Conductor

Hi @RobH1 

 

I am from CEE Environmental Scientists and Engineers.

 

There are several interesting points made in the post about the extent of travel of temperature and chlorine discharged at Crib Point.   This first response considers the travel, dilution and effects of the temperature discharge (which involves tidal flows, mixing and the energy balance for WPB) while a subsequent response will consider the travel, dilution and chemical transformations of chlorine.

 

Western Port is dominated by the inflow and outflow of seawater each day due to the tides.  The semi-diurnal tides are the largest (peak range of about 1.9 m) while the diurnal components are smaller (peak range of 0.7 m).  As a result, there are two tides each day (as the semi-diurnal components are the largest) but a pronounced spring tide to neap tide variation due to the diurnal components.

 

Currents have recently been measured across Western Port at Crib Point for the AGL project using acoustic doppler current profiling loggers. These instruments measure currents in 1 m layers from the seabed to the water surface. The plot below shows north/south (blue line) and east/west (red line) tidal currents measured in the mid-water layer near Crib Point Jetty from 19 March to 16 April 2019. The period of record starts during neap tides (low current speeds), progresses through the spring tide high current speeds), then neap, then spring and finishes during the third neap tides of the deployment. In spring tides, the north current reaches about 0.6 m/s (on the flood tide) while the south current reaches about 0.8 m./s (on the ebb tide).    In neap tides, the peak north current is 0.4 m/s while the peak south current is about 0.45 m/s.  

Image1.JPG

 

For this set of measurements, there is a net south current at Crib Point jetty averaging about 2 km/day: the southward water movement on the ebb tide are generally larger than the northward water movement on the flood tide.  There can be periods of a net northerly current in particular wind and tide conditions, although it did not show up in the month of current metering.

 

The current measurements also show a consistent east current, although much smaller, at about 0.38 km/day.  This is apparent in the figure where the east (positive red) currents are generally larger than the west (negative red) currents, resulting in a net movement of water to the east over a tide cycle.

 

The peak northward current speed of 0.6 m/s implies a net water movement to the north (in the flood spring tide) of 8.6 km.  This would transport water from Crib Point to about opposite the Bluescope Jetty.   The peak southward current would transport water from Crib Point to about opposite the Ventnor.

 

These movements are for water and a conservative constituent in the water.   But not temperature, which is a dynamic property of water and varies over the day (due to the combined effects of solar radiation, evaporation and transfer of heat between the atmosphere and the surface water).  Temperature also varies over the year (from about 10 to 24 degrees C in northern Western Port, which is shallow, to about 11 to 21 degrees C in southern Western Port, where the water is deeper and more affected by the temperature in Bass Strait waters).    On any day there can be a 1 to 4 degree C difference between the water temperature in northern Western Port compared to the water temperature in southern Western Port (check baywx.com.au for data on Western Port water temperature).

 

Often there is a 1 degree C variation in the water temperature over a vertical profile – particularly apparent when warm water from over the mudflats flows back into the deeper channels.   The point about this description about water temperature variations in Western Port is to demonstrate that water temperature varies from day to day and with distance along North Arm.  

 

Natural gas is liquefied to form LNG by lowering the temperature to approximately minus 160 degrees C, which I would term “super-chilled”.    AGL are considering returning the LNG back to a gas by heating it with seawater, and the heat-exchange process will result in about 5 m3/s of seawater being discharged back to Western Port via six discharge ports at seven degrees colder than the surrounding seawater.

 

The ports discharge horizontally at a velocity of 5 m/s, which creates intense mixing over the path of the seawater jets from each port.   The typical dilution over the path of each discharge jet is 20:1, which will change the temperature in the jets from 7 degrees C cooler than ambient at the discharge point to just under 0.4 degrees C cooler than ambient near the seabed.

 

The dilution prediction can be checked by considering the ratio of the volume of water in the jets to the volume of seawater passing the vessel in the path of the jets.  The jets discharge 5 m3/s of colder seawater (half on each side so there is 2.5 m3/s discharged on each side of the vessel).   The jets extend for 60 m from the vessel (see diagram below) in a water depth of 13 m.  At the mean tidal current speed of 0.3 m/s, the volume of seawater passing through the mixing volume is 234 m3/s (60 m by 13 m by 0.3 m/s = 234 m3/s).   The ratio of seawater flow to discharge is 93:1 (234 m3/s / 2.5 m3/s = 93).  If there was perfect mixing, the dilution would be 93:1.  But mixing is not perfect, and we estimate the actual mixing to be 20:1.  

 

In the jets at 60 m from the discharge point, the water temperature is just 0.4 degrees cooler than ambient – this could be termed “cooler” water.

 

So to clarify my terminology:

          Superchilled = LNG at -160 degrees C

          Colder seawater is 7 degrees C below ambient

          Cooler seawater is 0.4 degrees below ambient.

 

 

Image 2.png

Next we discuss what happens to the patch of cooler seawater.  It simply mixes with the adjacent seawater as it is carried up and down Western Port by the tidal currents

 

Attached are four videos representations of the rate of mixing of the cooler water from 0.4 degrees C to 0.1 degrees C.   This reduction in temperature occurs within about 200 to 400 m, within the confines of the Port area.

 

After that, the slightly cooler water continues to mix with the Western Port waters.   The measurements indicate the patch (which will become very large and have so little temperature difference, perhaps from 0.01 to 0.001 degrees C, that it will be very difficult to measure due to the larger natural variations in water temperature.

 

Even so, fundamental laws of physics tell us that energy must be conserved.So how will this be achieved?  The answer is that the ever-so-cooler Western Port water will radiate less energy into the atmosphere and evaporate less water into the atmosphere (counter global warming, but not nearly to a noticeable extent).    According to the CSIRO, evaporation is expected to increase by 2 % to 8 % with global warming  The theoretical reduction in evaporation due to the cooler water discharge will be a small fraction of the projected increase.

 

There is no chance that cooler water will accumulate in the Marine Park or at Flinders.   There are two reasons for this statement.

  1. Mixing in tidal currents will reduce the temperature in the ever-expanding patch to such a small difference that it cannot be detected (and certainly the patch cannot hang on as cold water in a dynamic environment with strong currents and mixing);
  2. The daily energy balance will marginally reduce heat transfer from Western Port waters to the atmosphere maintaining thermal equilibrium, with undetectable change from the present condition (not allowing for climate change in the future).

Neap Fall Slow

 

Neap Rise Slow

 

Spring Fall Slow

 

Spring Rise Slow

Consulting Environmental Engineers (CEE) Hydrodynamic Consultant