Physics

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Figure 5: Depth profile of salinity between sites H50 and G59 in the Fal Estuary.

Station G59 was closest to the sea, near black rock, and station H50 was the furthest upriver. This was very clearly reflected by the fact that the salinity of each station increased from H50 to G59. Salinity increased with depth at all stations and the greatest variation in a salinity depth profile was found at D53 where the salinity increased by about 1 psu with depth. The salinity was the most constant with depth at G59. There are some spikes in salinity, caused by the lag between the temperature and conductivity sensors on the CTD.

N.B. There was an error with the data for stations C52 and D53 provided by Group 4, resulting in identical data in Figure 5.

Figure 4: Depth profile of temperature between sites H50 and G59 along the Fal Estuary.

The temperature decreased with depth at all stations but the decrease was not constant between stations. There was no clear pattern from warmer to colder temperatures as the stations got progressively closer to the sea. At station G59, the temperature decreased by about 1.5°C over the water column, making it the station with the largest temperature change with depth. Conversely, at H50, the temperature only decreased by roughly 0.5°C with depth. At station F58, there was a sharp temperature increase at about 7m depth but this was down to instrumental error.

N.B. There was an error with the data for stations C52 and D53 provided by Group 4, resulting in identical data in Figure 4.

The flow in the Fal Estuary was measured along transects using the ADCP. To do this, the sampling sites down the estuary were selected. At each point, the transects were started just off the bank of the river. From there, the transect was recorded, taking note of the position and the distance from the shore, as the vessel slowly travelled across the breadth of the estuary. The ADCP recorded flow measurements along the transect. The recording was stopped and the distance from shore was noted once the vessel had reached the other side of the transect. This was done along four points down the estuary.

Figure 6: Transect of station 1, near D-56, from 50° 12.132N, 005° 02.589W to the opposite bank.

The transect of station 1 showed a maximum depth of 15.91m. The greatest flow at this station was shown to be around the surface of the water, reaching velocities of 0.8-0.9 m/s at some points. This could be due to the freshwater flowing over the seawater, while maintaining a higher velocity from further upstream. It also visually demonstrates the shape of the channel.

Figure 7: Transect of station 2, near E-57, starting at 50° 11.309N, 005° 03.287W and ending at 50°11.633N, 005° 01.794W.

The transect of station 2 showed the deepest point being 18.41m. At this point, the flow appears to be quite evenly spread, varying from less than 0.1 m/s up to around 0.9 m/s, but distributed throughout the transect, no one area seems to have significantly different flow to the rest.

Figure 8: Transect of station 3, near F-58, starting at 50°10.330N, 005°01.498W and ending at 50°10.012N, 005°02.768W.

The maximum depth shown by the transect of station 3 was 26.41m. By this point, the velocity of the flow appears to have slowed down more throughout, with a lot more of the cross-section showing the slower colours of the 0.002-0.740 m/s part of the scale. The channel also widens significantly at this site, compared to the previous ones.

Figure 9: Transect of station 4, near G-59, starting at 50°08.895N, 005°01.130W and ending at 50°08.799N, 005°02.489W.

The maximum depth shown by the ADCP along the transect of station 4 was 28.91m. The velocity of flow still appears to be mixed throughout the cross-section, similar to the previous station, with most of the flow appearing to be somewhere in the 0.001-0.666 m/s part of the scale. The main channel is also narrower than it was at site 3, returning to a width closer to the previous two sites.