Although the papers in this issue have no core theme, they cover some interesting applications over a range of topics.
Manning's empirical equation has been used for over 100 years to describe the mean velocity in open channels. The form of the equation raises questions such as ‘why does the velocity depend on the hydraulic radius?' and ‘why is the power two-thirds?' The first paper, by Horritt and Wright (2013), describes the use of a mixing length model for estimating channel conveyance and shows that this simple, physically based model explains the functional form of Manning's equation and the Colebrook–White formula. The model applies Prandtl's mixing length hypothesis, with the mixing length at each point in a river cross-section being proportional to the distance to the nearest solid boundary. The model is applied to three different geometries (planar beds and circular conduits, rectangular channels, and arbitrary cross-sections) and different numerical solution methods (one-dimensional Runge-Kutta and the Jacobi method) are used for each geometry. The results show that the mixing length model reproduces some of the well-known hydraulic behaviours of open channel flows, and how essentially empirical results such as Colebrook–White and Manning's arise from open channel hydraulic processes. It also reproduces the dependence of mean velocity on channel geometry, and produces further information such as velocity profiles and bed shear. The authors suggest that the method may have further usefulness in such aspects as modelling complex geometries, interactions between regions of flow, turbulence modelling, head loss through structures, and scour and sediment transport.
Marine outfall discharges of wastewater effluents typically contain contaminants even after treatment. The paper by Liu and Li (2013) addresses the question ‘To what extent is the receiving environment being exposed to effluents?' by describing a particle-tracking model of outfall flumes in a tidal channel. In the model, effluents are represented by a large number of particles, whose trajectories are tracked under various conditions of flow and density stratification. The technique is particularly useful for examining the undesirable scenario of effluents rising to the surface or coming into contact with the seabed. The method takes into account advection, non-Fickian horizontal diffusion and vertical diffusion. The paper describes an application of the technique to an outfall discharge in British Columbia, Canada, where it successfully captured the detailed structures of outfall plumes. Based on the technique, the paper details a number of important conclusions for modelling these conditions.
The paper by Pagliara and Carnacina (2013) addresses the effect debris collected on bridge piers has on the scour pattern. This can be a significant contributor to bridge failure. Previous work has shown that scour hole dimensions generally increase with debris accumulation and roughness. The paper describes a series of experimental runs in a hydraulic laboratory to analyse the flow field resulting from debris accumulation and roughness at a cylindrical bridge pier. Debris increased the velocity and turbulence. The contraction in the flow field caused by debris accumulation increases the kinetic energy of the flow at the base of the pier, and this kinetic energy increases both the capacity of the flow to scour sediment from around the pier and the ability of the flow to convey sediment. The flow is further accelerated by the roughness of the debris.
The paper by Liu and Wang (2013) derives explicit equations for flow in city-gate cross-sections, which are frequently used for free-water conveyance tunnels. These cross-sections have a flat floor, a vertical wall and an arch top, a shape which has good hydraulic and structural characteristics and is convenient to construct and maintain. However, the complex geometry means that for many practical sections, the governing equations for critical and normal depths are implicit and no analytical solutions exist. The paper describes the use of gradual optimisation fitting to derive explicit equations for standard city-gate sections.
The final paper by Benzaghta et al. (2013) compares the performance of several types of cover in reducing evaporation from water bodies in a humid climate. The tests were carried out in Malaysia using four PVC tanks and a class A evaporation tank. Tanks were stood on timber platforms, with one PVC tank uncovered and the other three covered, each by a different method. Cover types used were lightweight, easy to handle and available locally. Two floating covers (Mengkuang mat and pieces of plywood) were used, along with one structural cover (galvanised iron corrugated sheets). These gave average evaporation reductions over a year of 40%, 33%, and 26% respectively. Mengkuang mat gave the highest reduction and is renewable and sustainable. A series of laboratory analyses revealed no serious effects on water quality.
