Movies of Currents in Port Stephens

(Brian Sanderson 2013, [email-masked])

Introduction
A multiple-nested, control-volume, 3D
hydrodynamic model is used to model tidal currents in Port Stephens. The model
used a combination of third- and fourth-order accurate numerical methods. A
split-explicit scheme is used. Temporal differencing is third-order accurate
Lorenz (1971) N-cycle. Advection uses third-order upwind-weighted spatial
interpolations.

Here we only present results from the highest-resolution nested-model for
comparison with drifter measurements made offshore from Pindimar. Drifters
were released from the proposed location of an outlet pipe. The model is
forced at its eastern boundary using water levels measured at Tomaree.

Study Area

Depth (in metres relative to AHD) of Port Stephens is
shown by the coloured map above. The total area of the above map is
modelled using a three-dimensional hydrodynamic model with horizontal
grid-scale 150 m. The area shown by the bigger black box was modelled
using a grid-scale of 50 m with two-way nesting within outer model.
The smaller black box shows the area, near Pindimar, which was
modelled using a grid-scale of 25 m with two-way nesting.

KR, Karuah River

LP, Lower Pindimar

MR, Myall River

TC, Tilligerry Creek

SB, Salamander Bay

NB, Nelson Bay

Bathymetry of Karuah River has not been
measured. A guesstimate representation of the bathymetry of Karuah
River is included. A truncated version of Myall River is included.

Drifter Measurements
Drifters were deployed from the
outlet position and tracked for short periods (typically 20 minutes).

The top plot shows trajectories of drifters
released from the proposed outlet location (red circle). The intake is
indicated with a red cross. Contours and gray scale indicate depth.
Extent of the existing of Posidonia meadow is indicated with
green dots.

The following current velocities are calculated from the drifter
trajectories. Components of velocity are: u positive towards east and
v positive towards north.

Date, start time --- end time

u (cm/s)

v (cm/s)

6-Apr-2012 10:45 --- 11:10

-5.2

-8.0

6-Apr-2012 11:15 --- 11:30

-8.7

-13.3

6-Apr-2012 11:35 --- 11:50

-4.8

-12.8

6-Apr-2012 12:00 --- 12:20

-4.2

-9.4

6-Apr-2012 12:23 --- 12:38

-6.6

-8.0

6-Apr-2012 12:45 --- 13:00

-8.1

-13.1

6-Apr-2012 13:03 --- 13:18

-10.6

-14.7

07-Apr-2012 10:30 --- 10:50

6.1

0.8

07-Apr-2012 10:53 --- 11:13

2.6

-4.5

07-Apr-2012 11:15 --- 11:35

4.3

-6.4

07-Apr-2012 11:37 --- 11:53

-0.8

-8.7

07-Apr-2012 11:55 --- 12:20

7.1

-8.2

07-Apr-2012 12:22 --- 12:44

9.9

-5.2

07-Apr-2012 12:50 --- 13:08

6.9

-0.3

The middle plot shows water level measured at Tomaree. Red
dots indicate times of drifter deployment.

The bottom plot is a movie showing:

Modelled current as black arrows.

Blue arrows show measured drifter displacements normalized to
the displacement over an interval of 20 minutes.

Bottom depth is indicated by background colour.

The model coastline is drawn with a white line.

Outlet position is drawn with a red circle and intake position
with a red cross.

Modelled currents closely agree with
measurements on 6 April 2012. Measured currents are more offshore than
modelled currents on 7 April 2012. The model gave similar offshore
currents a little earlier at locations a little to the south-west.
Given the turbulent nature of the eddy fields, such differences are to
be expected.

The top plot shows trajectories of drifters
released from the proposed outlet location (red circle). Contours and
gray scale indicate depth. Extent of the existing of Posidonia
meadow is indicated with green dots.

The following current velocities are calculated from the drifter
trajectories.

Date, start time --- end time

u (cm/s)

v (cm/s)

14-Apr-2012 09:41 --- 10:20

0.46

-14.2

14-Apr-2012 10:23 --- 10:46

-6.7

-14.5

14-Apr-2012 11:11 --- 11:30

-12.4

-12.7

14-Apr-2012 11:34 --- 11:59

-12.1

-4.5

14-Apr-2012 12:02 --- 12:25

-11.7

-6.1

14-Apr-2012 12:27 --- 12:55

-8.6

-2.6

12-May-2012 09:16 --- 09:32

-12.7

-23.2

12-May-2012 09:36 --- 09:53

-14.6

-18.0

12-May-2012 10:00 --- 10:17

-9.5

-8.4

12-May-2012 10:20 --- 10:37

-7.6

-5.2

12-May-2012 10:39 --- 10:57

-5.0

-1.9

12-May-2012 10:59 --- 11:17

-4.5

0.93

12-May-2012 11:19 --- 11:37

-1.4

3.6

12-May-2012 11:39 --- 11:54

1.3

6.9

12-May-2012 11:56 --- 12:13

1.4

7.7

12-May-2012 12:15 --- 12:47

3.6

1.8

12-May-2012 12:51 --- 12:57

4.5

7.1

12-May-2012 12:59 --- 13:13

0.38

7.7

12-May-2012 13:15 --- 13:38

0.30

1.7

12-May-2012 13:42 --- 13:48

1.7

11.7

The two middle plots show water level measured at Tomaree.
Red dots indicate times of drifter deployment.

The two bottom plots are movies showing:

Modelled current as black arrows.

Blue arrows show measured drifter displacements normalized to
the displacement over an interval of 20 minutes.

Bottom depth is indicated by background colour.

The model coastline is drawn with a white line.

Outlet position is drawn with a red circle and intake position
with a red cross.

Modelled currents are generally consistent
with measurements on 14 April 2012.

Modelled currents are generally consistent with measurements on 12
May 2012. The model slightly underestimates the very weak onshore
current that was measured near slack water.

The top plot shows trajectories of drifters
released from the proposed outlet location (red circle). Contours and
gray scale indicate depth. Extent of the existing of Posidonia
meadow is indicated with green dots.

The following current velocities are calculated from the drifter
trajectories.

Date, start time --- end time

u (cm/s)

v (cm/s)

29-Jul-2012 11:30 --- 11:40

6.1

-12.0

29-Jul-2012 11:40 --- 11:48

4.5

-12.5

29-Jul-2012 11:37 --- 11:45

4.8

-13.0

29-Jul-2012 11:45 --- 11:55

3.1

-11.4

29-Jul-2012 11:55 --- 12:08

2.5

-5.07

29-Jul-2012 11:52 --- 11:59

1.4

-10.6

29-Jul-2012 11:59 --- 12:09

-0.31

-11.7

The middle plot shows water level measured at Tomaree. Red
dots indicate times of drifter deployment.

The bottom plot is a movie showing:

Modelled current as black arrows.

Blue arrows show measured drifter displacements normalized to
the displacement over an interval of 20 minutes.

Bottom depth is indicated by background colour.

The model coastline is drawn with a white line.

Outlet position is drawn with a red circle and intake position
with a red cross.

Modelled currents are generally consistent
with measurements on 29 July 2012, although the measured current is
directed slightly more offshore.

The top plot shows Trajectories of drifters
released from the proposed outlet location (red circle). Contours and
gray scale indicate depth. Extent of the existing of Posidonia
meadow is indicated with green dots.

The following current velocities are calculated from the drifter
trajectories.

Date, start time --- end time

u (cm/s)

v (cm/s)

4-Aug-2012 10:00 --- 10:26

6.7

6.9

4-Aug-2012 10:06 --- 10:29

6.8

6.8

4-Aug-2012 11:04 --- 11:31

11.8

9.1

4-Aug-2012 11:16 --- 11:37

11.2

10.6

4-Aug-2012 11:36 --- 11:48

12.4

14.2

4-Aug-2012 11:41 --- 12:01

18.0

13.0

4-Aug-2012 11:52 --- 12:07

18.7

14.7

The middle plot shows water level measured at Tomaree. Red
dots indicate times of drifter deployment.

The bottom plot is a movie showing:

Modelled current as black arrows.

Blue arrows show measured drifter displacements normalized to
the displacement over an interval of 20 minutes.

Bottom depth is indicated by background colour.

The model coastline is drawn with a white line.

Outlet position is drawn with a red circle and intake position
with a red cross.

Modelled currents are generally consistent
with drifter measurements on 4 August 2012, although the measured
current is a little stronger.

Dispersion of Discharged Material from the Outlet
The
three-dimensional model was run for a period of 2.6 days during spring tides
and a period of 3.5 days during neap tides. In both cases discharge with
normalized concentration 1 was advected into the cell adjacent the bottom at
the outflow location. The pipe flow was set to 50 ML/day, consistent with the
operations for the planned abalone farm. Below we show the dispersion of
material emmanating from the outlet pipe, as it is advected and mixed by the
modelled currents.

Discharge Plume during Spring Tides
Movie of the
vertically-averaged concentration of the plume during spring tides.
The source concentration was 1 (normalized). The first tidal
cycle is not shown, this being a spin-up period. Note that the
color scale is for the range 0 to 0.1 in order to resolve
concentrations in most of the plume. Concentrations higher than 0.1
are localized near the outlet position.

Discharge Plume during Neap Tides
Movie of the
vertically-averaged concentration of the plume during neap tides.
The source concentration was 1 (normalized). The first tidal
cycle is not shown, this being a spin-up period. Note that the
color scale is for the range 0 to 0.1