Numerical groyne layout optimisation for restoration projects in large rivers: An adaptive approach towards a desired morphodynamic equilibrium

Abstract

Integrative restoration measures at large rivers target the improvement of morphological and ecological conditions, under consideration of economic demands, specifically navigational ones. Alternative groyne layouts with e.g. reduced groyne spacing and lowered crest elevation reduce ecological deficits and have the potential to cease frequently encountered river bed incision of heavily modified rivers. On the other hand, the induced change in the morphodynamic equilibrium may interfere with navigation by reducing the water depth in the fairway. In 2009, a pilot project was realised on the Austrian Danube, including an alternative groyne layout. As a consequence the desired aggradations in the fairway became too large, leading to an increased dredging effort. In 2014, a numerical groyne optimisation, specifically a 3D numerical model in combination with a sediment transport model, was applied. In 2015, after implementing the optimised groyne layout in the field, morphodynamic equilibrium was reached reducing the need of extensive dredging. This equilibrium could be shown by analysing subsequently observed bathymetries until 2017. Moreover, the morphodynamic changes due to the groyne optimisation in 2015 were reproduced successfully with the numerical models. Thus they represent a cost effective tool for planning and optimising future restoration measures in large and heavily modified rivers.