Podcast: Sediment science, shipping and Liverpool Bay

Liverpool waterfront

Liverpool waterfront

25 June 2013 by Sue Nelson

This week in the Planet Earth podcast, Alex Souza of the National Oceanography Centre and Jon Rees of Cefas talk about how their work could save the dredging industry money, maintain coastal defences and protect salt marshes.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, using new research to improve the marine environment and save money in one of our most important commercial ports. I'm Sue Nelson and today we're alongside the River Mersey. The port of Liverpool handles around 700,000 containers each year and when a second terminal is built here some of the world's largest container vessels will be docking along the river and also in Liverpool Bay. Now, where I am at the moment is, sort of, down the estuary of the Mersey I can see Garston Docks right alongside me, across on the Wirral there is the oil refineries of Elsmere Port and I am on the rather lovely sounding Grassendale Esplanade with Dr Alex Souza from NERC's National Oceanography Centre in Liverpool and Jon Rees from Cefas. Well, Alex, we'll begin with you, let's describe exactly where we are in terms of what's going on with the water, the estuary here.

Dr Alex Souza: We're here close to the head of the estuary where the fresh water of the River Mersey starts mixing with the salty water of the Irish Sea. This is quite important because this helps drive different currents which gets as far away in the Liverpool Bay itself and this is important how it drives the currents and the sediment ultimately.

Sue Nelson: Now we can see a few vessels in the Mersey at the moment. One of them is probably more than likely to be a dredging vessel. What does dredging actually do?

Dr Alex Souza: Dredging - what it actually does, it digs out into the sediment of the estuary of the river or whatever in the water and takes all that sediment up into the vessel which is then disposed in a different area. The idea is that you deepen the channels so that boats can sail up. So if you have bigger boats coming in you have to dredge even more to let them into the port.

Sue Nelson: And what happens to that material that you do dredge? Where does it go?

Dr Alex Souza: It has to be disposed in different areas and these areas are selected in different ways. In the case of Liverpool it has been historically on different sides and in the Mersey we have three sides, one is just across the water from when we are sitting and there are two further up.

Sue Nelson: Is there any way here that you can tell? I'm looking across the Mersey now and there's nothing physically, or to my eyes, that makes me think, oh, there's a dumping site.

Dr Alex Souza: You couldn't see it with the eye. You can probably see some slight difference in the waves if there was too much dumping into it just at the time you are putting the sediment, or you can see clouds of what the sediment is doing but in reality it would be very hard to see it by eye.

Sue Nelson: And here in Liverpool, what materials is the sediment made up from?

Dr Alex Souza: It is basically sand which is a coarse material and we all have some [unintelligible], so it's a combination of fine and coarse particles that you have here and they kind of behave differently depending if you have really fine or heavier particles how they move.

Sue Nelson: And this is where we come to your work isn't it, because you've been tracking the way these particles move. Why?

Dr Alex Souza: Well the idea is to understand how the particles move, once the sediment moves, once you put it into the water and what we've been doing is what we call particle tracking, which is you put in a numerical model you put the small particles into it and give them properties of the sediment and see how it moves. What we're trying to do in Liverpool itself is to see what was the effect of the fresh water gradients into driving the sediment in the bay.

Sue Nelson: And what did you find?

Dr Alex Souza: We found that, at least, in areas like Liverpool Bay or the Dutch coast where fresh water is important in driving the physics of the system you have to put that into your model. If not the sediment is going to behave in a very different way of what we expect it to be doing.

Sue Nelson: Studies like these have been done before but what makes your study slightly different is that you've incorporated density. What difference does density make?

Dr Alex Souza: Density has primarily an effect in driving what we call the residual current which is the long-term currents, longer than tides. It is smaller than the tides but this has a residual impact while the tide is only coming backwards and forwards. It also modifies the tides but the most important thing, I think, is how your residual currents behave.

Sue Nelson: Now more recent work on the Mersey estuary here was commissioned by Cefas, the Centre for Environment Fisheries and Aquaculture Sciences, and Jon Rees you're here from Cefas' Lowestoft laboratory. How are you going to use this research?

Jon Rees: These numerical models and the particle tracking from these models are as essential in understanding the whole of the marine environment and the sediment regime within the estuary because we need to know what is happening to these particles over short times and long periods as well around the estuary. So the three main drivers associated with that are a sustainable sediment management plan for the whole of the estuary, understanding what is going to happen to the sediments and how they impact on biota, for instance, the estuary is a special protection area for birds, and the third reason that we're very keen to understand what's going on in the estuary on these very small scales and the long scales is that we want to make sure that industry don't spend too much money on actually dredging their areas and that the sediment that they dispose of in the licensed areas doesn't go straight back from the licensed area back into their dredged channel and therefore they have to do too much dredging in those locations.

Sue Nelson: So you're finding the optimum places in an estuary. Have you found any other places other than the three that Alex mentioned?

Jon Rees: We're looking at all of these existing licensed sites at the moment to make sure that they are robust sites for the future. We're making sure that we're future proofing these sites and looking to see if they're sustainable. At the moment we've not made any decisions on these sites but in terms of the amount of material that is being disposed of, which is half a million tonnes a year, which is a lot of sediment, compared with what goes on in the whole of the UK, we need to make sure that we have a good understanding of the fate of the sediment particles, both the silts and the sands.

Sue Nelson: Now, Cefas acts as technical advisors to the marine management organisation. You've touched on it with birds, but how is this work going to actually improve the marine environment in the Liverpool area?

Jon Rees: Well in terms of the environment around here we have quite a lot of salt marshes etcetera, so we need to make sure that they're maintained over the long-term, especially with climate change and changing in sea level rise around the coastline, the salt marshes are under a lot of pressure and we need to make sure that any dredging that goes on doesn't impact on these salt marshes or any of the coastal defences around the area as well, so making sure that homes and houses are protected. The overall aim of what we're trying to do is to make sure that we have a win-win situation. That we're protecting the environment and at the same time ensuring that industry doesn't have any excessive costs or monitoring they have to do associated with the dredging.

Sue Nelson: With the second port, Alex, that's going to mean even more ships coming along here and even more dredging then, I assume?

Dr Alex Souza: Well, yeah, it is part of the study that Cefas is looking at, at how much new sediment will be put into these areas and see how we can manage it in a better way and manage its long term impact into the system.

Sue Nelson: You did touch on, very briefly, you said Dutch estuaries. So have you been looking at other countries as well to see whether the same research can help them?

Dr Alex Souza: Yeah. These can be applied in any area where estuarine input into the coast is important and we're working together with Dutch colleagues in similar projects in looking at the Rhine outflows, so, yeah, definitely.

Sue Nelson: Dr Alex Souza and Jon Rees, thank you both of you for meeting me here. We'll put some pictures of today's recording on our Facebook page and you can also find us on Twitter, just search for Planet Earth online. I'm Sue Nelson and from near Garston Docks on the River Mersey that's the Plant Earth podcast from the Natural Environment Research Council.