High Definition Hydrographic Survey in Extreme Tidal Sites

To start us off, please give a short personal and professional introduction.

[Christian Armstrong] I'm a PhD candidate here at the University of Highlands and Islands based at the Scottish Association for Marine Science. I've been looking at how the seabed changes on the west coast of Scotland, and in short I design, organize, and implement bathometric surveys, looking at the seabed in what we call extreme tidal locations to determine the relationship between tidal flows and seabed change.

[Pim Kuus] My name is Pim Kuus and I'm a product manager and senior hydrographer at Teledyne Marine. Within Teledyne Marine, I'm responsible for the E20 single beam, the SeaBat multibeam, the Hydrosweep multi-beams and the ParaSound Sub Bottom Profiler. That involves the development of the product life cycle, but also looking forward at what's happening in the future.

Christian, can you give us an overview of the SAMS project?

[CA] The project looks to repeatedly map the seabed of extreme tidal sites by collecting time-series bathymetry data; essentially re-surveying the same area several times. Extreme, in this context, is defined by tidal flow speeds that exceed three meters per second or around six knots. So I'm particularly interested in quantifying how the seabed responds to these high-energy environments over the course of months to years.

These high-energy seabed environments can be very dynamic, especially where the elevated flow speeds transport sediment. As a result, these huge sedimentary structures are generated, not dissimilar to dunes that you might find in the desert. They can be several meters high, hundreds of meters long and mobile.

These bedforms are quite important, especially considering the increased interest in offshore energy development. There's a need to better understand these high-energy environments and how to build effectively and safely within them. Seabed instability induced by these mobile bedforms affects the seabed's suitability to construction. In essence, by better understanding the nature of these mobile bedforms we hope to improve the cost effectiveness of offshore construction.

Image courtesy The Scottish Association for Marine Science (SAMS)

The Falls of Lora is an awe-inspiring place. It's at the mouth of Loch Etive, a glacially over-deepened estuary, about 150 meters deep with shallow, narrow, constrictions turned sills resulting from past glacial activity … numerical simulations suggest that (water flow is) about eight knots. It really rips through there. Christian Armstrong,
PhD candidate,
University of Highlands and Islands.

I understand that the Falls of Lora, Loch Etive was chosen as a test site. Can you discuss the specific physical characteristics of the site with insights on what it was about this site that makes gathering information so difficult?

[CA] The Falls of Lora is an awe-inspiring place. It's at the mouth of Loch Etive, a glacially over-deepened estuary, about 150 meters deep with shallow, very narrow, constrictions turned sills resulting from past glacial activity.

The Falls of Lora occur because of one of these sills and because of the shallowing, the tidal flow becomes restricted. It's the difference in tidal height and the adjacent marine environment that causes the flow speeds to increase quite dramatically.

We don’t have an absolute quantification of this, but the numerical simulations suggest that it's between three and four meters per second, about eight knots. It really rips through there. Clearly, it's an energetic site and its location is ideal for us because we are about a kilometer away here at SAMS. During an initial survey in 2014, we identified some peculiar bed forms that attracted further investigations, so we chose this site as an ideal location for the development of novel methods (to study the site). It's not been without its challenges, and deploying the pole mounted RESON Seabat T50meant that we could only effectively survey at slack water (that lasts about an hour) given these flow speeds.

Despite these tidal dynamics, the biggest challenge came from the effects of rainfall. With Loch Etive having quite a large catchment area, a lot of rainfall funnels into the interlock and out through the Falls of Lora at the mouth. When the fresh water meets the sea water it becomes layered due to the different densities. This stratification is quite dynamic with the vertical position of the interface between the two water densitieschanging both spatially and temporally. We also had to fit into that 30-minute slack water window, numerous sound velocity profiles to correct for how the sound propagates from the sonar into the dynamically stratified water. So, a challenging place to survey, but worth it.

Images courtesy The Scottish Association for Marine Science (SAMS)

Images courtesy The Scottish Association for Marine Science (SAMS)

Images courtesy The Scottish Association for Marine Science (SAMS)

Images courtesy The Scottish Association for Marine Science (SAMS)

Why was Teledyne's RESON SeaBat T50-R Multibeam Echosounder chosen for the study?

[CA] The T50-R is a remarkable piece of kit. It's compactness was ideal, as we needed something that could be mobilized extremely quickly given our slack water window. This system had to be fitted on the end of a generic pull system used to mount other sonars. We've had problems in the past with calculating the offsets for between the transmitter, the receiver and the IU and the antenna system. With everything for the T50-Rbeing mounted first to a mounting plate then being put onto the sonar pole or the pull system, a lot of the offsets were already pre-calculated. That made deployment incredibly efficient and meant that we could deploy it on vessels of opportunity. It didn't matter which vessel we were deploying on, we could just turn up and go.

Then there's the data quality. The 3D waterfall display for this system revealed the seabed pre cleaned in incredible detail. And I really have to credit the high quality to the software's automated tracker functioning, where the sonar parameters are essentially tuned. The tuning that would normally be done by a professional surveyoris tuned autonomously, filtering out any water column noise and the like. So even an amateur like me can get it right, which is great.

Lastly, I must give credit to the normalized backscatter function, a really incredible software advance. When you're tuning all these or all these parameters, rather being tuned autonomously, the backscatter is also fed these tuning parameters and it normalizes the backscatter so that we get a nice clean, intensity data, a nice clean backscatter of data, which you can visualize in real time without any spikes of the data or without any irregularities caused by this autonomous tuning. So a really incredibly efficient system that required very little cleaning. Cleaning took minutes instead of hours.

Then there's the data quality. The 3D waterfall display for this system revealed the seabed pre cleaned in incredible detail. And I really have to credit the high quality to the software's automated tracker functioning, where the sonar parameters are essentially tuned.

Pim, can you give us an overview of the Teledyne RESON SeaBat, T-50R technology and its capabilities?

[PK] The work that Christopher's project had to do is something you do find a lot across different types of survey applications: repetitive surveying. When you do those type of surveys, it's really important that, ultimately, when you look at the differences, you look at the changes in the seabed. Like Chris was saying, setting it up, aligning it all, is super important, and that's what this system is suited for. If you want to see really fine scale geological features, you need to have really small beam widths and lots of data points eventually.That's what a system like a T-50 can do. It runs between four and 200 kHz, quite a wide range. That means you can go really shallow, to fairly deep. (Add to that) the normalized backscatter coupled with the tracker so that you don't need to worry about changing settings; everything is corrected in real-time.

Image Courtesy Teledyne Marine

For us as a manufacturer, we're interested in enabling users to use our high-spec, high-resolution equipment, as easy as possible. We don't want you to have to be a super user to use this type of kit. And like the automated features that Chris talked about before, those things do make it easier so that it enables more people to use our equipment to do these surveys, to gather this type of data, and come up with some really neat results. Pim Kuus, Teledyne Marine

So what do you consider to be the most interesting or unusual discoveries from this project?

[CA] We noticed some pretty peculiar dynamics in the bed forms that were inexplicable considering the tidal flows alone.Typically these mobile bed formstend to migrate and maintain their wave properties, like for example, their wavelength. However, in the last few years, there have been reports from around the world that have said that these wave properties have been observed to be dynamic. The wavelength changes and the amplitudes change if exposed to irregularities in the flow. For example, if the flow speed increases, you get more friction at the bed, increase sediment transport, and the wavelength stretches.

So Falls of Lora, we observed this change, but only on one side of the system. So if you imagine the shallow sill kind of centrally, then the Falls of Lora occurring above the tidal system, flowing bi-directionally and reversing every six hours, you'd expect then that any flow changes would affect both the flood and ebb tides, and therefore influence both sides of the sill somewhat similarly. However, we observed that the bed forms west of the sill dominated by the ebbing flow stretched and the bed forms east didn't really resemble this change. So there was an asymmetry here that caused a lot of confusion, a bit of head scratching, and it wasn't until we considered the glacial past of the site, that things started to come to light. So, perhaps we thought maybe the flow system still possesses some of its ancestry, predating the formation of the Fjord.

Falls East: We observed that the bed forms west of the sill dominated by the ebbing flow stretched and the bed forms east didn't really resemble this change. There was an asymmetry here that caused a lot of confusion, a bit of head scratching, and it wasn't until we considered the glacial past of the site, that things started to come to light, said Christian Armstrong, PhD candidate, University of Highlands and Islands.

Falls West.

[CA] So we set about developing a numerical simulation, a model of the system and using time series, water level data. So how the water level changes, collected for a month, started to analyze influenced the water levels within the loch specifically. And what we found was that the rainfall had a significant influence on the water level within the loch. That's something that we didn't conceive because we thought it was an estuary environment, it's tidally dominated, but that's not usually the case. Kind of like a river on flood, the rainfall affected the height of the water and therefore affected the tidal flows at the Falls of Lora. So the wavelength stretching that we observed in the bed forms west of the sill, was actually found to coincide with the sediment transport increase in response to rainfall as predicted by the numerical simulation.

Now, this result is pretty fascinating because it means that even in a tidally dominated system, the meteorology can affect the seabed stability. And that has all kinds of implications for sill flows, tidally dominated fjords in other latitudes that have variable freshwater influx. How does that freshwater modulate that tidal operation? That tidal system? And how do things like heat, for example, get transferred into these regions, let's say high latitudes where, where heat transfer is instrumental to like glacial melting, for example? So we've found a climate sort of based system or a climate-based effect from the seabed, and then worked our way back, which was quite an interesting way of investigating a system like this.

Is the project now complete? And if so, what are the results or the results to date?

[CA] My PhD has another eight months to go, so we're in the home stretch;the field work is complete. In addition to the results that I mentioned about the Falls of Lora, we also re-surveyed another extreme tidal site called the Gulf of Corryvreckan off the west coast of Scotlandbetween the islands of Scarba and Jura. (This site) hosts the world's third largest whirlpool, and we really tested the T-50 here, as opposed to the relatively shallow, 15-45 meter depths of the Falls of Lora, the Gulf of Corryvreckan is around 220 meters deep. The currents are even faster – 5.5 to 6 meters per second on spring tides, which is an incredibly energetic place.

We were really excited by the Gulf of Corryvreckan. In an initial survey in 2012 we found peculiar sedimentary structures – wave interference patterns, which is very peculiar for these sedimentary structures. So we want to know what would happen some 10 years later, and we expected that, as it was a highly energetic place that it wasgoing to be completely different. We went back and it is absolutely preserved 10 years later: no difference, exactly the same. You'd expect the bed forms to be more dynamic, but on the contrary, they are immobile, by and large. So this is a bit of a mystery to us, an we're still working this one out. For now all I can say is watch this space!