Claudio Arena

A simulation/optimization model for selecting infrastructure alternatives in complex water resource systems

The paper introduces a simulation/optimization procedure for the assessment and the selection of infrastructure alternatives in a complex water resources system, i.e. in a multisource (reservoirs) multipurpose bulk water supply scheme. An infrastucture alternative is here a vector X of n decision variables describing the candidate expansions/new plants/water transfers etc. Each parameter may take on a discrete number of values, with its own investment cost attached. The procedure uses genetic algorithms for the search of the optimal vector X through operators mimicking the mechanisms of natural selection. For each X, the value of the objective function (O.F.) is assessed via a simulation model. Simulation is necessary as the O.F. contains, besides investment costs, also incremental operation costs and benefits that depend on the incremental water amounts which the alternative can provide. The simulation model transforms a thirty-year hydrologic input at daily/monthly scale in water allocations, accounting for the usual nonnegativity constraints and using some simple, sytem-specific rules aimed at reducing spills and at sharing water deficits among demand centres. Different O.Fs and constraints have been tested, such as incremental financial cost/benefit minimization under various maximum water deficit constraints scenarios or cost/benefit mimization including scarcity costs. This latter approach has the advantage of implicitly allowing for the magnitude of deficits, but requires the assessment of deficit-scarcity cost relationships. The application of the procedure to a water resources system in south-western Sicily shows that the model is able to converge to results that are consistent with the planning options expressed by the selected O.Fs.

PLANNING REHABILITATION ACTIONS ON WATER DISTRIBUTION NETWORKS IN CONDITION OF DATA SCARCITY

The paper shows a viable approach to the selection of water networks rehabilitation actions, in condition of data scarcity and uncertainty, based on the use of four concise performance literature indices, allowing the analysis and identification of efficient, reliable and robust rehabilitation alternatives, even if assessed on the basis of a relatively small amount of data. A promising statistical sampling method of water consumptions is also described, that may be fruitfully employed within the proposed approach for drafting water balances at network district scale in order to quantify the extent of leakages. The illustrated case study shows that the selected indices are capable to describe suitably the performance the network would achieve, with respect to water demand satisfaction and to service efficiency, if the proposed rehabilitation options (e.g. leakage detection and reduction, cleaning and replacement of mains) were undertaken, as well as the reliability and robustness of the options themselves, and hence provide a good selection criterion of alternative rehabilitation options. The application of the statistical sampling method to water consumptions of a small municipality in Sicily, (Italy), shows that it may be applicable in practice and that it provides results having the accuracy level predictable by the theory.