Eco-morphodynamic modelling for gravel bed rivers (BASEveg)
Caponi, Francesco Siviglia, Annunziato
River managers and a society that aims at a sustainable interaction with the fluvial system are challenged by the need to ensure flood protection, water resources availability, and ecosystem health in a changing environment.
Understanding and predicting the interaction between riparian vegetation and river morphology is crucial to assess river evolutionary trajectories under different climate, flow regulation, and river restoration scenarios. In this context, the goal of BASEveg is to quantify important geomorphological processes that so far have been described only in qualitative way. This will be achieved developing a physically-based, quantitative tool integrating both eco-morphological and numerical expertise. Therefore, BASEveg aims to:
advance and further develop the vegetation dynamic model to be included in existing two-dimensional models capable of predicting river morphodynamic changes;
analyse the thresholds between vegetated and un-vegetated bed morphologies, as a function of flood intensity and the ratio between vegetation growth timescale and flood frequency, for different types of vegetation and different initial morphological configurations;
investigate and modelling the morphological and vegetation dynamics and their interactions in a reach of the highly dynamic braided Tagliamento River in NE Italy, which is worldwide known as an reference river system for the interactions between flow, sediment transport and riparian vegetation dynamics.
Re-establishment of the sediment continuum at an alpine reservoir – influence on river morphology, ecology and flood prevention
Facchini, Matteo Siviglia, Annunziato Boes, Robert
Sediment continuum in rivers is often interrupted by the presence of dams, which trap bed load and suspended sediments inside the reservoirs. In the light of the new water protection law in Switzerland, bypass tunnels are an effective countermeasure to this effect. They are used with a twofold aim: on the one side they convey the sediments downstream to the dam diverting parts of the most relevant floods and on the other side they recover the sediment river continuum.
This project focuses on this latter aspect for which is important to analyze and study the dynamics of sediments in the reach downstream of the dam. It is particularly relevant to detect the time and spatial scales over which the sediments propagate and predict the development of the new morphology. It will be also of paramount importance to understand if it is possible to manage (e.g. by controlled flood release) this extra load of sediments in order to improve the ecological conditions of the downstream reach.
This project will be conducted with the use of numerical models and with the fundamental help of field observations that are necessary to produce reliable results.
Sediment budget analysis and management in the Ötztal
Kammerer, Stephan Vonwiller, Lukas Ehrbar, Daniel Vetsch, David
In order to increase the hydropower utilization, the hydropower company TIWAG is planning to access new catchment areas to contribute to the existing reservoir “Gepatsch” which belongs to the hydropower plant “Kaunertal” in Tirol. For this purpose, two new water intake facilities shall be built at the main tributaries of the “Ötztaler Ache”. To counteract reservoir sedimentation, an annual flushing of trapped sediments is being planned.
The long-term effects of sediment flushing are investigated by means of 1-D hydro- and morphodynamic simulations using the software BASEMENT. To meet the conditions of the study area, bedload transport was modelled using a multi-grain extension of the Smart & Jäggi transport formula.
Firstly, the actual state was modelled for the period 1997-2012. Sediments are being excavated along the “Ötztaler Ache” to some extend in the scope of flood protection. These removals play an important role and where implemented by different approaches in the numerical model.
Secondly, the sediment flushing during the operational phase of the intakes (planned state) was simulated for the same period of time. A comparison of the results of the actual and planned states was performed in order to evaluate the effect within the diverted river section downstream of the intake facilities.
Sediment replenishment by gravel deposits and stream bank erosion in Swiss lowland rivers
Vonwiller, Lukas Vetsch, David Boes, Robert
Gravel-bed rivers in Switzerland are generally in a poor ecological condition. This is mainly due to increased river training measures and hydro power plant operations over the last centuries. The construction of dams, weirs, and sediment retention basins led to a disruption of sediment continuity. Hence, many rivers suffer from a sediment deficit and morphological degradation. In order to improve the situation sediment can be replenished by means of (i) artificial gravel deposits, (ii) bank erosion, or a combination of both. Pilot projects of artificial gravel deposits in Switzerland have shown some promising results. However, the behavior of such measures and their impact on the river morphology are still not fully understood. There is a knowledge gap concerning successful application of artificial gravel deposits. Lateral erosion is the relevant process to entrain sediment from gravel deposits and river banks. Therefore, the objective of this research project is to improve the numerical modelling of processes related to fluvial bank erosion. The current research project will be performed in close cooperation with the PhD project of Friedl at VAW conducting laboratory experiments. This hybrid model approach combines the advantages of experimental and numerical methods allowing for a comprehensive analysis and an improved prediction of bank erosion.
The current research project will be performed in close cooperation with the PhD project of Friedl at VAW conducting laboratory experiments. This hybrid model approach combines the advantages of experimental and numerical methods allowing for a comprehensive analysis and an improved prediction of bank erosion.
Two dimensional depth-averaged modelling of braided gravel-bed rivers
Vetsch, David Florian Siviglia, Annunziato Vanzo, Davide
This computational study aims at describing and quantifying aggradation, degradation and channel rearrangement during floods by means of numerical modelling of braided rivers. For the validation of the numerical model, sophisticatedly monitored flume experiments provide topographical data as well as information on sediment transport with a high resolution in time and space.
The numerical model used to simulate temporal extracts of the flume experiments is based on the two-dimensional shallow-water equations solved with a finite-volume technique and explicit time discretization. Regarding sediment transport the numerical modelling approach includes diverse treatment of suspended load and bed load transport of sediment mixtures based on an active layer approach.
As far as sediment transport and according empirical relations are concerned, the focus is on bed load transport and the effect of bed armouring, which often occurs in alpine gravel-bed rivers. The numerical simulations are carried out with an initially braided topography based on an extract of the flume experiments and a constant hydrograph as steady inflow boundary condition. The results are verified with the data of temporally corresponding flume experiments. The impact of different bed load formulae and a varying resolution of the computational grid are studied. Furthermore the aspect of the different time scales of water and sediment transport is of interest.