GeoLog

Geomorphology

Imaggeo on Mondays: Earthquake Lake

Imaggeo on Mondays: Earthquake Lake

Despite its alluring turquoise waters and rugged mountain backdrop the story behind this beautiful lake is rather more troubling. In today’s Imaggeo on Mondays, the first post since our short break from the traditional format during the General Assembly, Alexander Osadchiev writes about the shaky origins of Sarez Lake.

Lake Sarez is situated in Tajikistan, deep in the Pamir Mountains. In 1911 a local earthquake caused a large landslide which blocked the valley of the relatively small Murgab River (which discharge is only 100-150 m^3/s). The valley is relatively young, on the geological scale at least, meaning it is deep and narrow and has steep sided slopes. This is the reason why the moderate volume of the landslide (about 2 km^3) was enough to form the tremendously high Usoi dam (about 550 m) – the tallest in the world either natural or man-made. The length of the Usoi dam is about 500 m which is almost equal to its height. However, lakes formed by landslide dams blocking river valleys are not uncommon in the Pamir Mountains or elsewhere around the world.

Most blocking dams are not high or solid enough to remain in place for extended periods of time. Initially, a river will seep through the dam eroding it, but usually the outflow discharge is less than the river inflow into the lake. Together with active sedimentation and silting, the water level in the lake steadily increases until it reaches the dam height. Eventually water starts flowing over the top of the dam and intensively destroys the dam. Yet due to a number of circumstances the behavior of the Sarez Lake was significantly different. On the one hand, the Usoi dam is solid enough not to have been significantly eroded in the more than one hundred years since it appeared. At the same time, it is porous: outflow and inflow volumes of water across the dam balance each other.  Crucially, this balance was obtained for a very high water level, close to the height of Usoi dam itself. Lake water levels oscillate near 500 m height, just 50m away from the top of the of 550 m dam. The height of the dam resulted in the large size of the Sarez Lake – its length is about 60 km and its volume exceeds 16 km^3.

This large volume of water (and potential energy!) situated high in the mountains (3263 m above the sea level) presents a hazard for millions of people in Tajikistan, Afghanistan, and Uzbekistan living below the Sarez Lake and along the banks of the Mugrab, Panj and Amu Darya rivers. The Usoi dam is solid enough to resist erosion and create such a big lake, but it is not known if it can withstand a big earthquake, which are not uncommon in the area. Not only can an earthquake directly destabilize Usoi dam, but an earthquake-induced landslide into the lake could cause a lake tsunami and result in the dam overflowing. Particularly, an area of friable soil forming a unstable slope, has been particularly identified as a risk. Following a large earthquake (8-9 on the Richter scale) it could presumably form a landslide.

The levels of monitoring and investigation of landslide hazards in the region and the risk presented by Lake Sarez itself are still largely understudied. Limited funding availability in Tajikistan and the remoteness of the lake – it can only be reached on foot, after several days of strenuous mountain trekking through an almost uninhabited, but unbelievably beautiful area – are amongst the main reasons this is so.

“The view of the Sarez Lake was the best prize for me and Zhamal Toktamysova at the final part of our 2-week trekking through the Pamir Mountains”, explains Alexander.

 

By Alexander Osadchiev, Shirshov Institute of Oceanology, Physical Oceanography, Moscow

Imaggeo on Mondays: Drumlins Clew Bay

Imaggeo on Mondays: Drumlins Clew Bay

During ice ages landscapes are sculpted by the power of advancing glaciers. From rock scratches, to changing mountains and the formation of corries, cirques and aretes, through to the formation of valleys and fjords, the effects of past glaciations are evident across the northern hemisphere landscape.

Perhaps not so familiar, drumlin fields are also vestiges of the erosive power of ancient ice sheets. Glacial deposits tend to be angular and poorly sorted, meaning they come in lots of different sizes and shapes. The extreme of this are glacial erratics. Drumlins are are elongated hills made up of glacial deposits and they represent bedforms produced below rapidly moving ice. Our Imaggeo on Monday’s image this week is of Clew Bay in western Ireland and shows the streamlining of drumlins into an extensive drumlin field of glacial sediment. The drumlins here formed during the rapid thinning of the fast moving central parts of the western sector of the British-Irish Ice Sheet, in a process known as deglacial downdraw – probably between 18,000 and 16,000 years ago. The ice was streaming through bays in western Ireland both during and at the end of the Last Glacial Maximum (also known as LGM). This was the time in which the ice sheets covered most of northern America, Europe and Asia. In Clew Bay the ice was a minimum of 800m thick and flowing out into a series of tidewater glaciers situated along the length of Ireland’s western shelf.

By Prof. Peter Coxon, Head of Geography, School of Natural Sciences, Trinity College Dublin & Laura Roberts

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

Communicate your Science Video Competition finalists: time to get voting!

For the second year in a row we’re running the EGU Communicate Your Science Video Competition – the aim being for young scientists to communicate their research in a short, sweet and public-friendly video. Our judges have now selected 3 fantastic finalists from the excellent entries we received this year and it’s time to find the best geoscience communication clip!

The shortlisted videos will be open to a public vote from now until midnight on 16 Apri; – just ‘like’ the video on YouTube to give it your seal of approval. The video with the most likes when voting closes will be awarded a free registration to the EGU General Assembly 2016.

The finalists are shown below, but you can also catch them in this finalist playlist and even take a seat in GeoCinema – the home of geoscience films at the General Assembly – to see the shortlist and select your favourite.

Please note that only positive votes will be taken into account.

The finalists:

Inside Himalayan Lakes by Zakaria Ghazoui. Like this video to vote for it!

 

Glacial Mystery by Guillaume Jouvet. Like this video to vote for it!

 

Floods by Chiara Arrighi. Like this video to vote for it!

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The winning entry will be announced during the lunch break on the last day of the General Assembly (Friday 17 April).

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