Tag dredging

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.

 

Where and what. Dissertation takes shape.

Steve Hall

Having had a boat in the marina for six years and having worked at the marina for a year prior to starting my bachelor’s degree, I knew the management team. We had an informal chat about the possibility of a research project to define and monitor the currents within the marina, and the quantity of suspended sediment entering and leaving the marina at different states of the tide. Unfortunately, Premier Marinas felt the administration and bureaucracy that they would have to adhere to would make the project unworkable for them. A blow for me as I really wanted to try to make a difference to the marinas functionality and environmental impact.

Undaunted, and using contacts I had gained from working at Brighton Marina, I contacted a friend at Shoreham Port Authority about my idea. To my surprise he told me that Shoreham Port was already trying alternative methods of sediment management, and following a few emails, I sat down with the harbour master and it became obvious to me very quickly how enthusiastic and helpful the team at Shoreham would be.

Over the course of a few months I developed a sampling project that Shoreham Port Authority would help me complete. The enthusiasm and positivity of everyone at Shoreham was energising and gave me renewed vigour in the project and in the hope that the project really could make a difference.

Now that I had an idea of the direction I wanted my dissertation project to take, and I had managed to secure support from the port authority, I needed to define what the project would consist of?

To research and design an alternative to backhoe dredging was completely beyond the scope of a bachelor’s degree, and I had to keep reminding myself of this as lecture after lecture inspired me to do more and more. The quality and enthusiasm of the teaching staff at the University of Brighton made deciding exactly what the scope of the project would be incredibly difficult.

As I discovered the amazing characteristics of salt marshes (of which Shoreham has many) and their ability to capture and lock away carbon, out performing even the Amazon rainforest per square metre, I wanted to bring this in to the project. As I learnt about the strength, and complexity of ocean currents, tides and amphidromic points, I wanted to explore and develop these inputs. As I discovered the unique and varied flora and fauna that lives in estuary environments, I wanted to bring this magical world and its distinct battles that it wages every tide in to my project. But I had to keep reminding myself of the constraints and expectations of a bachelor’s degree dissertation project.

So it was that the final project was devised. Using three transects on the western arm of the stretch of the River Adur up to almost to the Sussex Yacht Club from the mouth of the river. I would take samples of water at one metre intervals of depth in three places across the river. This would allow me to build up a picture of the volume of suspended sediment at three distinct locations within the estuary. By sampling at different times of the year, at different states of the tide, and after differing weather phenomenon, I could expand the picture of suspended sediment to try and identify key inputs and factors affecting the type and volume of sediment being carried and deposited in the estuary.

With the boundaries of the project decided it felt like I had moved away from the original concept completely, but upon reflection I realised I could not look at what to do with the sediment until I understand completely the origin, volumes and dynamics of the material I hoped to control.

Following discussions with what became my supervising professor, I also added Acoustic Doppler Current Profiling (ADCP) to the project. This compact and portable piece of equipment can analyse a cross section of the river and show you the direction and speed of the flow. This additional information would allow me to show what the currents in the estuary are doing and help to expand on the energy regimes present in the water. (More to follow on the ADCP in another blog piece.)