Those pursuing a career in the engineering sciences are normally called upon to confront a broad spectrum of theoretical and practical issues and the papers presented in this month's Water Management address an interesting range of subjects which are of current concern to water engineers and scientists. The four papers included here deal with the capture and storage of rainwater, the sustainable use of groundwater resources, the dewatering of an unconfined aquifer at a construction site, and the discharge of water from a side weir on a curved channel. The contributions come from the West Indies, Australia and Turkey.
Rainwater harvesting has become an important feature of water management in many parts of the world, whether it is the voluntary collection and storage of water from rooftops for gardening and horticulture or the mandatory provision of rainwater-harvesting structures at buildings, now required in many Indian cities. The first paper, by Peters (2012), takes us to the islands of the Grenadines in the eastern Caribbean. These islands have no surface water flows, limited groundwater and depend almost exclusively on rainwater harvesting to meet household and agricultural requirements. The study focuses on the small island of Carriacou (34 km2).
Where rainwater harvesting is the dominant form of water supply, the low, seasonal and unpredictable distribution of rainfall presents a considerable challenge. Peters examines the drought, influenced by El Niño, of 2009–2010 at Carriacou and its impact on a typical range of household cisterns. The author notes the low consumption of water (18 l/c/d) during periods of drought in low- and middle-income households and the impact on sanitation. The paper discusses the impact in the Grenadines of the local government water crisis committee which was set up to manage the water shortage, the expensive measures taken to bring water by barge to the islands and other coping strategies.
Recognising the need for drought monitoring and forecasting, Peters analyses alternative drought indices and suggests how they can be used in conjunction with a simulation model of the available storage to improve decision making processes. Peters' conclusions include a recommendation for a high degree of community participation and liaison with government agencies to mitigate the impact of water shortage. This is a theme that is expanded upon in the second paper (Elmahdi and McFarlane, 2012) which discusses the management of the depleted water resources of the Gnangara groundwater system in Western Australia, an important source of groundwater for the city of Perth (population 1·5 million).
Elmahdi and McFarlane describe what is now becoming a familiar story in many parts of the world, where abstraction exceeds groundwater recharge and the groundwater levels have declined. At Gnangara, groundwater levels fell by up to 6 m between 1979 and 2003. The paper revisits participatory approach principles (Asian Development Bank, 2003) and describes a multi-agency framework methodology that utilises a decision support system (DSS) relating to land and water management planning processes. The application of these to the Gnangara groundwater system, an area of about 2200 km2, is described. The approach adopted, with inputs from a number of government and other agencies, allowed the issues and impacts of pine forest cover, horticulture, climate change, urban planning and water supplies to be addressed. The DSS model was used to predict the effects of a number of possible future land and water use scenarios. The authors recognise the complexity of adopting a multi-agency approach but see this as the best way to optimise what they call the ‘triple bottom line'; the environmental, economic and social values required to achieve sustainability.
Whereas, at Gnangara, restoring and maintaining groundwater levels is the objective, the paper by Demirbaş et al. (2012) deals with optimising the dewatering of an unconfined aquifer at a site excavation. This is an important subject as there is an increasing demand for the construction of underground and sub-surface structures and there are both capital and operating costs of dewatering to be considered. The authors make use of models, developed some 20 years ago, which combine a finite-difference groundwater model, Modflow (McDonald and Haubaugh, 1988) and an optimisation module, ModMan (Greenwald, 1998), to generate a response matrix. They explore the most efficient dewatering scheme for two hypothetical areas, a circular area with a radius of 1000 m and an excavation with an plan area of about 2·5 ha, similar to one studied by Degirmenci (1997).
The study applies a method, which uses a linear response matrix approach, to an unconfined aquifer that shows a non-linear response to pumping. The aim of the study was, by introducing different iterative methods, to remove the errors arising from the non-linearity. The authors conclude that the modified linear optimisation procedure used provides an efficient tool for analysing dewatering options for unconfined aquifers. The results of applying their findings to the selection of pumping facilities and optimisation of costs for dewatering in a real life excavation are awaited with interest.
The final paper in this issue adds another chapter to the saga on discharge coefficients for weirs. Agaccioglu et al. (2012) have looked at and derived a coefficient for a sharp-crested side weir discharging from a curved rectangular channel. Ten discharge coefficients, derived between 1972 and 1999, are cited by the authors for discharge coefficients equations for rectangular side weirs in straight channels but few studies have reported on discharge over a weir from the side of a curved channel.
The authors performed over 1500 tests in a physical model with flow under sub-critical flow conditions rounding a 180° bend of 3 m radius in a rectangular channel 0·5 m wide by 0·5 m deep. Three different curved lengths of weirs were tested (0·25, 0·5 and 0·75 m long) and three different heights. The tests were made for individual weir configurations at a variety of points on the outside of the bend, subject to the nappe height being greater than 30 mm to avoid surface tension effects. The discharge coefficient (Cd) was derived for different weir levels, weir length, Froude number and position of the weir on the bend. An equation for Cd, derived from the tests and based on dimensionless parameters for a sharp crested weir located on a curved channel is given in the paper. Agaccioglu and Yuksei (1998) reported on similar but less extensive tests and a comparison is made between the Cd derived then and now. The general conclusions include comments on the lateral flow, stagnation and separation effects in the bend, indicating that the point on the bend where the weir is placed is an important parameter influencing the discharge coefficient, and that the discharge coefficient of a rectangular side weir located on a bend was found to be 1·2 to 1·9 times higher than one on a straight channel. Irrigation, land drainage, water and wastewater engineers may wish to take note.
