Tamar Estuary
Physical Findings…
Overview
Salinity increases sharply with depth in the upper estuary where we can see very
steep haloclines indicative of the interaction between riverine freshwater carried
by the Tamar River carrying fresher lighter water which spills out over denser more
saline water in the estuary. Some salinity structure is noticeable in the mid-
The lowest salinity recorded was that of the surface of station a1 with a salinity of 6 PSU, as opposed to the highest salinity of 35.3 recorded at station j19 furthest seaward.
Detailed Analysis
Figure 10 shows all the salinity data collected from the CTD up the estuary. At stations
a1 to a6 we mostly see a steep increase in salinity with depth down to the bottom
of each profile. Station a7 and a8 show a sharp increase in salinity down to a depth
of 4 and 2m depth respectively, after which there is an inversion and salinity decreases
for around 1-
SALINITY
The vertical density profile up the estuary displayed in Figure 3 shows the potential
density calculated from the in-
The density profiles observed in the Tamar estuary when compared with salinity are almost identical suggesting that salinity is mostly driving the observed density structure in the Tamar estuary. However, at station a10 and b14 the similarity between salinity and density reduces somewhat suggesting that vertical mixing of properties of temperature and lower salinity water maybe affecting this region. Overall, the density structure shows that there is some strong stratification further up the estuary which is mostly controlled by salinity but is also likely to be enhanced by surface warming as a result of increased light levels around this time of year and especially warm weather on the day of sampling. This stratification however, seems to begin to weaken from station a12 where the River Tamar meets the River Tavy and enhanced turbulent mixing is likely to occur by the converging rivers. By c15 this stratification seems to have mostly broken down which coincides with a constricting of the water channel at this location and turbulent flows which are likely to result from fluid interaction around the Royal Albert Bridge marginally south of this location. Further down from c15 the estuary tends to more partially mixed conditions. Stratification does not reform after c15 and any density structure formed up river is progressively lost further down the estuary likely due to the dominance of tidal mixing in the lower estuary.
DENSITY
The following ADCP transect and ship track was taken at 10:20 UTC on the estuary sampling day and shows how the flow velocity is varying across the b14 transect. This ADCP profile gives a good indication of the flow conditions at this location and exemplifies partly why we observe such high Ri numbers at this location.
From the ADCP profile the water depth is below 6m across the channel and that the
tidal currents are weak. Additionally, it is apparent from the velocity vectors shown
on the ships stick track that various vectors point in opposing directions showing
that the tide was starting to turn as this transect was taken. This coincides with
the state of the tide given that high water occurred just before the transect was
taken at 09:56 UTC. The shallow water depth, and the large increases of salinity
with depth, and as a result, a large increase in density (seen Figure 17), with the
added effect of the slower moving water due to weaker tidal currents, mean that it
is possible that this thin layer of water heats sufficiently quickly to develop stratified
conditions which coincides with the high calculated Ri values calculated. It is important
to mention that the Ri values for stations a1 to b14 were calculated using the difference
in density between the uppermost and lowermost water layers and the average flow
velocity at b14 (0.05ms-
The high Ri numbers from stations c15 to j19 were calculated at depth intervals of 1m with velocity data extracted from the ADCP measurements and the density calculated from the Falcon Spirit CTD measurements. The following ADCP transect is taken from d13 at 09:41 UTC which shows larger flow velocities present and an increased water depth than in the b14 transect further upstream where the tide is still flooding.
Further upstream from this location the river constricts and a bridge likely results in mixing and homogenising of stratification. Additionally, the tide is still flooding seen from the ship stick track (see Figure 20) which will be opposing the freshwater influx generating further turbulence and mixing. The homogenisation of stratification resulting from mixing further upstream, the flooding tide and higher flow velocities at this location may be contributing to the observed Ri values <1.
RICHARDSON NUMBER
Above Figure 15. A profile of temperature plotted against salinity for the upper Tamar estuary sampled by RV Winnie The Pooh on 05/07/18.
Above Figure 16. CTD data of temperature plotted against salinity for the lower Tamar estuary. Sampled by RV Falcon Spirit on 05/07/18.
Figure 9. A profile for temperature plotted against depth for the Tamar estuary from
RV Winnie the Pooh and RV Falcon Sprit-
Above Figure 10. Temperature plotted against depth for the upper Tamar estuary, as sampled by RV Winnie the Pooh on 05/07/18.
Above Figure 11. A profile of temperature plotted against depth for the lower estuary, as sampled by RV Falcon Spirit on 05/07/18.
Above Figure 14. A profile of salinity with depth, for the lower Tamar estuary, as sampled by RV Falcon Spirit on 05/07/18.
Above Figure 13. Graph to show the variation of salinity with depth in the upper estuary, as sampled by Winnie The Pooh on 05/07/18
Above Figure 12. A profile of salinity (PSU) against depth (m) for the entire Tamar estuary, as sampled on 05/07/18.
Above Figure 17. Potential density calculated from temperature (°C) and salinity (PSU) plotted against depth (m) from measurements taken along the Tamar estuary collected on 05/07/18.
Above Figure 18: Scatter plot of Richardson Number (Ri) against depth (m) for the lower Tamar estuary, Plymouth, UK, as sampled by RV Falcon Spirit on 05/07/18.
TEMPERATURE
Overview
Temperature data collected from the CTD profiles reflect that, further upstream temperatures were higher than those downstream (see Fig. 9). The range in temperature was 6.4°C, with the highest temperature of 22.2°C recorded in surface waters at Station a7 in the upper estuary, and the lowest temperature of 15.8°C, recorded at Station j19 in the lower estuary at around 10.3m depth near to the breakwater (see Fig. 1 for location of each station). In general warmer, less dense water sits above cooler, denser water throughout the estuary until in the lower estuary we observe areas of complete homogenisation of temperature throughout the water column.
Detailed Analysis
At stations a1 to b14 (except for stations a7 and a14) the surface layer is considerably warmer than in the lower estuary (see fig. 10) and there seems to be more stratified conditions which is observed by the sharp decrease in density. Temperature enhances this stratification and we observe ranges in temperature between 1 and 3°C from the surface to near the sea bed. Stations a7 and a14 are in deeper water where some deviation away from mostly stratified conditions occurs around occurs around 2 to 4m depth where there is a short interval where temperature starts to increase with depth suggesting vertical mixing is taking place around these depths. From stations c15 to g17 the estuary shows temperature profiles where some partial mixing of the estuary may be occurring where stratification seems to break down somewhat as previously eluded to.
This is reflected in the calculated values of the Richardson’s number where up the estuary there are high Richardson’s (Ri) numbers indicative of dynamically stratified flows between station a1 to b14 (see Table 1) and relatively low Ri values in the lower estuary (see Fig. 18) where stratification seems to be breaking down (see Richardson number and stratification).
Furthermore, stations d13 and e16 show three distinct layers of water each with mostly constant temperature through each layer suggesting the presence of mixed layers at different depths. Beyond these more partially mixed areas of the estuary, stations f12 to i18 become far more homogenous in temperature with depth (most apparent for h11) with only slight reduction in temperature with depth most likely a result of strong tidal mixing in this area. The final station, j19 shows that a warmer layer of mostly homogenous water sits above a cooler and mostly homogenous layer of water.
Above Figure 19. Ship Stick track from a horizontal transect near to station b14.
Above Table 1. Table of Ri values for Winnie the Pooh calculated using difference in density between upper and lower water layer and velocity taken from the nearest station for which tidal velocity was available at b14.
Above Figure 20. Ship Stick track from a horizontal transect near to station d13.