Source: \Dropbox\web\raspberry_pi\temperature_analysis\

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Salcombe Estuary 2021-11-13

Water Temperature, Salinity and Turbidity Measurements


An up and down trip, in two kayaks, was made along the Salcombe Estuary starting from North Sands, and turning round at Kingsbridge Quay.

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This trip was an opportunity to try out a prototype turbidity sensor. The turbidity sensor and the conductivity sensor were temporarily attached using thick fencing wire. This seemed to result in considerable drag.

Both sensors were used on the upstream segment and the average speed was about half of normal. On the return trip both sensors were removed from the water for much of the time.

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The trip was undertaken on a Neap Tide with a Tidal Coefficient of 37 (i.e. its height was 37% of the mean height of all Salcombe Spring Tides).

      Trip Timings
  06:47 UTC     Low Tide Salcombe  
  11:09 UTC     leave North Sands  
  13:05 UTC     arrive Kingsbridge Quay  
  13:12 UTC     High Tide Salcombe  
  13:55 UTC     leave Kingsbridge Quay  
  15:15 UTC     arrive North Beach  
  19:42 UTC     Low Tide Salcombe  

The red bars in the graph below show the timing of the two sections of the trip. The upstream section was with the weak incoming tide and the second downstream section was with the outgoing tide.

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Outline Observations

The image below shows (from left to right) temperature, salinity and turbidity.

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Water Temperature Measurements

Temperature measurements were made from both kayaks. As both kayaks followed similar courses the temperatures were very similar.

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At North Beach the water temperature was around 13.5°C, going upstream it decreased to about 12.6°C at Kingsbridge. Returning the profile was very similar.

Using the links below you can zoom and pan the map - clicking on a coloured circle shows the actual temperature measurement at that point.

Going upstream there were a number of sharp decreases in temperature:-

The first was among the pontoons between Ox Point and Black Knob Point. The second was adjacent to Toss Noss Point, and the third was adjacent to the point at the southern end of Park Bay and just north of Kingsbridge Sewage Works.

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Salinity Measurements

Measurements of salinity were estimated at 10 second intervals using a electrical conductivity probe. The conductivity measurements were temperature corrected and converted to salinity values.

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The graph above shows the observed variation of salinity with time on the upstream leg.

The salinity remained relatively constant at around 35-36 ppt (parts per thousand) until about 12:30. After this it dropped erratically to nearly 24 ppt.

The graph shows a number of significant 'dips'.

The first three of these salinity dips occurred at the same time (and place) as the dips in temperature. This implies that we passed from warm saline water into cooler less saline water.

The third salinity dip between 12:46 and 12:52 is shown below. It persisted for the whole width of Park Bay.

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The final two dips occurred on the approach to Kingsbridge Quay. They are shown in the image below as white markers.

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Turbidity Measurements

The following links show the turbidity data overlaid on maps.

The turbidity sensor outputs a frequency which depends on the turbidity measured. The sensor was calibrated using dilute suspensions of diatomaceous earth with concentrations in the range 40 ppm to 20,000 ppm by mass.

The table below shows how the frequency measured corresponds to diatomaceous earth concentrations and to the colours used in the turbidity maps above. The sensor was not optimised for such relatively low turbidities, it will need to be modified to measure such low values with greater resolution.

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The turbidity remains low in the lower parts of the estuary but spikes and increases nearer to Kingsbridge.

The 12:47 peak is just north of the Kingsbridge Sewage Works.

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