Source: \Dropbox\web\raspberry_pi\temperature_analysis\

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Water Temperature and Salinity Measurements - River Dart 2021-08-24


A down-and-back trip, in two kayaks, was made along the River Dart starting from Dittisham, turning round at Mill Bay, before returning to Dittisham, with a break at Warfleet Cove.


The trip was undertaken on a Spring Tide with a Tidal Coefficient of 92 (i.e. its height was 92% of the mean height of all Dartmouth Spring Tides).

      Trip Timings
  07:24 UTC     High Tide Dartmouth  
  10:17 UTC     leave Dittisham  
  11:27 UTC     turn at Mill Bay  
  11:48 UTC     arrive Warfleet Cove  
  13:13 UTC     Low Tide Dartmouth  
  13:42 UTC     leave Warfleet Cove  
  14:50 UTC     arrive Dittisham  
  19:33 UTC     High Tide Dartmouth  

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

Given our timings the outgoing tide was considerably stronger than the incoming one.

      Click image for larger version

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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Heading downstream the water temperature declined slightly from about 16.7°C at Dittisham to about 16.3°C at Mill Cove, before recovering to 16.5°C at Warfleet Cove.

Heading upstream from Warfleet Cove the temperature increased from around 17.1°C to 18.4°C at Dittisham.

      Click image for larger version

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

Salinity Measurements

Measurements of salinity were estimated at 10 second intervals using a electrical conductivity probe.

Given measurements of the electrical conductivity and temperature of water it is possible to estimate its salinity.

Electrical conductivity is the inverse of electrical resistance, and salt water has a much higher conductivity than fresh water.

The probe was installed in a section of plastic tube which was designed to protect the probe from damage. This was attached to the side of the kayak using adhesive tape. Unfortunately on the upstream trip this plastic tube came loose and disappeared. This was only noticed after passing Dartmouth.

Without the support of the cover the probe was bobbing near the surface and this appears to have affected the measurements between 13:49 to 13:58. If the sensor is partially uncovered (i.e. part is of it is in the air) it will record a substantially lower conductivity value.

After 13:58 the cable was lengthened and the probe sank to about 10 cm below the surface, and the measurements stabilised.

As shown in the graph below the relationship between salinity and conductivity of salt water is very dependant on the water's temperature.

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At 11:00:00 UTC the conductivity probe measured 42,220µS/cm when the water temperature was 16.25°C.

The following graph shows the 'raw' salinity measurements assuming that the water temperature was constant at 25°C.

As well as the erroneous measurements between 13:49 to 13:58 there are a number of 'drop outs' affecting individual 10 second readings. These are assumed to be as a result of the conductivity sensor being briefly exposed to the air.

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These 'drop outs' were removed, and clearly erroneous values were removed from the period between 13:49 to 13:58. The salinity was then calculated using the temperature observed at the same time.

This results in the following graph (note the change in vertical salinity scale)

      Click image for larger version

Wikipedia: On average, seawater in the world's oceans has a salinity of about 3.5% (35g/l, 35 ppt, 600 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams of dissolved salts (predominantly sodium and chloride. Average density at the surface is 1.025kg/l. Seawater is denser than both fresh water and pure water (density 1.0kg/l at 4°C) because the dissolved salts increase the mass by a larger proportion than the volume..

The salinity increased slightly on the downstream trip. At Dittisham the salinity was about 32 ppt (parts per thousand), and alongside Mill Bay it was about 34 ppt. It dropped slightly on returning to Warfleet Cove.

On the upstream trip the salinity drops markedly. At Warfleet Cove it was about 33 ppt, and at Dittisham it dropped below 27 ppt.

There are significant wiggles in the salinity graph at the northern end of both legs - this might be associated with the turbulence observed near the Dittisham bend.

      Click image for larger version

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

Salinity Variance

The variance (variability) of the observed salinity was calculated over 2 minute periods (using the 10-second values).

      Click image for larger version

At, and below, the bend at Dittisham the water was quite turbulent with a number of quite large eddies. It was thought that this flow might be responsible for mixing sea and fresher water.

There were some large variances in this area - but also in other areas where the flow appeared to be quite smooth.

Plotting temperature and salinity for the 15 minute period between 14:33 and 14:48 gives the following graph (note that the salinity axis values increase downwards).

      Click image for larger version

In this region it seems that higher temperatures are associated with lower salinity. Traveling upstream the water temperature increased, as would be expected in summer when warmer water flows off the land into a cooler sea. This warmer water starts with low or no salinity but increases in salinity as it mixes in the estuary with the cooler saline sea water flowing in with the tide.

Thus it seems possible that in this region, near the Dittisham Bend, is where water of different temperatures and salinity was being actively mixed.