The papers in this issue of Water Management address two distinct worlds of water problems – the world of direct and immediate needs for improving efficiency of the existing technical water systems, and the world of our understanding of flow phenomena.
The first paper (Chanan et al., 2011) belongs to the first of these categories and focuses on economic assessment of water recycling policy on a national scale. The paper, based on an Australian case, highlights the lessons from the solid waste recycling industry and applies them to the water recycling sector. A sound water pricing regime that reflects the true costs of water and a competitive water industry is offered as a better policy alternative to setting recycling targets. Instead of promoting large-scale uneconomical water reuse megaprojects that may result from a target-based policy, this approach is likely to encourage decentralised reuse initiatives with long-term economic viability.
Although on a smaller, urban, scale, the paper by Vaez Tehrani et al. (2011) addresses the same issue of optimising the water supply. Increases in population and water demand and limited investments in the water sector in the city of Rasht, Iran have placed significant pressure on authorities and infrastructures in the provision of water. In this case, the efficiency of groundwater abstraction utilities is the concern of the authors. Using the system approach and dynamic model of well rehabilitation, dependency of the groundwater abstraction system performance on technical parameters and relevant causal loops is demonstrated. The model also considers the effects of management scenarios for different methods of rehabilitation. The paper presents general methodology that can be applied elsewhere.
The paper by Kabiri-Samani et al. (2011) on labyrinth side weirs describes the results of an extensive array of hydraulic experiments aimed at structural optimisation of weirs. The experimental work has a direct effect on the efficiency of irrigation facilities. Until quite recently, approximate methods based on experiments conducted over a limited range of the many geometric and hydraulic variables have been used. In many cases, the use of such methods caused substantial errors in the calculated spill discharge. In the experiments described by the authors, the employment of new devices – namely piles and vane plates with different configurations in the flow field – do influence the flow direction and are therefore applicable for improving the hydraulic performance of side weirs. During a comprehensive set of tests it was observed that, technically, if the vanes or piles were attached to the flow field at the downstream end of the weir, the flow was more uniform and the head loss was less than in other cases.
Finally, two papers that belong to the second, more theoretically oriented category of water issues add to the literature on flow in estuaries and in open channels. The paper by Hartnett et al. (2011) on residence time of tidal waterbodies aims, in practical terms, to assess the average residence time of pollutants that have been discharged into an estuary. The theoretical study shows that it is possible to relate average residence times to a combination of basic, easy-to-determine estuarine physical characteristics of primarily tidally-flushed estuaries. The new formula was developed by the authors on the basis of numerical model simulations of several Irish coastal water bodies. The values of the formula were compared with residence time estimates provided by other researchers and predictions obtained from the application of other existing methods. The new method also compares favorably with methods such as the jet–sink circulation model and Guo and Lordi's method. The formula is based on the definition of residence time proposed by Takeoka (1984), which, when coupled with the decay of the contaminant predicted by the numerical model, gives highly accurate estimates of contaminant concentration in the estuary.
The second paper of this category by Vatankhah and Easa (2011) addresses the old hydraulic issue of gradually varied flow (GVF). The authors present a new solution to the GVF equation that allows tracing of the flow depth along a channel length. Direct solutions of this differential equation are available in the technical literature only for the special case of wide rectangular channels and for general rectangular channels based on the simple Chezy formula. No direct solution is available for the case of general rectangular channels based on the complex, but more realistic, Manning formula. The paper presents an approach for establishing the GVF profile for general rectangular channels through application of the Manning formula. By yielding a very accurate formula for gradually varied flow the formula can become a useful tool for direct quantitative analysis and designs of general rectangular channels.
