Falmouth Field Course 2017

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Estuarine Physics

Time Series

In Figure 4 salinity rapidly increases along the River Fal between from around 32.8-34, when the river meets the estuary at ~12.55 UTC the increase in salinity slows. In the estuary, the salinity continues to increase moving southward towards the mouth, moreover, between 12:55 – 14:05 UTC salinity increases from ~33.6 – 34.8. After 14:10, as can be seen in Figure 3, the Conway heads back inshore, therefore on the time series salinity is seen to decrease slightly. Salinity is seen to increase towards the mouth of the estuary as fresh, less dense water sourced from the River Fal and other tributaries mixes with denser more saline seawater. The Fal Estuary is dominated by the tidal cycle, therefore at high tide ~14:00, salinity is at its highest in the estuary, this can be seen when analysing the time series plot. After 14:00 as discussed above, due to the Conway returning to dock, salinity was recorded as decreasing. The results observed are to be expected as (when referencing to the salinity plot from Conway) the data recorded suggests the estuary has a salt wedge structure. Furthermore, fal estuary is a complex system comprising of the Fal and Helford ria systems (drowned river valleys) with both river systems receiving low freshwater inputs (Natural England, 2017). Therefore, we can assume the estuary has a stratified structure with a halocline of ~1.2 observed over the time series.

On the other hand, temperature can be seen to be highest at the head of the estuary where the surface water is warmer, after 13:10, temperature rapidly decreases as the Conway travelled further south where colder seawater is mixing at depth with the warmer river water. As seen previously with the salinity, the temperature increases as the Conway heads back inshore. Over the time series conducted, a thermocline can be seen with ~1.8 °C change indicating a stratified structure which is within the expected thermocline range in UK coastal waters.

Figure 3. Time series ships track plotted every ten minutes. Location, salinity and temperature variables were recorded every five minutes.

Figure 4. Line graph showing the temperature and salinity conditions over the time series.

References

Natural England. (2017). Designated Sites View. Retrieved from Natural England: http://designatedsites.naturalengland.org.uk/Marine/MarineSiteDetail.aspx?SiteCode=UK0013112&SiteName=acle&countyCode=&responsiblePerson =&SeaArea=&IFCAArea=07860630937

Galperin, B., Sukoriansky, S., & Anderson, P. S. (2007). On the critical Richardson number in stably stratified turbulence. Atmospheric Science Letters, 8(3), 65-69.

Temperature

Results:

Figure 1 shows the data collected from the depth profiles and demonstates the change in temperature (°C) with increasing depth (m) for each of the 5 stations. All sample stations display an overall decrease in temperature from the surface measurements to the deepest measurement of 25.76m which was recorded at station 9. Station 13 had the highest surface temperature of 16.3050°C while station 10 had the lowest recorded surface temperature of 15.5810°C. Station 9 shows the most dramatic decrease in temperature, from 15.8790°C to 13.8050°C, whereas station 12 had the most vertically linear trend, with a small decrease from 15.9830°C to 14.8950°C.

Salinity

Results

Figure 2 shows the data collected from the vertical profiles and demonstates the change in salinity (PSU) with increasing depth (m) for each of the 5 stations. All sample stations display an overall increase in salinity from the surface measurement to the deepest measurement of 25.76m which was recorded at station 9. Station 10 had the highest surface salinity of 33.8500 PSU while station 12 had the lowest recorded surface temperature of 32.7180 PSU. Station 13 shows the most dramatic decrease in salinity, from  34.34 to 33.1560 PSU, whereas station 9 had the most linear vertical trend, with a small decrease from 34.212 to 33.773 PSU.

Figure 2. Salinity depth profiles for the Fal Estuary Stations.

Figure 1. Temperature depth profiles for the Fal Estuary stations.

Discussion:

Temperature decreases with depth as we had hypothesised as stratification causes warmer, less dense fresh water to sit over denser, more saline water. Therefore the denser underlying water body absorbs less heat energy then the overlying water body as irradiance decreases exponentially with depth. Additionally temperature changed along the surveyed horizontal transect, from station 12, 13, 11, 10 and 9. Mean surface water temperature in coastal waters is usually 16°C in the summer and high irradiance leads to greater buoyancy potential. The warmest three surface temperatures, Station 11, 12 and 13, were situated the furthest inland up the estuary (towards the riverine end), these locations have a lower water volume and so require less energy to reach higher temperatures. The seaward end stations had lower recorded temperatures due to the higher energy reguirements to heat the larger water body.

Discussion

Salinity increases with depth as denser more saline water sits below less dense water bodies of riverine origin which possess an overall lower salt concentration. The further down the estuary we travelled the greater the salinity due to the tidal imputs. Tidal mixing increases with distance to the mouth of the estuary and so decreases stratification and creates more vertical salinity profiles.

Richardson Number

The Richardson number gives an indication of stratification and therefore the strength of density gradients in the water column (Galperin, et al., 2007). Ri values <0.25 refer to turbulent flow and therefore the destruction of density gradients, values >1 indicate laminar flows with anything between 0.25-1 suggesting a transition between states.

Station 9 at the mouth of the estuary is the only site where at all depths, Ri<0.25 therefore water is well mixed; Turbulence is to be expected due to vertical mixing of seawater and estuarine water masses resulting in the breakdown of gradients. All other stations show a range of Ri values from turbulent to laminar flow suggesting further up the estuary some stratification still exists where warmer, less dense riverine water sits on top of denser, cooler seawater.

Figure 5. Scatter graph showing the Ri values at each estuary site with depth