Tag Suspended sediment concentrations

Dissertation findings.

Steve Hall Leave a comment

Now that the sampling, lab work and project writeup is all finished, I have detailed below some of my key findings and interpretations of the data. As has been mentioned in previous blog pieces, the biggest single finding from my research has been the paucity of data relating to the processes at play within the Adur Estuary, and how much more research is needed.

So what did I find?

Suspended sediment concentrations (SSC) within the samples were quite consistent across all samples collected. In the end, I collected ten full sets of samples across the three transects, five during Spring tides, and five during neap tides.

I also collected two sets of samples from a single location across an entire tidal cycle. Whilst sitting in the middle of the estuary for 13 hours may not be everyone’s idea of fun, the results I gained from these two data sets (again from both Spring and Neap tides) was really interesting.

The first thing to say about SSC from all twelve data sets is that they are incredibly low. Research conducted in macro tidal estuaries suggested SSC levels should have been in the range of 1000 to 10,000 ppm. The difference in tide heights between high and low water (tidal range) means that there is lots of energy in the water as it flows into and out of the estuary, thus enabling larger quantities of SSC to be present.

My findings for the Adur estuary showed typical results of around 30ppm, an incredibly low figure for a macro tidal estuary.

I would imagine that initial reactions to this will be surprise and relief, after all less SSC means less dredging and restriction to shipping channels. However, it would also mean less sediment available to the salt marshes and mud banks within the estuary to maintain their heights in a world of rising sea levels though.

The type of sediment carried within the water column is very interesting too. Analysis of the sediment contained within the samples, by passing it through a Laser Particle Size Analyser (LPSA) shows that the majority of SSC are silts and clays, with a surprisingly small amount of sand size particles contained within the water column.

LPSA results

The further right on the table the larger the grain size of the SSC gets. The red line demonstrates SSC at the mouth of the river, showing a small increase in sand sized grains.

 

The closer to open water samples were collected, so the concentration of sand size particles increased, but even as far downstream as the RNLI slipway, concentrations of sand particles were very low (<30% by volume).

This lack of sand size particles would perhaps suggest minimal marine sediment intrusion, or a much lower than expected energy regime within the estuary meaning the larger sand particles just cannot be picked up and carried by the water flow.

Data sets produced from a complete tidal cycle show a clearly defined and significant increase in SSC during the middle to later sections of the ebb tide. These spikes in SSC were almost identical during both Spring and Neap tides, also, high water slack tide and the flood tide both showed very reduced SSC levels.

SSC concentrations over an entire tidal cycle, with peak SSC values during the later stages of the ebb tide.

 

These results show that the estuary is an ebb tide dominated regime, with the falling tide being the period during which entrainment of sediment and its transportation within the estuary is at its greatest. These results imply that the Adur estuary is an asymmetrical tidal regime, with the flood tide occurring quickly, followed by a much more extended ebb tide due to the constricted shape of the estuary which extends ~8km inland. This long thin shape of the estuary (exacerbated by modern seawalls and flood defences) means that the retreating tide produces a ‘log jam’ effect upstream and causes the strength and period of the ebb tide to become extended.

These findings need much more empirical research to confirm, but personal anecdotal evidence of the delay between high tide at the estuary mouth and the beginning of the ebb tide upstream from Bramber supports the asymmetrical hypothesis. I found a delay of ~2 hours before the tidal flood waters started to recede after the tide table declaration of when it should have begun.

A Map showing the survey location and the confined but extended length of the river influenced by the tides.

My Dissertation on the Adur Estuary is finished.

Steve Hall Leave a comment

It has felt like a long time coming, but my undergraduate dissertation project is finally finished and submitted. After months of collecting samples, analysis in the lab, and then pondering and considering the results, I feel like I finally start to understand the Adur Estuary a little bit better.

The biggest single realisation after over a year’s work is how much more research is needed to fully understand the estuary and the processes underway there:

  • Accurate and detailed logging of the tidal regime within the estuary and it’s changing dynamics dependant upon the tidal range are needed.
  • The impact the turbidity maximum has on the estuary and the suspended sediment concentrations (SSC), and how the turbidity maximum migrates along the estuary during the tidal cycle.
  • How do SSC from upstream, beyond tidal influences, differ to estuarine waters, and how will recent changes to sea defences change the estuary.

Considering the economic, social, and aesthetic importance of the Adur estuary to the larger area I am amazed how little research has been done into the characteristics and behaviour of the estuary. As a society we are happy to spend millions of pounds on defences, sea walls and protection measures, but do we truly understand the consequences of these changes?

The river Adur channel is highly constricted as it approaches the sea, and abstraction of water from the river seems to be at unprecedented levels and only increasing. Environment Agency data from Beeding Bridge at Bramber shows that freshwater discharge has dropped significantly three times in the last six years (Figure 1), with typical river levels dropping by 30% in just six years. This is at a time when rainfall levels for the South East of England are higher than historical levels, with the last three years (2015-2018) being 105% of the average for the last 100 years.

Figure 1. Freshwater discharge flow in the River Adur, West Sussex. With the black circles highlighting dramatic reductions in flow volumes.

 

So, less water is coming down the river even though more rainfall is falling within the Adur’s catchment.

Why should we care?

Freshwater discharge within the Adur can be seen to have two crucial roles:

1) it helps to bring sediment from upstream down the river (along with nutrients and phosphates from agricultural run-off) thus helping to supplement areas suffering coastal erosion, and help the accretion of areas being inundated by sea level rise (particularly salt marshes and mud flats, both essential habitats);

2) high discharge rates help to flush sediment through the estuaries navigable channels, keeping these working corridors operational, and producing net outflow of sediment into the marine environment, rather than net inflow and ingress of sediment from the sea.

The exact impact of changes to freshwater discharge in the Adur, and in macro-tidal estuaries generally, is still unclear. Our growing demand for freshwater, both for agriculture and as potable water is only going to increase. Should this be at the expense of our estuarine environments?

Filtering out suspended sediment.