Research | IGB | 12-04-2023

“To researchers, this river is like a textbook”

Dr. Alexander Sukhodolov of the Leibniz Institute of Freshwater Ecology and Inland Fisheries has been carrying out a research on the Tagliamento River in the north-east of Italy for almost a decade and a half. Scientists are fascinated by this river because it has a number of features that makes it a unique object of study. IGB operates a River Lab at the Tagliamento, where researchers study river hydrodynamics and its effect and interactions with aquatic biota.

Clip from the video "Biodiversity at IGB"

The Tagliamento has its source in the Southern Alps in north-east Italy and flows into the Adriatic Sea some 170 kilometers away. The largely unregulated river is considered the most important last remaining intact river in the Alps. This is due to a number of characteristics: the river system makes up an enormous corridor, covering more than 150 square kilometers, that connects land and sea and two important habitats: the Alps and the Mediterranean. This corridor provides a “dynamic mosaic of aquatic-terrestrial habitats,” as once described by former IGB Director Professor Klement Tockner, who started researching the Tagliamento even before the turn of the millennium, and his co-authors: Deeper and shallower river branches are connected over an extensive floodplains separated by hundreds of islands and moveable gravel bars because of mobility of the composing sediments containing different types of rock – metamorphic, limestone and volcanic. The Tagliamento provides a habitat for unique flora and fauna, and is the ideal place to study how natural processes occur in a largely free-flowing river.

Experiments and measurements at the River Lab

The idea behind the River Lab is to better understand how a natural fluvial ecosystem works and how we can then use this knowledge to develop effective river protection and management programs. According to Alexander Sukhodolov, the River Lab serves as a platform for combining traditional fieldwork and laboratory research: using the theoretical models, the researchers integrate physical laws that affect the fluvial system. The River Lab is used to verify whether those theories fit. For this purpose, the scientists use a shallow tributary of the Tagliamento, which is separated from the main river by a gravel bar in summer and is fed solely by groundwater. “Stable hydraulic conditions prevail here during the summer months,” the researcher remarked.

Experiments and measurements are carried out in a shallow in-stream flume measuring 40 meters long, 5 meters wide and 35 cm deep. This flume can be altered as required, such as for the conduction of specific experiments. For example, Sukhodolov and his team created in the flume a patch of aquatic vegetation made of silicone models of S. sagittifolia plants. This vegetation patch can be used to study the interactions of their real-life prototypes with turbulent flows at different flow rates.

Flow structure has an impact on fish

Another project carried out in recent years explores the flow behavior of two confluent rivers. “In the channel, we constructed a model of river confluences. We are able to vary different parameters, such as the volume of the incoming water or the junction angle of the confluence,” reported Alexander Sukhodolov. After all, the flow behavior that occurs when two rivers merge into one is sometimes different in nature to what we might expect from the simple laboratory experiments because sometimes rivers mix over a very short distance, while at some confluences, such as where the Danube meets the Sava in Serbian Belgrade, it can be observed that water masses often remain unmixed over long stretches. “A mixing interface forms between the two rivers, along which the mixing can be very complex and governed by different factors,” stated the researcher. According to the findings of the IGB team, the length of time for which this interface remains stable depends to a great extent on the depth of the river in the area downstream of the confluence and to what extent flow velocity differs. The riverbed roughness also plays a key role: it can inhibit the lateral turbulence, preventing merging flows from mixing for many kilometers downstream.

“This phenomenon is ecologically important. Salmon and sturgeon, for example, use their ability to smell mineral composition of waters and to use that for returning to the place where they hatched upstream, which is then in turn used as a spawning ground,” explained Alexander Sukhodolov. Each river has its own specific mineral composition; if two rivers do not mix over a long distance, migratory fish are better able to navigate because they can swim through their “native” waters.

Findings such as these help to protect river biodiversity in a world marked by climate change. So far, according to Alexander Sukhodolov, global warming has not had a noticeable effect on the Tagliamento: “Rivers in other regions of Italy are now more prone to flooding than in the past, but this is not the case for the Tagliamento river system.” However, the growing agricultural use of land in the river catchment
area is problematic.

Also Dr. Oleksandra Shumilova belongs to the research group “Ecohydraulics” of Alexander Sukhodolov. She reports about the research of IGB in the Science Clip “Biodiversity at IGB.” The film also features the River Lab at Tagliamento.

Text: Wiebke Peters

The article was published in Verbundjournal 119 | 2022 with the focus on "30 years of FVB."

Video / Science Clip