GeoLog

sediment transport

Imaggeo on Mondays: how short-term storms can impact our landscapes

Imaggeo on Mondays: how short-term storms can impact our landscapes

In the Sierra de Aconquija, a mountain range in the southern Central Andes of Argentina, strong storms often come and go at a moment’s notice, but they can have a long-lasting impact on the Earth’s surface.

The thunderstorm cell featured in this photo formed in less than half an hour, giving all those nearby only a few minutes to take cover. Mitch D’Arcy, a geomorphologist and postdoctoral researcher at the University of Potsdam and the GFZ German Research Centre for Geosciences, had the opportunity to witness this storm (and snap this picture!) while carrying out field work in the area.

“It was a spectacular experience, pouring heavy rain onto a very localised part of the mountain range, but it was also a hazard because the storm was quickly moving towards us with a lot of lightning. Without any trees around, we were likely targets for lightning strikes!” said D’Arcy. Luckily, he and his colleagues were able to find shelter in their truck while the huge downpour passed over them.

These kinds of thunderstorms are short-lived, but have intense precipitation rates. In this case, the temperature dropped by 14 degrees Celsius, and the storm was accompanied by heavy hail and lightning. And while these natural hazards are transient, they can have a long-term impact on the region’s landscape. Severe storms are capable of triggering landslides and floods and can relocate large amounts of sediment and debris in a short period of time.

D’Arcy is part of an international research programme called StRATEGy (Surface processes, Tectonics and Georesources: The Andean foreland basin of Argentina), which looks into how past and present climate change makes a mark on the terrain of the Argentine Andes, among other topics.

This research is essential for understanding and predicting how human-caused climate change will alter weather patterns and impact surface processes (such as how quickly sediments are eroded and transported across landscapes), according to D’Arcy. Having a better understanding of these surface processes and their sensitivity to the climate could help scientists better inform the public about how to prepare for natural hazards, such as flooding, erosion and landslides.

D’Arcy notes that it’s also important to assess how climate and weather trends will impact the sedimentary record, since it is one of the only physical records that scientists can use to examine how the Earth’s surface has change through time.

“North-western Argentina is a fascinating place to study how climate change affects surface processes, because it has experienced pronounced and abrupt changes in hydroclimate through time,” said D’Arcy. Their research has found that even subtle changes in the region’s climate have produced large changes to the surface environment, impacting how rivers take shape and how sediments move.

For example, while the Sierra de Aconquija is a semi-arid environment today, more than 12,000 years ago it used to be much wetter as a result of global climate changes. In fact, back then the mountain range was covered in glaciers and many of the basins were filled with lakes.

“It’s really important that we understand how different landscapes function and how they react to changes in climate. When we look at places like the southern Central Andes in Argentina, we find that the landscape records interesting signatures of ancient climate changes in Earth’s past. However, one of the big questions we still don’t have a good answer to, is how important are these very intense but rare storms for shaping landscapes and creating the sedimentary record from the geological past,” said D’Arcy.

By Olivia Trani, EGU Communications Officer

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: Severe soil erosion events and how to predict them

GeoTalk: Severe soil erosion events and how to predict them

Geotalk is a regular feature highlighting early career researchers and their work. In this interview we speak to Matthias Vanmaercke, an associate professor at the University of Liège in Belgium who studies soil erosion and land degradation across Europe and Africa. At the EGU General Assembly he received the 2018 Soil System Sciences Division Outstanding Early Career Scientists Award.

Thanks for talking to us today! Could you introduce yourself and tell us about your career path so far?

Hi! So I am Matthias Vanmaercke. I’m from Belgium. I’m studied physical geography at the University of Leuven in Belgium, where I also completed my PhD, which focused on the spatial patterns of soil erosion and sediment yield in Europe. After my PhD, I continued working on these topics but with a stronger emphasis on Africa. Since November 2016, I became an associate professor at the University of Liege, Department of Geography where I continue this line of research and teach several courses in geography.

At the 2018 General Assembly, you received a Division Outstanding Early Career Scientists Award for your contributions towards understanding soil erosion and catchment sediment export (or the amount of eroded soil material that gets effectively transported by a river system).

Could you give us a quick explanation of these processes and how they impact our environment and communities?

We have known for a long time that soil erosion and catchment sediment export pose important challenges to societies. In general, our soils provide many important ecosystem services, including food production via agriculture. However, in many cases, soil erosion threatens the long term sustainabilty of these services.

Several erosion processes, such as gully erosion, often have more direct impacts as well. These include damage to infrastructure and increased problems with flooding. Gullies can also greatly contribute to the sediment loads of rivers by directly providing sediments and also by increasing the connectivity between eroding hill slopes and the river network. These high sediment loads are in fact the off-site impacts of soil erosion and often cause problems as well, including deteriorated water quality and the sedimentation of reservoirs (contributing to lower freshwater availability in many regions).

Matthias Vanmaercke, recipient of the 2018 Soil System Sciences Division Outstanding Early Career Scientists Award. Credti: Matthias Vanmaercke.

What recent advances have we made in predicting these kinds of processes?

Given that we live in an increasingly globalised and rapidly changing world, there is a great need for models and tools that can predict soil erosion and sediment export as our land use and climate changes.

However, currently our ability to predict these processes, foresee their impacts and develop catchment management and land use strategies remains limited. This is particularly so at regional and continental scales and especially in Africa. For some time, we have been able to simulate processes like sheet and rill erosion fairly well. However, other processes like gully erosion, landsliding and riverbank erosion, remain much more difficult to simulate.

Nonetheless, the situation is clearly improving. For example, with respect to gully erosion, we already know the key factors and mechanisms that drive this process. The rise of new datasets and techniques helps to translate these insights into models that will likely be able to simulate these processes reasonably well. I expect that this will become feasible during the coming years.

 

What is the benefit of being able to predict these processes? What can communities do with this information?

These kinds of predictions are relevant in many ways. Overall, soil erosion is strongly driven by our land use. However, some areas are much more sensitive than others (e.g. steep slopes, very erodible soil types). Moreover, many of these different erosion processes can interact with each other. For example, in some cases gully formation can entrain landslides and vice versa.

Models that are capable of predicting these different erosion processes and interactions can strongly help us in avoiding erosion, as they provide information that is useful for planning our land use better. For instance, these models can help determine which areas are best reforested or where soil and water conservation measures are needed.

They also help with avoiding and mitigating the impacts of erosion. Many of these processes are important natural hazards (e.g. landsliding) or are strongly linked to them (e.g. floods). Models that can better predict these hazards contribute to the preparedness and resilience of societies. This is especially relevant in the light of climate change.

However, there are also impacts on the long-term. For example, many reservoirs that were constructed for irrigation, hydropower production or other purposes fill up quickly because eroded sediments that are transported by the river become deposited behind the dam. Sediment export models are essential for predicting at what rate these reservoirs may lose capacity and for designing them in the most appropriate ways.

At the Assembly you also gave a presentation on the Prevention and Mitigation of Urban Gullies Project (PREMITURG-project). Could you tell us a bit more about this initiative and its importance?

Urban mega-gullies are a growing concern in many tropical cities of the Global South. These urban gullies are typically several metres wide and deep and can reach lengths of more than one kilometre. They typically arise from a combination of intense rainfall, erosion-prone conditions, inappropriate city infrastructure and lack of urban planning and are often formed in a matter of hours due to the concentration of rainfall runoff.

Urban gully in Mbuji-Maji, Democratic Republic of Congo, September 2008. Credit: Matthias Vanmaercke

Given their nature and location in densely populated areas, they often claim casualties, cause large damage to houses and infrastructure, and impede the development of many (peri-)urban areas.  These problems directly affect the livelihood of likely millions of people in several countries, such as the Democratic Republic of Congo, Nigeria, and Angola. Due to the rapid growth of many cities in these countries and, potentially, more intensive rainfall, this problem is likely to aggravate in the following decades.

With the ARES-PRD project PREMITURG, we aim to contribute to the prevention and mitigation of urban gullies by better studying this problem. In close collaboration with the University of Kinshasa in the Democratic Republic of Congo (DRC) and several other partners and institutes, we will study this underestimated geomorphic hazard across several cities in DRC. With this, we hope to provide tools that can predict which areas are the most susceptible to urban gullying so that this can be taken into account in urban planning efforts. Likewise, we hope to come up with useful recommendations on which techniques to use in order to prevent or stabilise these gullies. Finally, we also aim to better understand the societal and governance context of urban gullies, as this is crucial for their effective prevention and mitigation.

Interview by Olivia Trani, EGU Communications Officer

Imaggeo on Mondays: Sedimentary record of catastrophic floods in the Atacama desert

Imaggeo on Mondays: Sedimentary record of catastrophic floods in the Atacama desert

Despite being one of the driest regions on Earth, the Atacama desert is no stranger to catastrophic flood events. Today’s post highlights how the sands, clays and muds left behind once the flood waters recede can hold the key to understanding this natural hazard.

During the severe rains that occurred between May 12 and 13, 2017 in the Atacama Region (Northern Chile) the usually dry Copiapó River experienced a fast increase in its runoff. It caused the historic center of the city of Copiapó to flood and resulted in thousands of affected buildings including the University of Atacama.

The city of Copiapó (~160,000 inhabitants) is the administrative capital of this Chilean Region and is built on the Copiapó River alluvial plain. As a result, and despite being located in one of the driest deserts of the world, it has been flooded several times during the 19th and 20th century. Floods back in 2015 were among the worst recorded.

The effects of the most recent events are, luckily, significantly milder than those of 2015 as no casualties occurred. However, more than 2,000 houses are affected and hundreds have been completely lost.

During this last event, the water height reached 75 cm over the river margins. Nearby streets where filled with torrents of mud- and sand-laden waters, with plant debris caught up in the mix too. Once the waters receded, a thick bed of randomly assorted grains of sand  was deposited over the river banks and urbanized areas.

Frozen in the body of the bed, the sand grains developed different forms and structures. A layer of only the finest grained sediments, silts and clays, bears the hallmark of the final stages of the flooding. As water speeds decrease, the finest particles are able to drop out of the water and settle over the coarser particles. Finally, a water saturated layer of mud, only a few centimeters thick, blanketed the sands, preserving the sand structures in 3D.

The presence of these unusual and enigmatic muddy bedforms has been scarcely described in the scientific literature. A new study and detailed analysis of the structures will help better understand the sedimentary record of catastrophic flooding and the occurrence of high-energy out-of-channel deposits in the geological record.

By Manuel Abad and Tatiana Izquierdo, Universidad de Atacama (Chile)

 

Dust in the desert: The Skeleton Coast – Foggy, dusty & demanding – part 3 of 3

In this third instalment in this series our journey takes us into the Skeleton Coast. Synonymous with shipwrecks and known as “The Land God Made in Anger” to indigenous Bushmen this coastal desert has been protected as a National Park since 1971. Similar to many of Namibia’s National Parks, the Skeleton Coast does not allow anyone to stay overnight within its boundaries. However at over 16,000 km2 rarely any of the public is able to experience much of the landscape on the ground beyond the 120 km salt road in its southern section. This coastline is inundated with fog for most of the year owing to the cold Benguela upwelling, which cools the overlying air. In contrast, dry conditions prevail when ridging high pressure systems south of southern Africa cause strong, offshore winds.

The point of no return: entrance to the Skeleton Coats National Park. (Credit: F. Eckardt)

The point of no return: entrance to the Skeleton Coats National Park. (Credit: F. Eckardt)

From Swakopmund with a support vehicle full of supplies one vehicle started the journey north up to the top of the Skeleton Coast on the lonely salt road to meet with the Park Ranger to go over recommended routes and safety concerns (you can get a feel for the drive watching this time-lapse video of drive from Swakopmund to Skeleton Park Gate). The following morning the support vehicle, with the now fully functional trailer, headed up the coast packed with 6 full weather stations to meet with the other vehicle on its way back from Möwe Bay at a prearranged location near the mouth of the Huab River. However, without cell phone signal since the early hours of the morning, assumptions had to be made on how each group were getting on. As it turns out, the group coming back from the ranger station had experienced an incident with their vehicle where the rear passenger wheel had ripped off the vehicle completely  around 20 km south of Terrace Bay. The group coming from the south decided to look for them around midday after realising that something must have gone wrong for them to be so late. They found them on the side of road with a 60 m long trench coming out of the back of the car from the hub dragging along the ground. Through some quick reorganising and a call to a mechanic back at Terrace Bay, the vehicle was restored to a working condition and we headed south to the Huab River the following day.

The Huab River Valley 30 km in from the coast looking towards the east. (Credit: A. Dansie)

The Huab River Valley 30 km in from the coast looking towards the east. (Credit: A. Dansie)

The Huab River is a very remote and narrow river system that has one of the larger drainage systems of the dry valleys along the Skeleton Coast. The system is composed mainly of the Karoo-aged Etjo sandstones in the interior and volcanic-derived dolerite sills near the coast (including many exposed dolerite pipes). T here is a multitude of other landforms in the valley though, including nebkhas, barchans dunes, coastal sabkhas, quartz gravel plains, and gravel/boulder covered desert pavements. A most amazing scene to be located for fieldwork, but also the most demanding – after a long day of hauling equipment around you are on your own to build a campfire to cook food and brave the cold dry desert air (or in a couple cases the cold foggy coastal air) in tents overnight.

A foggy morning in the Huab River Valley (Credit: J.King)

A foggy morning in the Huab River Valley (Credit: J.King)

The dust monitoring weather stations installed here follow a narrow single-track ranger road that is almost completely erased by the wind-driven shifting soil and sand.  This required very careful 4×4 driving that had the vehicles going up and over active sand dunes, alluvial fans, two to three metre high river terraces, and along coastal beaches. The site farthest up the valley (about 30 km from the ocean) takes around 2.5 hours to get to from the public coastal road and is located on an extensive desert pavement soil intended to be a clean and dust–free upwind site.

The treacherous driving conditions along the Huab River, here navigating the older river terraces. (Credit: A. Dansie)

The treacherous driving conditions along the Huab River, here navigating the older river terraces. (Credit: A. Dansie)

The stations down the valley consisted of one on the older active fine river sediments now surrounded by desiccated nebkha dunes and other sparse vegetation remnant of a wetter period in time. It also provided an area for our campsite just across the most recent active river channel. Its location, still quite inland, should help test that the majority of dust transported from this valley originates from recent river deposited sediments showing the reliance of arid processes on river sediment delivery. A visit the next week did just this as seen in this time-lapse video of the site from the car of a local dust event.

The rising moon over one of the weather stations photographed from the campsite. (Credit: A. Dansie)

The rising moon over one of the weather stations photographed from the campsite. (Credit: A. Dansie)

The sites closest to the ocean consisted firstly of another sandy nebkha site close to an active spring; which could prove a good test of the razor wire to protect the instruments from inquisitive animals. Another site located in the much wider groundwater fed channel system was installed the following week and included one of the Cimel Photometers that measures the amount the sun is obscured by aerosols. And lastly, a beach site located only 50 metres from the ocean surf at high tide that may well be an area of high sediment transport(but not from the continental dry easterlies rather from the predominant coastal on-shore winds never known for producing dust emissions).

The weather station closest to the active spring. Dust emissions in the background are from a south-westerly wind never previously thought to be able to drive dust emissions. (Credit: F. Eckardt)

The weather station closest to the active spring. Dust emissions in the background are from a south-westerly wind never previously thought to be able to drive dust emissions. (Credit: F. Eckardt)

This route of sites will be visited monthly over the austral winter to download data, change dust monitor filters, empty sediment traps, inspect for damages, and keep a record of the dusty days to coordinate with the others on the project. One such visit was just performed and within two weeks all sites were successfully visited, although not without getting the 4×4 stuck for an afternoon in the Tsauchab River sands, taking a day to repair damages from a Hyena, and enduring a series of cold, foggy, and windy days in the Huab.

Come October the project will hopefully join the very lonely group of projects that have successfully collected dust storm emissions data at a remote source location!

You can follow the project’s progress through their twitter account @DO4Models.

The installation crew enjoying the camp fire after a long hot day in the field. (Credit: R. Washington)

The installation crew enjoying the camp fire after a long hot day in the field. (Credit: R. Washington)

By James King, University of Oxford

Want to catch up on the rest of the Dust in the Desert series? Take a look at Part 1: Measuring it is only half the battle and Part 2: The Kuiseb and Tschaub Rivers.