Southampton University Falmouth 2015

Group 13

© B Carter

Figure 58 - CTD profile at station 1

Figure 59 - CTD profile at station 2

Figure 60 - CTD profile at station 3

Figure 61 - CTD profile at station 4

Temperature

The data shows a small temperature range from stations 1 -8. The largest vertical temperature range could be seen in the mid estuary and could be related to the degree of mixing. There are generally higher levels of mixing in the upper (riverine) and lower (tidal) estuary. This creates more uniform temperatures in these regions. More uniform temperatures were particularly evident in the upper estuary at stations 1 and 2 with ranges of 0.3 and 0.8 degrees. Higher temperatures could also be seen in the upper estuary. This could be related to riverine inputs and solar heating. A thermocline was apparent at most stations at 5 metres. The thermocline is more obvious in transects closer to the upper estuary.

Salinity

Highest salinities were present towards the mouth of the estuary and the lowest in the upper. The largest salinity ranges was found at station 1, this could be due to a high input of freshwater. The freshwater lies over the top of the saltwater due to differences in density; creating a salt wedge estuary. Indicated by the strong halocline present at station 1. Haloclines were found at all stations and correspond to the thermocline at around 5m. Variations in salinity could also be explained by local variables such as weather or mixing within the estuary.

Richardson number

Figure 62 - Change in Richardson number at Station 2

Figure 64 - Change in Richardson number at Station 4

Figure 63 - Change in Richardson number at Station 3

Figure 65 - Change in Richardson number at Station 5

Figure 68 - Change in Richardson number at Station 8

Figure 67 - Change in Richardson number at Station 7

Figure 66 - Change in Richardson number at Station 6

ADCP Data

Start Time: 08:21:57  End Time: 08:23:33

Transect 1

This transect was approximately 2 minutes in length and 109m in physical length. The net flow was in a southbound direction on the east side of the transect but flowed in a northerly direction and with increased strength in the western side of the transect.

Transect 2

Start Time: 09:13:54 End Time: 09:15:58

For this transect had to average every 5 ensembles to remove areas the ADCP received no acoustic waves back which are shown by white squares in the 3 ensemble transect.

The flow was strongest on the western side of the transect between 7.5m and 11.25m depths. The net flow went in a northbound direction across the transect suggesting that the incoming tidal flow was the dominant flow.

Transect 3

Start Time: 09:50:17  End Time: 09:53:52

Transect 4

Start Time: 10:24:48

End Time: 10:27:17

This transect was 180m in length and went from the eastern side of the estuary to the western. The average flow was in a northerly direction. The velocity of the flow was greatest in the centre of the estuary. There is an area of flow with a low velocity at the surface on the eastern side of the estuary. This could be due to riverine flow acting against tidal flow and therefore decreasing the overall flow velocity of that area.

This transect was 260m long and the ship moved in a north easterly direction. The ship track shows that the net flow had a northerly direction throughout the entire transect. The flow had the greatest velocity in the centre of the transect between 5m and 14m depth.

Transect 5

Start Time: 11:35:25 End Time: 11:38:46

Transect 6

Start Time: 12:15:11

End Time: 12:38:13

This transect was 1760m in length and went from the eastern bank to the western bank of the estuary. On the western bank the net flow was in a southern direction whereas the centre and eastern section of the transect has a net northerly flow. The greatest flow velocities are in the centre of the channel. The channel has two areas of low flow velocity near to the sea bed which could be caused by vegetation or rocks.

Transect 7

Start Time: 13:06:16

End Time: 13:21:32

This transect is 1460m in length and went from the western bank to the eastern bank. The flow velocities across the transect are very low, but the greatest flow velocities are in the western area of the transect which have a southern flow direction. This is because high tide was at 13:17 UTC so the tidal flow will have changed from a northerly direction to a southerly direction. The state of the tide also explains the low velocities. The deepest measurements the ADCP data could take in the deep channel of the estuary were unusually high off the sea bed.

Transect 8

Start Time: 13:58:03

End Time: 14:18:47

This transect is 1700m in length and went from the eastern side of the estuary to the western side. The flow here is in a southern direction. The flow velocities were low in this transect but greater than in transect 7. This is because the southern tidal current is becoming stronger. The greatest flows are in the eastern area of the transect. This could be due to the Coriolis force acting on the current and pushing the current to the right, which will push it towards the eastern side of the estuary in this case.

Disclaimer: The views and opinions expressed are those of the individual and not necessarily those of the University of Southampton or the National Oceanography Centre, Southampton.

Physical Analysis

Calculations

For the calculations a simple equation was used:   In all of the stations the first depth calculated was from 3m as this was the shallowest the ADCP measure the velocities. Then the deepest point was determined by the deepest CTD readings which were used to determine the density values. As a consequence of this, in some stations there are fewer calculated the Richardson numbers. At station 1 only a single Richardson number could be calculated. The change in depth was set to 1m at all stations.

Station 1

There was only a single data point on this graph so it was determined that the graph was not necessary. The Richardson number for this station was 1.74 so the flow was Laminar however this is only for the 3-4m layer.

Station 2

At station 2 there was only a single point where the Richardson number was below the laminar boundary of 1.0. All the other points are well above the laminar boundary with varying values from 5 to over 100. The top and bottom sections of the water have laminar Richardson numbers whereas the middle section is almost turbulent.

Station 3

At this station the top section of the water column has laminar Richardson numbers, which increase in the middle section of the water column. In the lower water column the Richardson numbers decrease significantly. Between 11m and 12m the flow becomes turbulent and remains turbulent for the rest of the measured water column.

Station 4

This station shows a similar trend to above. The top section of the water column has Richardson numbers well above the laminar boundary however with increasing depth the Richardson numbers decrease till they are below the turbulent boundary. The middle section of the water column at depths ranging from 8 -10m have Richardson numbers in the transition zone between the 2 boundaries then there is a single depth where the Richardson number is laminar before becoming turbulent at the last 2 meters of the CTD measurements.

Station 5

The top section of the water column has high Richardson numbers around 10. The middle section is turbulent and then the Richardson number increases into the transition zone in the depths 10-12m. In the bottom section there are 2 points at 12.5 and 13.5m depth that are 0 however these cannot be plotted on the graph as the logarithmic axis cannot show values of 0. The last data point at the deepest depth has a laminar value.

Station 6

At this station there is a general trend of increasing Richardson numbers as you increase depth. In the top section the values start in the transition zone then decrease to the turbulent zone, there is then an increase from 6m to Richardson numbers above 10. After 8m the values begin to decrease with fluctuations, to Richardson numbers in the transition zone between 11m and 13m. After this they then increase to numbers over 100. Again there is a point where the value is 0 at 17m and this cannot be plotted on a logarithmic scale.

Station 7

This station has high Richardson numbers. Starting off in the laminar numbers the Richardson numbers decrease into the transition zone at depths between 8 -9m and then increase back into the laminar zone at depths above 12m. At 17 and 18m there are values of 0 that cannot be plotted on the graph, this is a small spike down to very low Richardson numbers caused by no change in the density at these depths. All the other values around these 2 are over 10.

Station 8

In the upper water column there are no changes in density, this causes Richardson numbers of 0 which cannot be plotted on the graph. As a result the graph does not show much in the upper water column, however when you get past 10m you see that the majority of Richardson numbers are very high, some reaching 1000+ values which indicate that this water is very laminar.

This transect was 238m in length from the southern to northern bank. The pattern that can be seen here is due to Coriolis. The greatest velocities in the centre of the estuary at 10-15m depth and there is an area of high velocity joining the centre area of high velocity on the northern bank of the estuary. The flow is easterly due to a bend in the river.

Figure 69 - Ship track plot for transect 1

Figure 70 - ADCP profile for transect 1 showing the velocities in the water column

Figure 71 - Ship track plot for Transect 1

Figure 72 - ADCP profile for Transect 1

Figure 73 - Ship plot track for transect 2

Figure 74 - ADCP profile for transect 2

Figure 75 - Ship plot track for transect 2

Figure 76 - ADCP profile for transect 2

Figure 77 - Ship plot track for transect 3

Figure 78 - ADCP profile for transect 3

Figure 79 - Ship plot track for transect 4

Figure 79 - ADCP profile for transect 4

Figure 80 - Ship plot track for transect 5

Figure 81 - ADCP profile for transect 5

Figure 82 - Ship plot track for transect 6

Figure 83 - ADCP profile for transect 6

Figure 84 - Ship plot track for transect 6

Figure 85 - ADCP profile for transect 6

Figure 86 - Ship plot track for transect 7

Figure 87 - ADCP profile for transect 7

Figure 88 - Ship plot track for transect 8

Figure 89 - ADCP profile for transect 8

The flushing and residence time of the water is calculated using the Tidal Prism method. For every tidal cycle the ebb tide removes a volume of estuary water: This method assumes that none of the water removed from the estuary during the ebb tide returns on the estuary during the flood tide. It is important to note that the tidal flushing period is often considerably less than the residence time. This is due to a tidal prism removed every tidal cycle, and this is often not the case as water may be trapped in embayments within the estuary. The Tidal Prism method’s flushing time is likely to give to a more accurate reading.

Flushing and Residence Time

Average depth along estuary = 5.892 m

Area of Estuary from Google Earth Pro = approx. 24 475 129 m^2

Volume estuary = 144 207 462.7 m^3

Tidal Range = 4.20m-1.70m = 2.5 m

Tidal Period = 13.17 GMT – 07.18 GMT = 5h 59 min = 359 minutes

The Average depth was worked out by looking at the ADCP transects that we took in the estuary.

This allowed us to work out that the flushing time was 861.1 minutes for the estuary.

To work out the residence time we also needed to have the average salinity of the water in the estuary and in the waters off the shore. This was taken from the CTD profiles we took both in the estuary and offshore on the Callista. We also needed information on the volume of water that was entering the estuary from the Fal and the Kenwyn rivers.

After finding out all these variables we concluded that the residence time of the Fal estuary is 90 days.