Modelling of scour processes due to eco-friendly low-head restoration structures in natural streams

In recent decades there is a general degradation of the atmosphere and geomorphological systems of the earth because of global warming, industrial pollution, and other anthropogenic activities. The various water resources of the world are no exception. Therefore, there is a pressing need to maintain and restore the natural flowing conditions of rivers and other water bodies and preserve the natural aquatic habitat in these ecosystems. In this context, low-head eco-friendly grade-control structures like block ramps, cross vanes, different types of logframes, bendway weirs and chevrons are particularly important. These structures help to control the sediment load carried by the river and to dissipate flow energy to protect the downstream channel basin from excessive erosion. Moreover, they enable fish migration and create localised scour holes in the stilling basin which act as refuge for fishes and other aquatic animals. These structures are placed in a variety of field conditions. Particularly, in mountainous regions the river channels are characterised by significant curvature which causes excessive scour in correspondence with the outer bank due to secondary accelerations acting on the flow. In these cases, design criteria need to be developed focussing on bank protection and asymmetric nature of flow distribution in the downstream stilling basin. Moreover, often in both straight and curved channels, these structures can be associated with natural growth of various types of riverine vegetation in the downstream stilling basin. These vegetation significantly modify the equilibrium scour morphology due to low-head structures and can act as natural barriers to stabilise the outer bank and basin. Therefore, the aim of this thesis is to study the scour mechanism and equilibrium bed morphology due to double winged log-frames, arch shaped sills, and chevrons in straight and curved channels. In case of sills and log-frames the effect of in-stream vegetation on the scour process is also studied. The first chapter of the thesis deals with the description and design features of the commonly used low-head river restoration structures and a detailed review of the state of art. The second chapter discusses the various experimental models used in this study along with the instruments used to measure flow and scour characteristics. The third chapter provides the theoretical framework to carry out dimensional analysis using Buckingham’s theorem to identify the crucial hydraulic and geometric parameters governing the scour phenomena. In the fourth chapter, the results obtained from the different experimental models are discussed. In particular, a thorough classification of equilibrium scour typology due to different structures are presented to develop efficient design guidelines for the stilling basin. Empirical equations are proposed to estimate major scour features such as maximum scour depth, maximum scour length, maximum dune height and length with reasonable accuracy. Finally, the fifth chapter reports the conclusions of the study highlighting that the empirical equations along with the knowledge of resulting bed morphology are valid tools to optimise structure and stilling basin design.