Editorial

Abstract

The papers in this year's final issue of Water Management not only all reflect important features of modern hydraulic engineering practice, but also have an underlying unity. To begin with, the majority highlight the importance of wellfocused experimental studies to elucidate key hydraulic phenomena that are not readily amenable to investigation by purely analytical or numerical models. The studies therefore required well-equipped laboratories, with suitable facilities for measurement and analysis. In no way wishing to diminish the role of modelling or computational fluid dynamics (CFD) in the analysis and solution of many practical problems, or as a means of understanding fundamental hydraulic processes, these papers are a reminder that CFD, or modelling in general, is not the answer to everything. Even today, physical studies are still required in order to fully understand some of the fascinating phenomena within fluid flows in open channels and pipes.The paper by Martín-Vide et al. (2010) describes experiments designed to understand the morphological evolution of a river inside a sharp bend where the footings of vertical walls require protection; a not uncommon requirement in urban waterfront developments. Riprap placed near the toe is known to protect such walls, but the way in which it subsequently deforms and develops into a sloping apron, thereby changing the features of the mainstream flow, is described. The authors not only show how such riprap can be an effective tool against scour at the toe of the outer bend wall, but also explain the mechanism by which the thalweg is pushed away from the outer wall and the mainstream velocities and boundary shear stresses are modified. Such flexible and falling aprons that respond to the river flow are then shown to provide a potentially simple solution to a practical problem.The two other papers on riprap and scour in this issue further illustrate the value of experimental studies. The paper by examines the four possible failure mechanisms of riprap placed on an apron, with particular relevance to characterising the size of riprap blocks to be placed on aprons at the base of spill-through abutments. Extensive studies on shear and erosion failure mechanisms, undertaken in two moderate-sized experimental flumes in the leading laboratories of two countries are described, leading to the validation of an equation for sizing stone riprap to prevent both shear failure and erosion failure over the abutment side slopes. The concept of critical approach velocity or intensity is also highlighted as being of some practical use.The paper by Cardoso and Fael (2010) differs in that it uses existing data from a particular set of long-lasting flow experiments on clear water scour (already reported in the literature) to re-examine the reasons why time to equilibrium scour in the field is often longer than that based on laboratory experiments, and why consequently field studies tend to report observed scour depths smaller than those calculated from equations based on equilibrium lab...