. As mentioned previously (see Section 2.2.3) the instrumentation setup for this measurement station changed during the survey:

• 2021-03-01 to 2021-07-14, at 1.6 m intervals from 6.0 m to 39.6 m below sea surface

• 2021-07-17 to 2021-09-27, at 1.6 m intervals from 3.6 m to 37.2 m below sea surface

The analysis presented in the following treats these two periods individually.

Figure 5.34 shows examples of the measured vertical current speed profile at the Hesselø F-LiDAR for 12 randomly selected output times between 01 March 2021 and 14 July 2021. The vertical structure is not coherent, and the largest current speeds may occur at almost any position in the water column; hence, it may not be suitable to adopt a standard vertical current profile such as the 1/7^th power law for tidally dominant flows (e.g., as described by equation 2.31 in Section 2.4.8.3 of [25]).

Another way to illustrate the variation in the vertical current profile is by means of a statistical analysis of the measured current speed and direction relative to the depth-averaged values. Figure 5.35 and Figure 5.36 display normalised current profiles during normal conditions (herein represented by all profiles for which the depth-averaged current speed is larger than the 50% quantile). The following general set of characteristics were inferred:

• The ratio of the current speed at bin level to the depth-averaged current speed shows a large variation at all levels and is largest in the upper part of the water column

• In the upper part of the water column (i.e., from the near-surface layer to

~20 m below the surface) it is more likely that the current speed is larger than the depth-average current speed (U/CS > 1). At lower levels (i.e., from ~20 m below the surface to near-seafloor) the current speed is more likely to be less than the depth-averaged current speed (U/CS < 1).

• At all levels, the phase shift in the current direction relative to the depth-averaged current direction can be ±180°

• The most probable phase shift is close to 0° in the upper part of the water column (i.e., from near-surface to ~20 m below the surface) with more deviation from the depth-averaged current direction at lower levels (±30°) Figure 5.37 and Figure 5.38 display normalised current profile plots conditioned on depth-averaged current speeds that are larger than the 99% quantile. The most probable values of the current speed ratio and the phase shift are similar to those presented in Figure 5.35 and Figure 5.36, however the variability of the values is markedly lower.

Figure 5.39 and Figure 5.40 display normalised of the current profiles

conditioned on near-surface current speeds (i.e., the depth bin that is closest to the surface) that are larger than the 99% quantile. In the upper 10 m of the water column the ratio of the current speed at bin level to the depth-averaged current speed was, in all cases, larger than 1. The most probable value increased towards the surface, suggesting the presence of wind-generated surface currents (e.g., see equation 2.3.2 in Section 2.4.8.3 of [25]). The phase shift in current direction in the upper 10 m displayed less variability than at depth.

Figure 5.34 Measured current speed profile at Hesselø F-LiDAR

The images show 12 randomly selected profiles between 01 March 2021 and 14 July 2021

Page 123 Figure 5.35 Normalised vertical current profiles (50% depth-averaged current exceedance) at

Hesselø F-LiDAR (2021-03-01 to 2021-07-04)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-03-01 to 2021-07-14, conditioned on depth-averaged current speeds larger than the 50% quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each level

Figure 5.36 Normalised vertical current profiles (50% depth-averaged current exceedance) at Hesselø F-LiDAR (2021-07-17 to 2021-09-27)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-07-17 to 2021-09-27, conditioned on depth-averaged current speeds larger than the 50% quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each level

Page 125 Figure 5.37 Normalised vertical current profiles (1% depth-averaged current exceedance) at

Hesselø F-LiDAR (2021-03-01 to 2021-07-04)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-03-01 to 2021-07-14, conditioned on depth-averaged current speeds larger than the 99% quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each level

Figure 5.38 Normalised vertical current profiles (1% depth-averaged current exceedance) at Hesselø F-LiDAR (2021-07-17 to 2021-09-27)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-07-17 to 2021-09-27, conditioned on depth-averaged current speeds larger than the 99% quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each level

Page 127 Figure 5.39 Normalised vertical current profiles (1% near-surface current exceedance) at Hesselø

F-LiDAR (2021-03-01 to 2021-07-04)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-03-01 to 2021-07-14, conditioned on near-surface current speeds larger than the 99%

quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each level

Figure 5.40 Normalised vertical current profiles (1% near-surface current exceedance) at Hesselø F-LiDAR (2021-07-17 to 2021-09-27)

Statistical representation of measured current profiles at the Hesselø F-LiDAR for the period 2021-07-17 to 2021-09-27, conditioned on near-surface current speeds larger than the 99%

quantile value. Left hand panel: current speed at bin level (U) normalised by the depth-averaged current speed (CS). Right panel: the phase shift of the current direction at bin level () to the depth-averaged current direction (CD). The red markers show the most probable value for each bin

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