Tag sediment

Acoustic Doppler Current Profiler

Steve Hall

Acoustic Doppler Current Profilers (ADCP) are an amazing (and expensive) piece of kit. Mounted on to a float in this case, the unit works by sending acoustic ‘pings’ through the water, and then records the echoes that rebound off the sediment and other objects suspended in the water. The clever part of the ADCP is in its ability to identify not only the presence of items in the water, but also the direction that the water is flowing in down through the water column to the riverbed, as well as the speed at which the water is flowing. When linked to the differential GPS (dGPS) system built in to the unit (and a static GPS base station of the river bank) you can then see a cross-section of the river and identify exactly what is happening below the surface.

In a dynamic and volatile environment like an estuary, water currents direction and speeds are complicated. With salt water and fresh being different densities, you can have a situation (particularly on rising tides) where fresh water is flowing down the river, whilst the heavier salt water is driving upstream beneath the fresh water.

The level of discharge of the river (the physical volume of water flowing down the river) and the tides state will all affect how quickly and how thoroughly these two distinct water bodies mix and combine. Whilst the ADCP cannot tell us the salinity of the water being analysed, the direction and speed of flow can be a good indication of the waters source, especially when combined with salinity readings from the water samples already collected.

Once surveyed, the data collected is downloaded in to GIS (Geographical Information Systems) software. This allows the data collected to be overlaid on to a three-dimensional map, so that visual presentation of the processes underway beneath the surface is straightforward and easy.

Through the support of Dr Annie Ockelford, my supervising professor for my project, and her contacts at the University of Hull, I was able to use their equipment on two separate occasions in May and June of 2018.

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The data collected supplemented and built on the data provided by the water sampling and will hopefully help to produce a more insightful and complete analysis of the processes underway in the estuary.

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Dissertation equipment and practicalities.

Steve Hall

Sampling the river at fixed depths and positions was the first major hurdle to overcome. The equipment I needed, a Van Dorn sampler, is an open tube that descends in to the water, and when it reaches the specified depth, releases the catch to seal the tube trapping the water from that specific depth within it. The issue I had with this was that the equipment cost in excess of £1000.00, money I didn’t have.

After some consideration, modification and improvisation, I built my own. Made from clear PVC piping, bungee straps and toilet plungers, the finished product was effective and had cost a tenth of the professional equivalent.

Because of the limited depths and strong currents within the estuary, lowering the sampler on a rope and hoping that the depth would be correct seemed unscientific and would add too big a margin of error to the results to be satisfactory. Again, online retailers came to the rescue, and a 5 metre long decorators retractable pole provided the perfect combination of lightweight manoeuvrability and precise depth monitoring. So, we were set.

Out on the survey vessel Capella, provided by the port authority, we began sampling. Logging location, time, total depth and sample depth, I began collecting samples to then analyse in the lab (I also added salinity and water temperature to these readings after the first sampling trip). The process was straightforward, aided particularly by the skills of Felix, the pilot of the vessel (an employee of the port authority, and a good friend of mine) and his ability to keep the boat steady and stationary whilst sampling was underway.

Sampling the Adur

Dependant on the condition of the tide, and therefore the total depth of the water, we could collect between 20 and 40 samples in total.

To add a further baseline comparison sample, each time I collect samples from the estuary I also collected a water sample from Bramber Village, approximately 8 Km upstream. Whilst Bramber is still a tidally influenced part of the River Adur, Its distance from open ocean meant that it would provide an interesting comparison to the suspended sediment levels found further downstream.

So the sampling began in May 2018 with regular trips to the river expected over the next eight to ten months.

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A brief history of Shoreham Port.

Steve Hall

The following is an extract from a report for my degree dissertation, on the history of Shoreham Port:-

Image 1. Western arm of Shoreham port, which encompasses the lower reaches of the River Adur.

Shoreham port sits at the mouth of the river Adur in West Sussex. It is a location that has had a working port sited in the area for many hundreds of years (Baggs et al, 1980). Because of the requirement to maintain a working depth of water for vessels to access the port (Image 3), historical attempts to manage the river channel have produced the present format of the port and coastline (Image 2) (Pritchard, 1843: Kleinhans et al, 2010). Consistent and extensive building of a shingle bar from the effects of longshore drift (Bird, 2008) produced what is now Shoreham Beach, and the present location of the river mouth.

Image 2. Map of New Shoreham, circa 1833. Source http://shoreham.adur.org.uk

The continued marine sediment input (Bird, 2008), and movement of the coastline (Dike and Agunwamba,2012) is the first aspect of the sediment budget for the area investigated. Tidal flow continues to provide a sediment input to the area being examined (Pacheco et al, 2007), and beyond as far North as Steyning and Upper Beeding, well beyond the area being surveyed (Environment Agency, 2009).

Image 3. Painting of the River Adur and Shoreham in the background. Circa 1879 by James R Knnear. Royal Pavilion & Museums, Brighton & Hove ©

Fluvial input of sediment is then added to the sediment budget for the lower reaches of the River Adur (Owens et al, 2005: Kirby, 2013). The exact volume, type and particle size of the sediment within the water column will hopefully enable us to quantify the sources of the sediment currently within the water column.

Whilst large areas of the River Adur’s lower reaches encompass mud flats, and these have been examined extensively, we will not be specifically looking at aspects of these (Law et al, 2002; Mudd et al, 2010). Precipitation levels will have an impact on the fresh water volumes passing through the survey area, and will therefore be monitored in the days leading up to surveys, and duly recorded from the nearby meteorological office at Shoreham Airport, or through Environment Agency monitoring stations along the river.

Image 4. Current River mouth and port entrance. Image source S.Hall

Current research at a number of European ports are looking at methods of managing the sediment flow within the water column (Cappucci et al, 2011), and the concept of “Keep Sediment In the System” (KSIS) as developed by Kirby (2013), either by current deflecting walls (CDW) or in-situ conditioning and the use of ‘fluid muds’ (Kirby 2011). These methods are in stark contrast to the more traditional methods of hard engineering (Image 4), dredging and silt pumping (Manning et al 2011).

The scope of the project is to merely assess and try to quantify the sediment transported within the water column. The implications for future development of both the Port Authority’s sediment management regime, and the current or future attempts to manage the sediment within the surveyed area will hopefully be something that can be developed from our findings (Bates et al, 2015).

References

Baggs, A.P., Currie, C.R.J., Elrington, C.R., Keeling, S.M. and Rowland, A.M. “Old and New Shoreham,” in A History of the County of Sussex: Volume 6 Part 1, Bramber Rape (Southern Part), ed. T P Hudson (London: Victoria County History, 1980), 138-149.

Bates, M.E., Fox-Lent, C., Seymour, L., Wender, B.A. and Linkov, I., 2015. Life cycle assessment for dredged sediment placement strategies. Science of the Total Environment, 511, pp.309-318.

Bird E.C.F. 2008. Coastal geomorphology. Wiley. 2nd Edition

Cappucci, S., Scarcella, D., Rossi, L. and Taramelli, A., 2011. Integrated coastal zone management at Marina di Carrara Harbor: sediment management and policy making. Ocean & coastal management, 54(4), pp.277-289.

Kirby, R., 2013. Managing industrialised coastal fine sediment systems. Ocean & coastal management, 79, pp.2-9.

Dike, C.C. & Agunwamba, J.C. 2012, “STUDY ON THE EFFECTS OF TIDE ON SEDIMENTATION IN ESTUARIES OF THE NIGER DELTA, NIGERIA”, Journal of Urban and Environmental Engineering, vol. 6, no. 2, pp. 86-93.

Environment Agency, 2009. Adur Catchment Flood Plan. Available from https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/293867/Adur_Catchment_Flood_Management_Plan.pdf (Accessed 29 March 2018)

Kleinhans, M.G., Weerts, H.J.T. & Cohen, K.M. 2010, “Avulsion in action: Reconstruction and modelling sedimentation pace and upstream flood water levels following a Medieval tidal-river diversion catastrophe (Biesbosch, The Netherlands, 1421–1750 AD)”, Geomorphology, vol. 118, no. 1, pp. 65-79.

Law, R.J., Kelly, C.A., Baker, K.L., Langford, K.H. & Bartlett, T. 2002, “Polycyclic aromatic hydrocarbons in sediments, mussels and crustacea around a former gasworks site in Shoreham-by-Sea, UK”, Marine Pollution Bulletin, vol. 44, no. 9, pp. 903-911.

Manning, A.J., Van Kessel, T., Melotte, J., Sas, M., Winterwerp, H. & Pidduck, E.L. 2011, “On the consequence of a new tidal dock on the sedimentation regime in the Antwerpen area of the Lower Sea Scheldt”, Continental Shelf Research, vol. 31, no. 10, pp. S150-S164.

Owens, P.N., Batalla, R.J., Collins, A.J., Gomez, B., Hicks, D.M., Horowitz, A.J., Kondolf, G.M., Marden, M., Page, M.J., Peacock, D.H. and Petticrew, E.L., 2005. Fine‐grained sediment in river systems: environmental significance and management issues. River research and applications, 21(7), pp.693-717.

Pacheco, A., Carrasco, A.R., Vila-Concejo, A., Ferreira, Ó. and Dias, J.A., 2007. A coastal management program for channels located in backbarrier systems. Ocean & Coastal Management, 50(1-2), pp.119-143.

Pritchard, W.B., 1843. The report of W. B. Pritchard, esq., C. E., to the commissioners of shoreham harbour, on the cause of the existence of the shingle bar at the mouth of shoreham harbour, and the proposed mode of keeping the mouth of the said harbour permanently free. (1843). Architect, Engineer and Surveyor, 4(42), 206-211. Retrieved from https://search-proquest-com.ezproxy.brighton.ac.uk/docview/6705695?accountid=9727

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.)

My dissertation project origins

Steve Hall

Having lived and worked in Brighton Marina for over five years, I had seen at first hand the problems of silting-up of a marina. Accumulation of silt is a problem that almost every port, harbour and marina the world over suffers from.
When the high energy ocean waters enter the calm sheltered safety of a port, the drop in energy levels of the water means that it can no longer carry the same amount of suspended sediment, and therefore deposits its suspended load on to the bed of the port.

It’s important to differentiate here between dissolved and suspended. Content that is dissolved in the sea water has undergone a chemical reaction and will remain dissolved regardless of the energy levels of the water. Suspended material however is merely carried by the water and will be deposited as soon as energy levels drop.
In the few years since moving on to my boat, the silting at Brighton Marina has become progressively worse with access to the marina becoming more and more restricted as the attempts to dredge the fairways and berths fails to keep pace with the sediment build-up.

Currently the standard solution to sediment build-up is to scoop it out using a large excavator (backhoe dredging) and dump it in to a specialised (split) barge, that then transports it out to sea and drops it somewhere they think won’t matter.

This method of sediment management raises a number of issues:-
I} discharge of sea toilets, engine grease, litter, and antifoul are all deposits that are particularly concentrated in marinas and ports. None of these contaminants are positive for the marine environment, and in their often-concentrated forms in port silts can be devastating to marine life.
Ii}as silt builds up, anoxic (low or no oxygen present) mud is produced as there is no penetration of the top few centimetres of the mud. This dark, stinking mud is almost devoid of life, but when disturbed (during the dredging process) quantities of methane can be released; a gas that can be over twenty times more effective as a greenhouse gas than CO2.
Iii} a typical split barge (a vessel that is amazing in itself, as it literally splits in two and allows its load to fall down through the gap), can carry up to 600 tonnes of silt in a single load. When dropped on to the sea bed, away from the marina, this sudden and dramatic inundation of silt can be catastrophic to filter feeders and benthic (bottom dwelling) species.
In Brighton the above considerations must be viewed whilst remembering that the marina lies at the western edge of a marina conservation zone, and that all the silt being removed from Brighton marina is then being dumped in to what should be a protected and conserved area.

Backhoe dredging is not a cheap process, with a typical annual bill of £300,000 to £500,000 not being unrealistic, and at these levels of expenditure silt is still accumulating quicker than it is being removed.
With my personal experience of trying to get my own yacht in and out of the marina, with her 1.8 metre draft, and the knowledge I was gaining during my University of Brighton Earth & Ocean Science Bsc (Hons) course, I felt that there had to be a better solution?
I began to read research papers on sediment management, I learnt about the intricacies of suspension times, currents, coastal littoral cells, and particles sizes. With this growing knowledge and interest in how sediment built up, I approached the marina management at Premier Marinas Brighton to discuss a potential dissertation project for my bachelor’s degree.