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Ocean Sciences

Imaggeo on Mondays: Indonesian mangroves and tsunamis

Imaggeo on Mondays: Indonesian mangroves and tsunamis

Pictured here is a solitary mangrove tree, rooted off the northern coast of the Indonesian island Flores. While this tree has the shallow sandy reef to itself, mangroves are often found clumped together in large forests covering tropical and subtropical coastlines. The propped-up roots of mangrove trees often tangle together, creating a dense natural barrier that can weaken the coastal impact of ocean tides, currents and storms. As a consequence, islands with mangrove forests on their coastlines experience less erosion and less damage from storm surges compared to barer shorelines.

Mangroves are also often said to provide protection against tsunami destruction. Indeed, there have been several cases in which mangroves trees were believed to have curtailed the devastating effect of tsunami waves. Recent research suggests that extensive mangrove forests hundreds of metres wide have been able to reduce tsunami wave heights by 5-30 percent.

Unfortunately, over the past decades, these environmental benefits are now under threat due to deforestation. About half of the global mangrove population (32 million hectares) has been wiped out, often to make way for fish farming operations. In Indonesia, mangrove ecosystem decline has been largely attributed to developing shrimp ponds and logging activities. There are now a number of places where mangrove plantations are supported by local individuals and governments.

Jörn Behrens, a professor of numerical methods in Earth sciences at the University of Hamburg in Germany, captured this shot while on a field trip in Indonesia. He and his colleagues were looking for traces of the powerful 1992 tsunami that struck the coast of the Indonesia island of Flores and other nearby smaller islands.

The tsunami, triggered by a magnitude 7.9 earthquake, sent waves reaching 4 to 27-metres high on the island’s northeastern coast, even destroying a whole village situated on the nearby island Babi. About 2,500 residents and tourists died from the event, with hundreds more injured, and thousands more homeless.

The 1992 Flores tsunami was also one of the first such events documented by an international survey that adhered to internationally accepted post-tsunami assessment standards.  On their field trip Behrens and his colleagues revisited some of sites assessed by the 1992 post-tsunami survey, spoke to eye witnesses, learned about the region’s current mitigation measures, and exchanged latest results from modeling and experimental tsunami research.

While on this field trip, Behrens came across this solitary mangrove, surrounded by what appears to be young mangrove propagules growing out from the water.

By Olivia Trani, EGU Communications Officer

References

Spalding M, McIvor A, Tonneijck FH, Tol S and van Eijk P (2014) Mangroves for coastal defence. Guidelines for coastal managers & policy makers. Wetlands International and The Nature Conservancy.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Back for the first time: measuring change at Narrabeen–Collaroy Beach

Back for the first time: measuring change at Narrabeen–Collaroy Beach

Narrabeen–Collaroy Beach in New South Wales, Australia, just north of Sydney, is home to one of the longest-running shoreline-measurement programmes in the world. With colleagues at the University of New South Wales (UNSW) Sydney, Eli Lazarus, an associate professor in geomorphology at the University of Southampton, UK, has been analysing over 40 years of data from Narrabeen–Collaroy to better understand how shorelines recover from major storm events.

In this blog post, Lazarus shares a glimpse of the programme’s history and describes his experience of visiting a field site that for him is both familiar and brand new.

“Want to see what an old GPS unit looks like after it’s been up and down the beach a thousand times?”

Mitchell Harley, a Scientia Fellow and coastal researcher at the UNSW Sydney Water Research Laboratory (WRL), in Manly Vale, Australia, handed me a battered, corroded, steel-cased receiver the size of a grapefruit. “It’s also seen a lot of Duct Tape.”

He loaded a carbon-fibre survey staff and a yellow Pelican case containing a new, a top-of-the-line Trimble GPS handset into the back of a WRL vehicle. With two visiting masters students – Tim van Dam from TU Delft, and Yann Larré from École Polytechnique – we set off on our afternoon excursion, to Narrabeen.

View of Narrabeen Beach, looking south from Narrabeen Headland. Credit: Eli Lazarus

Facing the open South Pacific, Narrabeen and Collaroy are the northern and southern halves of an embayed beach, a reach of sand framed at either end by rocky promontories, that extends approximately three-and-a-half kilometres between Narrabeen Headland and Long Reef Point. Narrabeen is the keystone of the Northern Beaches, a chain of sandy pockets defining the coastal peninsula north of Sydney. The beaches darken in colour with each embayment, from dun in the south to a reddish ochre in the north, representative of the ancient sandstone bedrock units in which they sit.

Narrabeen is a legendary surf break and home turf to a roll of world champions, where, to date, the locals have successfully prevented the installation of anything that resembles a surf cam. But the beach is also home to one of the longest-running and most complete beach-survey programmes in the world (Turner et al., Sci Data 2016).

In 1976, the renowned coastal scientist Andy Short, who used to live in Narrabeen, began the programme from the beach across the street from his house. He and family members, colleagues, friends, and volunteers diligently measured a set of cross-shore profiles along the full Narrabeen–Collaroy embayment every month for 30 years.

All long-term monitoring endeavours are labours of love. But frequent, detailed measurements of beach morphology, maintained consistently over long time scales, are exceptionally rare, and they offer essential quantitative insight into coastal events, changes, and cycles that occur more rapidly than most records tend to capture.

Harley took over the measurement programme in 2006, along with Ian Turner, who now directs the Water Research Lab, and recorded more than 120 monthly surveys of the full beach with a quad-bike Harley would trailer back and forth from Manly Vale.

Harley’s quad-bike – and shoreline-survey workhorse – at the UNSW Sydney Water Research Lab. Credit: Eli Lazarus

The Water Research Laboratory team has continued to experiment with different measurement methods for the Narrabeen–Collaroy system. Mounted on the top floor of the Flight Deck, a beachfront hotel where Narrabeen blends into Collaroy, is an array of five cameras, known as an Argus station, that takes time-averaged photos of the shoreline and surf zone. Tucked in among the cameras is a smoked-glass dome that looks like a space helmet: a lidar unit that uses a laser to measure wave swash and a cross-shore profile of beach elevation five times every second.

On our outing, Harley first drove us up Narrabeen Headland, to get an unobstructed southerly view of the bay. At the overlook was a stainless-steel post with a frame to hold a smartphone. This was the Narrabeen CoastSnap station.

In 2017, Harley, along with collaborators from the New South Wales State Government, launched the CoastSnap programme to collect crowd-sourced observations of beach dynamics (Harley et al., 2019). The process is simple: take a photo, post the image on social media with the station hashtag (#CoastSnapNarra, for example), and if you don’t post it right away, then write in the date and time of the image. With some clever analytical tricks, an algorithm finds the shoreline in your photo. Harley installed the first CoastSnap station at Manly Beach, above the Manly Surf Life Saving Club. There are now more than 35 CoastSnap stations in nine countries around the world.

Harley pointed out the various permanent features the algorithm uses to identify the shoreline position in every #CoastSnapNarra photo: an inlet hazard sign, the corners of prominent buildings in the foreground and distance. “We get about an image a day from people up here,” he said. Watching a sparse line-up of surfers work a peeling break at Narrabeen Inlet, we stood eating steak pies from The Upper Crust – like the surfers, another local institution.

Pies finished, we looped back down to the north end of the beach and assembled the GPS. The four of us would take turns walking the GPS receiver down the five main cross-shore transects still sampled at Narrabeen and Collaroy every month, and the three visitors would get our names added to the dataset’s long list of contributors.

Harley, Larré (holding GPS) and van Dam working through a beach profile. Credit: Eli Lazarus

In a reversal of cart and horse, I had written a scientific article about Narrabeen but never seen it. In fact, I was there in Sydney to visit people I had co-authored with but never met in person.

Earlier this year, Harley, Chris Blenkinsopp (of Bath University in the UK, and a former postdoc at WRL), Turner, and I published a paper in the EGU journal Earth Surface Dynamics about the information that shoreline records retain or destroy regarding the environmental conditions that shape them (Lazarus et al., 2019).

Extreme storm events, for example, can inscribe dramatic changes in the shape of a coastline. A detailed, high-frequency record of shoreline position presumably should reflect something about the magnitude of those events. But sedimentary systems can be very effective at obscuring or erasing their own histories, and not all evidence of conditions that impact a shoreline gets preserved. This phenomenon is known as ‘signal shredding’. The exceptional data catalogue for Narrabeen–Collaroy enabled us to pursue the first empirical test of signal shredding at a sandy beach, an idea I’d puzzled over since geomorphic signal-shredding was first described for other sediment-transport systems almost ten years ago (Jerolmack & Paola, 2010).

Among our survey crew, I asked to take Profile 4, near the middle of the embayment, because that was the record I had used the most when working through the signal-shredding analysis. To me, Profile 4 seemed to best capture, in a single line, the spatially variable character of the beach overall.

As we leapfrogged our way south, the beach profile became steeper and narrower. Harley mentioned an article that he had published with Turner and Short (Harley et al., 2015) that described, among other patterns at Narrabeen, a spatial pattern in the beach slope. If one end of the beach was steeply sloping toward the water, then the other end would be flat. The steep stretches of the beach tended to be narrow, and the flat stretches tended to be wide. Under certain wave conditions, the narrow, steep end of the would switch to being wide and flat, and vice versa – a pattern typical of embayed beaches called ‘rotation’.

As Harley described the slope pattern, the observation struck me as the kind that comes from investing time at a field site: the intuition internalised by surveying the beach over and over again in the seat of a quad-bike, from tipping sideways in the steeps and tracing the long meanders of the shoreline across the flats.

Standing astride the sharp break in beach slope at Collaroy, looking south toward Long Reef. Credit: Eli Lazarus

We finished the day with a walk around Long Reef, at Collaroy, looking back into the embayment we’d spent the afternoon traversing. Hang-gliders drifted in slow figure-eights above us. I was headed back to the UK the next day. There is more work to be done at Narrabeen, for sure, and we talked about what’s coming next: algorithms for predicting shoreline position (Davidson et al., 2017), fresh insights into beach recovery after major storms (Phillips et al., 2019), identifying shorelines from catalogues of satellite imagery (Vos et al., 2019). We talked about possible funding avenues to keep fuelling our collaboration.

The wind picked up, and the waves set to work rearranging the shoreline we had just measured.

Day’s end and hang-gliders at Long Reef, looking northwest toward Collaroy and Narrabeen. Credit: Eli Lazarus

By Eli Lazarus, University of Southampton, UK

Dr Eli Lazarus (@envidynxlab) is an Associate Professor in Geomorphology in the School of Geography & Environmental Science at the University of Southampton, UK.

 

Imaggeo on Mondays: Penguins – a biogeochemical link between sea and land

Imaggeo on Mondays: Penguins – a biogeochemical link between sea and land

A couple of Chinstrap penguins (Pygoscelis antarctica) at their nesting site on Deception Island, maritime Antarctica. Sea birds contribute importantly to biogeochemical cycles in coastal ecosystems and on islands. Feeding on the marine food chain and nesting on land, they carry large amounts of marine nutrients into terrestrial ecosystems. This might be of particular importance for the nitrogen (N) cycle of terrestrial ecosystems in the antarctic. In the form of ammonia, marine derived N can travel far inland with the wind, and perhaps represent an important nutrient source for the growth of mosses.

Description by Daniel Wasner, as it first appeared on imaggeo.egu.eu.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

GeoTalk: “Grown-ups are not focusing on the plastic problem, not as much as I want them to”

GeoTalk: “Grown-ups are not focusing on the plastic problem, not as much as I want them to”

Lucie Parsons, a ten-year old girl from the small village of Walkington, in England, is on a personal mission to save the environment from plastic pollution. After seeing on the BBC Blue Planet II documentary how litter in the ocean is damaging ecosystems, she decided to take action. Now she gives talks and is co-researcher in her mother’s PhD on climate change and the youth voice. Lucie has come to the European Geosciences Union General Assembly in Vienna with her mother, Katie Parsons, to tell scientists that children want to be involved in addressing environmental issues.

Unless the flow of plastics and industrial pollution into the world’s oceans is reduced, marine life will be poisoned by them for many centuries to come.

David Attenborough, The Blue Planet II: Episode 4, BBC One

 

How did you learn about the impact of plastic pollution in the oceans and marine life?

L: Through Blue Planet. I saw an episode about a whale and her calf, and how the contamination poisoned the whale’s milk. When I saw that I got really, really upset so I wrote a poster about it. Then I asked my mummy and daddy to photocopy it so that I might be able to put it around the village. I read and watched documentaries to learn more, and I found out that it is a big problem. I wanted to do something about it.

So you started giving talks… 

L: Katy Duke, the head of the [aquarium] Deep in Hull, got in touch with daddy because she saw my poster.

K: I tweeted Lucie’s poster to show what she had done after she was so moved by the documentary. The CEO of the Deep saw that and contacted us to ask if Lucie would like to give the opening talk at the European Union of Aquarium Curators Conference, which the Deep were due to host.

What do you tell people in your talks?

L: I have done two conferences and talks, also at schools. I have also been interviewed for the radio and profiled by the Earth Day Network. In my talks I basically tell people how bad the problem is, what it is doing to the animals and what they can do to help.

Here at the EGU General Assembly people were really touched by your presentation. Do you think your talks make people take action? 

K: Gilles Doignon, from the European Commission for Environment, was really moved about what Lucie had said at the Deep. He promised her that he would get the aquariums to sign up to a plastic pledge.

L: And he managed to do it.

K: He said that, thanks to Lucie, thousands of turtles will be saved. This is where she got her inspiration from to carry on. If she can talk and say the things she has done, even if just one or two people do something about it, that creates a knock-on effect.

Why do you think children should be involved in the fight against climate change?

L: Children are the next generation; when they get older they will take over the work grown-ups have done. So they should start now. Children can do the same things as grown-ups, there is not really a difference with helping, you need to get as many people to help as you can get.

K: Getting schools and individual children involved in science will make it real and manageable, part of life. Otherwise much science ends up in dusty journals. We need people to live it and understand it.

Are grown-ups doing enough?

L: I think they should be doing a tiny bit more. They are not really focusing on the problem, not as much as I want them to.

You have talked to politicians before, why do you think it is important to talk to scientists also?

K: When Lucie was affected by Blue Planet she luckily had me and her dad to help her. But other children will have their passion stopped unless they have an adult who supports them. Some schools don’t do environmental education, it is not within many curriculums, and some parents might not carry on informing their children.

There is amazing science going on and some scientists who communicate get through to the children. There is a youth rising at the moment. Children are interested, they want to know and they want to be involved. But, how? Scientists have to continue feeding the information to the children and involve them in citizen science so they will carry on with that passion.

What can people do to help?

L: Inform other people, go on litter picks and map the areas where they found the litter to help prevent more litter. With my friends and my family, we have cleaned three areas so far in my village and we are mapping them to feed in the data about where we found the litter. Also, stop using single-use plastics.

Is there any other documentary, book or podcast you would recommend to people who want to learn about plastic pollution in the oceans?

L: Drowning in Plastic. We have watched about three quarters of that.

K: You are enjoying that, aren’t you?

L: Yes… Well, I wouldn’t say we are enjoying it.

Interview by Maria Rubal Thomsen, EGU Press Assistant