Imaggeo on Mondays: The polje between sky and Earth

Imaggeo on Mondays: The polje between sky and Earth

Poljes are one of the most impressive features existing in karst landscapes. These large flat closed depressions are prone to regular flooding that can form temporary lakes on their surface. Planinsko Polje [in Slovenia] is surely the most famous example of polje existing. The highest floods can reach up to 8 metres above the gauging station and last for more than two months. The lake is then more than 10km squared large, which strongly modify the daily life of the inhabitants living in the polje surroundings.

Description by Cyril Mayaud, as it first appeared on

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

Artificial floods: Restoring the ecological integrity of rivers

Artificial floods: Restoring the ecological integrity of rivers

“You can never step into the same river,
for new waters are always flowing on to you.”
—Heraclitus of Ephesus

Rushing rivers, with their unremitting twists and turns and continuous renewal, are often used as a metaphor for life, but the analogy is just as appropriate for scientific research, I reflected as I walked along the banks of a sparkling, turquoise-blue river in the heart of the Swiss National Park. The never-ending cycle of formulating, testing, and modifying evidence-based hypotheses is a hallmark of how humanity acquires new knowledge.

Conducting experiments with rivers is especially challenging because they can never be isolated from their social and ecological contexts. Worldwide, people have appropriated more than half the globe’s accessible surface water by erecting hundreds of thousands of dams. Although these dams provide many societal advantages, including hydropower, water storage, and flood control, they also severely disrupt the ecosystems within which they’re placed. Recently, however, there has been a growing focus on using intentional water releases from the very dams that disturb rivers as ecological restoration tools.

The Spöl River flows through the heart of the beautiful Swiss National Park. (Credit: Terri Cook)

Thanks to the support of an EGU Science Journalism Fellowship, I was hiking next to the Spöl River, a beautiful ribbon of crystal-clear water winding through a deep gorge carved into a soaring limestone upland in the Rhaetian Alps, which are tucked into the country’s southeastern corner. The craggy peaks and towering spruce, pine, and golden larch trees provided a startling contrast to the arid, high-desert scenery along the Colorado River in the Grand Canyon where, several years earlier, I had witnessed the Colorado River’s rapid rise following a so-called “artificial flood” unleashed from Glen Canyon Dam.

Multiple manmade floods have been conducted in the Grand Canyon to benefit the corridor’s physical, cultural, and biological resources, most notably endangered native fish and the disappearing sandbars upon which many organisms, as well as the multimillion-dollar rafting industry, depend. Following years of intensive scientific study and negotiations between the numerous stakeholders, the U.S. government recently implemented a long-term strategy for releasing manmade floods following large sand inputs from tributaries that join the main stem below Glen Canyon Dam. The reason for this timing is to move the recently introduced sand up onto the banks to replenish the shrinking sandbars.

Although these events have been widely reported in the press, few people realize that one of the most important models for designing the Grand Canyon experiments was the Spöl River. I had thus traveled to Switzerland to report on the globe’s best example of how, using carefully designed and monitored floods, scientists and managers have collaborated for a decade and a half to restore—and sustain—this river’s ecological integrity.

Of Fish and Floods

From the Livigno Reservoir on the Italian-Swiss border, the Spöl flows through Switzerland’s only national park before joining the Inn River, a tributary of the Danube, 28 kilometers downstream. Inside the park, the Spöl is sandwiched between two dams, the 130-meter-high Punt dal Gall on the Italian border and the 73-meter-high Ova Spin downstream. Built in the 1960s following a contentious vote, the dams are towering concrete barriers that seemed to me to be out of proportion to the river’s modest size.

Punt dal Gall Dam: The 130-meter-high Punt dal Gall Dam was built in the 1960s on the Swiss-Italian border. (Credit: Terri Cook)

Studies in the national park in the late 1980s confirmed that two decades of reduced flows had severely altered the stream, Ruedi Haller, the park’s Research and Geoinformation Manager, told me as we hiked. The riverbed had become choked with fine-grained sediments, reducing brown trout spawning grounds and changing its assemblage of fauna.

In 1990, a mandated flushing of the safety release gates at the base of the upper Punt dal Gall Dam noticeably improved the ecological conditions downstream, flushing out many of the fine-grained sediments and decreasing the accumulations of mosses, algae, and bottom-dwelling fauna that had taken advantage of the low and steady dam-controlled flows. Within months, however, the Spöl returned to its prior condition. As Chris Robinson of the Swiss Federal Institute of Aquatic Science and Technology explained to me, this first experiment indicated that a single artificial flood could not sustain the river’s ecological integrity over the long term.

Following this initial success, park authorities, researchers, and representatives of the Engadin Hydropower Company, which operates the dams, gradually overcame their former distrust and began to work together to design and implement a flood release program to improve the river’s long-term health. Since then, operators have unleashed more than 25 experimental floods that, by mimicking the seasonally variable conditions to which native fauna and flora have adapted, have recreated an ecosystem much more typical of an Alpine stream. The current flood release program incorporates two artificial floods per year, with the magnitudes determined by annual monitoring.

The Sarine

I also visited a second managed river, the Sarine, near Bern, to watch scientists assess the results of an artificial flood that had just been completed. Among the team working at the site was Michael Doering of the Zurich University of Applied Sciences. He was using a drone to snap post-flood photographs to compare with images taken just before the event to provide a bird’s-eye view of the changes the water had wrought.

Michael Doering uses images taken by a drone to determine the amount of sediment relocated during an artificial flood. (Credit: Terri Cook)

Once analyzed, these and other data will show whether the Sarine flood was large enough to achieve the goals of moving sediment from the banks into the stream and raising the water level high enough to benefit the aquatic and terrestrial ecosystems straddling its banks. Both are necessary, explained Doering, to support a healthy amount of biodiversity, which dammed rivers typically lack.

Through a revision to its Water Protection Act, Switzerland has committed to eliminating the negative impacts of hydropower plants on all of the country’s rivers. Of the more than 700 facilities that need to be mitigated by 2030, it is envisaged that up to about 40 will use artificial floods, according to Martin Pfaundler of the Swiss Federal Office for the Environment. To accomplish this, scientists and water managers will rely on the experience obtained not only from Swiss rivers, but also—as part of the ever-flowing research cycle—from the new knowledge gained from the Colorado.

By Terri Cook, a science and travel writer and winner of the EGU’s 2016 Science Journalism Fellowship.

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)


GeoSciences Column: Mapping floods with social media

GeoSciences Column: Mapping floods with social media

Picture this: you are on your commute home, smartphone or tablet in hand, surfing the internet. You might quickly catch up on the latest news, check in with your friend’s on Facebook, or take to Twitter to share a morsel of information with your followers.

This scenario is common in the modern era of technology. No doubt we are all guilty of indulging in a serious session of internet navigation every now and then (and nothing wrong with that!). But what if your online persona could also make a contribution to better natural disaster management?

One of the many challenges during, and in the immediate aftermath of, natural disasters is being able to provide local populations with timely and reliable information about the extent of damage and/or disruption expected. Flooding events are a prime example: minimising and managing the financial, human and emotional cost of floods is key for researchers, local communities, policy makers and authorities alike.

Contributing to this effort, a team of German researchers have designed a tool which harnesses our desire to share snippets of our lives via social media to support the creation of rapid inundation maps during flooding events. The research was recently published in the EGU open access journal, Natural Hazards and Earth System Sciences.

Currently, measurements of flood water heights made by river gauges, hydrodynamic-numerical models and remote sensing data – such as before and after images acquired by satellites – are used to create rapid response flood maps. Despite their successful and wide-spread use, they are not without limitations.  River gauges only allow for narrow point information on water heights during a flood and require detailed topographical data to be validated. Hydronamic-numerical models aren’t very flexible: it is difficult to build unforeseen incidents into them (e.g. a dike breach). Remote sensing techniques have limitations when it comes to providing real time information; it can take up to 48 hours for the images to be delivered and processed before they can be used.

The study authors argue that eyewitness information about flooding events shared via social media can fill in some of the gaps. Using quantitative data, such as geographical location and flood water height, held in images shared via Twitter and Flickr, can provide information to make more detailed and accurate flood maps in almost real-time. The researchers put the theory to the test for the June 2013 Dresden floods.

The city of Dresden, with its 800,000 inhabitants, sits on the banks of the River Elbe, known for its long history of flooding. This means the city’s population is more aware of the hazard and, being an urban area, likely has a large number of social media users, making it a good case study candidate.

Location of useful photos retrieved with PostDistiller and inundation depths estimates. (Photos by Denny Tumlirsch (@Flitz- patrick), @ubahnverleih, Sven Wernicke (@SvenWernicke) and Leo Käßner (@leokaesner). Taken from J. Fohringer et al. (2016))

Location of useful photos retrieved with PostDistiller and inundation depths estimates. (Photos by Denny Tumlirsch (@Flitz-patrick), @ubahnverleih, Sven Wernicke (@SvenWernicke) and Leo Käßner (@leokaesner). For instance, photos 1 and 2 show inundated roads but a dry sidewalk. This context en- ables the analyst to estimate inundation depth in the order of approximately 5 cm Taken from J. Fohringer et al. (2016))

The research team created an inundation map using only information from photos filtered from Twitter and Flickr. To collate the flood data from social media, the team designed a computer programme. In the first instance a search for key words (in both English and German) related to floods was ran: “Hochwasser”, “Flut”, “Flood”, “inundation”, to name a few. The results were then filtered by the time frame of interest (from May 5th to 21st June 2013) as well as the geolocation of the posts. This yielded a total of 84 posts from which five inundation depths were derived (see the figure caption for details of how the team achieved this), in the space of no more than four hours. The depths calculated were then used to create the inundation map.

To test the robustness of the map, the team created a second map relying only on online data acquired from the Dresden river gauge. Comparing the two maps shows that the social media created map overestimates inundation height by decimetres as well as the geographical extent of the flooding. Despite that, the study authors argue that the errors are acceptable when it comes to providing rapid inundation maps, particularly in situations when no other information is available.

Inundation maps and inundation depths derived from online water level observations (a) and social media content (b) ; inundated area derived from the reference remote sensing flood footprint (c) ; and differences between inundation depths for overlapping areas in scenarios (a) and (b) (panel d ). J. Fohringer et al. (2016))

Inundation maps and inundation depths derived from online water level observations (a) and social media content (b); inundated area derived from the reference remote sensing flood footprint (c); and differences between inundation depths for overlapping areas in scenarios (a) and (b) (panel d). J. Fohringer et al. (2016))

The case study also highlighted some of the method’s shortcomings. It will be important to improve the vertical and horizontal accuracy of the social media created maps by supplementing them with more detailed topographical terrain data. The current method of acquiring data via social media is relatively passive and relies on users sharing images from a flooding event. Crowdsourcing data, where citizens are actively encouraged to share images, would improve the reliability of the data as well as the spatial coverage.

So when you next take a selfie or capture a stunning landscape to share on social media, who knows, the data held in your images and geolocation could have even more value than you might have originally thought!

By Laura Roberts Artal, EGU Communications Officer




Fohringer, J., Dransch, D., Kreibich, H., and Schröter, K.: Social media as an information source for rapid flood inundation mapping, Nat. Hazards Earth Syst. Sci., 15, 2725-2738, doi:10.5194/nhess-15-2725-2015, 2015.

Rimkus, S. et al.  A Century of UK Flood Losses (conference abstract) Geophysical Research Abstracts Vol. 18, EGU2016-11905, 2016, EGU General Assembly 2016

Trejo Rangel, M.A., et al. How Can Flood Affect the Real Estate Market? (conference abstract) Geophysical Research Abstracts, Vol. 18, EGU2016-8977, 2016, EGU General Assembly 2016