RAISE – Research on Alpine sediment processes, morphodynamics and ecosystem behaviour considering extreme events

Extreme floods have occurred more frequently in Austria in recent years, highlighting the vulnerability of humanity to such events. River geometry responds to heavy flooding with massive morphological rearrangements. Houses and infrastructure in the affected areas are thus at risk. Therefore, an improved understanding of sediment transport processes is crucial for the analysis of morphodynamics and thus for flood risk management. Also, the success of river restoration projects depends on sediment transport and morphodynamics as boundary conditions, but there is a significant lack of knowledge regarding these processes and the related impact on ecosystems.

Sediment monitoring intends to close these knowledge gaps, but its analysis has so far primarily focused on the river sections where the respective data were collected. The RAISE project now aimed at combining long-term and short-term sediment research (e.g. extreme floods) in order to relate and integrate these data for the first time.

A holistic hydromorphological assessment, which considers the sediment connectivity, was further developed. The evaluation, which is based on the Hydromorphological Evaluation Tool (HYMET, Klösch and Habersack, 2017), now includes the creation of a map which displays the connectivity of the entire river network. This map allows decision-makers to quickly get an overview of the larger-scale effects of measure scenarios on the hydromorphology of downstream reaches. An exemplified application to the Drava River shows the effect of an additional hydropower plant on the hydromorphological status. According to the evaluation, the restored reach lost the “good” hydromorphological status given the construction of the hydropower plant at the Schwarzach River, despite the long distance far upstream in the catchment.

The state of the art measuring methods regarding transport of suspended solids and bed load were evaluated. The wide variety of hydraulic, morphologic and sedimentologic conditions in natural rivers require the usage of specialized monitoring equipment to fulfill the wide range of site-specific requirements. A laboratory test in the BOKU research flume was used to determine the strengths and weaknesses of the individual bedload samplers. All basket samplers used in Austria have advantages in special catchment areas and situations. The model tests now help to make the results comparable with each other by using correction factors.

Sediment related processes, influences and initiating factors in the catchment area

Sediment budget calculation variants were compared and discussed and a summary of the shares of suspended sediment and bed load in the total sediment transport was prepared for all stations in Austria. The larger the catchment area, the more a homogenous ratio can be calculated; in small catchment areas, extreme events can often lead to very large fluctuations in one year. A further influencing factor are transverse structures, which accumulate especially suspended sediment over a long period of time, which is then released during major events (this is also relevant for the Danube). Based on a flood event in October 2018, an attempt was made to calculate a sediment budget from the Gail. Here, a river that had been subject to a sediment deficit over long distances in the past decades suddenly produced a sediment surplus through the mobilisation of material, which led to management problems in many areas. 1.4 million m³ of sediments were transported in the course of the event and possible sources and sinks could be identified. This example shows how important measurements, a good process understanding and finally a sediment management adapted to the catchment area are to meet the challenges of extreme events in the coming years. Through these and many other analyses in the RAISE project, the need for river resilience regarding sediments was identified. Thus, within the framework of a sediment management, both the lack and the surplus of sediments must be addressed.

The larger the catchment area, the more likely is a direct relationship between discharge and bedload transport. In small catchment areas, morphological events or a certain hydrological history can often lead to fluctuations in sediment transport of several orders of magnitude at the same discharge. This is often a major problem when evaluating measurement data, especially with regard to management issues. For this reason, the term Sediment Forensics was established within the project. The aim is to analyse occurring phenomena concerning sediment transport in detail in order to find the reasons for the high fluctuations. Five examples were given to show which events lead to otherwise unexplainable fluctuations in sediment transport. A sediment peak at the measuring station in Vent, for example, could be explained through the monitoring by the Bavarian Academy of Sciences. The melting of snow and a rainfall event at the Vernagtferner led to extensive morphological rearrangements directly below the glacier, which finally became visible with a time delay of five days in the measurement data of the geophone system in Vent. At the Urslau river and at the Drava/Falkensteinsteg, a high flow event led to a mobilisation of sediment during the first peak flow. Even after the peak flow has decreased, the bedload transport remained at a high level. At the Isel, the differences between a spring event and an autumn event were pointed out. In addition to the different characteristics between the seasons, the Isel also shows a bedload transport hysteresis, because the peak in bedload transport always occurs before the peak in discharge. This has effects on monitoring but also on the interpretation of the data. On the Drava/Dellach, a more detailed analysis of the data showed that a flood event in 2018 had a major impact on the bedload yield for the entire next year (2019). The annual yield of over 30,000 tons was not only much higher than the average yield, but also far exceeded the yield of the year with the corresponding event (2018: approx. 20,000 tons).

Based on the knowledge gained through process analysis (sediment forensics), management tasks can now be considered and improved through process understanding. Again, some examples have been worked through during the project and five of these examples are presented in more detail. Accordingly, at the Drava/Falkensteinsteg geophone impulses display a scatter of over three orders of magnitude for similar hydraulic conditions. The cause is to be found about 2 km upstream the monitoring station, where a 24 km long river section can be classified as residual flow reach. Due to these anthropogenically altered conditions, the water level in this river section is often not sufficient to mobilize and transport bedload material downstream. This leads to sediment deprived conditions with the formation of a distinct armor layer directly upstream of the monitoring station. During flood events, the sediment is mobilized in the residual reach, which, combined with the sediment influx by tributaries, leads to the formation of bedload pulses. With the detailed analysis of a long data series it was now possible to determine the time difference of the bed load pulse occurrence for different flow situations and thus improve the river management. At the same measuring station it was possible, due to the long data series and some findings gained from sediment forensics, to recalculate a data gap that occurred during a flood and thus to obtain a continuous budget, which in turn is decisive for good sediment management. At the Urslau river, the long data series and sediment forensics made it possible to classify bedload transport in terms of its relationship to flow. Four different event types were defined which occur depending on sediment availability and stream power. By associating the event types the calculation of the yearly yield and sediment management can be improved significantly. By continuously monitoring the bedload transport at the Drava/Dellach, the transport speed of a bedload influx from a tributary (Gailbergbach) could be determined. With this information a transport formula was calibrated and thus the speed of another bed load influx was predicted. This information is of great importance for a functioning bed load management. The increased understanding of the bedload transport processes and monitoring data have led to a better understanding of the sediment balance between the individual bedload measurement stations on the Drava and Isel. The methods used made it possible to draw up a conclusive balance, and further monitoring should clarify the exact sources and sinks of the sediments

The RAISE project made it possible for the first time to also apply hydrodynamic numerical models in the area of bed load measuring stations and to use their possibilities for extrapolation of data and scenarios. For example, the sediment transport model iSed was calibrated using the extensive monitoring data at the Drava river in such a way that it was possible to simulate a flood wave with an unsteady simulation. With the now calibrated tool, further waves can be simulated and thus sediment management can be improved. At the Rofenache, the sediment transport model iSed was used to calibrate bedload transport formulas to extrapolate rating curves. In the range of measured data this works very well; whether reasonable values could be estimated also for non-sampled high flow rates should be clarified by monitoring in the upcoming years. Moreover, the sediment transport model iSed was coupled with the habitat evaluation model HEM. Hence, habitats on the micro-scale as well as the meso-scale can now be evaluated under consideration of the morphological changes taking place during flood events. An example was given by modelling a flood wave with the new coupled module, showing that crucial morphodynamical processes creating habitat diversity can now be modelled and quantified.

Using the example of the Urslau, a methodology was applied that could have an impact on the future design of restoration measures. Large boulders may be implemented in alpine rivers were glacial deposits are evident in form of terminal, lateral or hummocky moraines. Structural features, such as boulders, have the advantage that specifically during high (scouring) flows, they provide sheltering habitats in the wake zone accompanied by reduced flow velocities and/or bottom shear stress. As exposed large roughness elements due to the glacial history were present at some sections of the Urslau, the option of implementing boulders may be of great importance for future sediment management in similar alpine rivers.

Physical laboratory experiments were used to better understand basic hydraulic processes. For example, the initiation of motion of a Danube gravel was investigated with a tomographic particle tracking velocimeter (TOMO-PTV). For most experiments particle dislodgment took place at the peak of positive kinematic energy. The importance of sweep events was emphasized - although sweeps and ejections cause the highest turbulent kinetic energy peaks, the movements were observed almost exclusively at sweeps. These findings are of great importance especially for the Danube, where hydraulic structures are to be used for a later initiation of motion. The delta formation of a typical Austrian small waterpower plant was studied in another laboratory experiment. The delta formation experiments revealed that virtually all incoming sediments accumulated at the head of the reservoir. Over time, the resulting delta formation grows in height and moves further into the reservoir. The delta has the potential to increase the flood risk for high floods. It also has ecological impacts, as important spawning gravel fractions might accumulate on and in the delta and are missing further downstream. Recommendations for the drawdown of a reservoir were derived to improve the management situation.

a) Coherent flow structures of a TOMO-PTV test, b) Grain size distribution of the top layer using various coloured grain sizes, c) evaluation of the percentage proportions of the size classes

Furthermore, methods for predicting sediment supply rates in ungauged river systems were developed and tested with focus on integrative analysis (flood protection and ecology), which are important to future socioeconomic strategies in river basin management. The role of sediments in alpine rivers was discussed with a novel perspective on a differentiation between non-fluvial, semi-fluvial and fluvial sources; - including aspects of habitat modelling and river restoration.

Many further analyses within the framework of the RAISE project showed the importance of well thought-out and functioning sediment management in rivers. Often damage is reacted to after an event instead of acting before it through a proper sediment management. In order to find out which questions have to be clarified for a functioning management and which data should be collected to set up such a management, a questionnaire was developed with a transdisciplinary approach, which addresses the crucial questions. With the involvement of different actors at the state governments and other responsible authorities, the questionnaire was set up and then tested for its functionality using the example of the Gail. People from different disciplines and activities related to river management were interviewed and it was shown that many valuable insights into morphological changes and relationships as well as extreme events could be gained. When interviewing people completely unfamiliar with the subject (neighbouring residents or similar), the questionnaire could only be used as an impulse, and valuable information could rather be obtained from personal conversations.

Through the research cooperation with the Bavarian Academy of Sciences (BAdW), which was started and established within the project, first interdisciplinary approaches could be implemented in the catchment area of the Vent measurement station. The linkage of the bedload measurement data with the monitoring data of the BAdW at the Vernagtferner offers the possibility to link long-term effects of climate change with the development at the bedload measurement station. The first promising results from the project are the starting point for a more extensive research cooperation with the BAdW in the future.

In summary, an important step was taken by the integrative consideration of data obtained through the sediment monitoring stations in Austria. The understanding of processes concerning sediment transport could be increased and occurring phenomena could be explained. The term sediment forensics describes well the degree of detail that is sometimes necessary to obtain more clarity about the transport processes in a catchment area. But it could also be shown how important a functioning sediment management is and that such a management is often missing for the Austrian rivers. Often people only react after damages instead of being able to act before they occur. It was shown that already a single extreme event, on a river that is basically suffering from a sediment deficit, can alter the sediment balance into a surplus of sediment, which in turn leads to problems. A sediment management system should therefore promote a certain sediment resilience so that extreme events can take place without causing high damage.  The analyses of the various catchment areas have shown how different the problems and approaches to solving them are in the individual rivers. The developed questionnaire serves as a tool that enables the transdisciplinary identification of problems and weaknesses in sediment management in a river. Now it is a task of the next years to establish sediment management concepts underpinned by measurement data at Austrian rivers.