| publications-4581 |
article |
2001 |
Tanyimboh, Tiku T. and Tanyimboh, Tiku T. and Kalungi, Paul and Kalungi, Paul and Xu, Chengchao and Xu, Chengchao and Goulter, I. C. and Goulter, Ian C. |
Reliability-Based Optimal Design of Water Distribution Networks |
Journal of Water Resources Planning and Management |
10.1061/(asce)0733-9496(2001)127:4(278) |
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A new approach for reliability-based optimization of water distribution networks is presented. The approach is capable of recognizing the uncertainty in nodal demands and pipe capacity as well as the effects of mechanical failure of system components. A probabilistic hydraulic model is used in the model to account for uncertainty in nodal demands and pipe capacity. The primary innovation of the model is the use of a first-order reliability-method-based algorithm to compute approximate values of the capacity reliability of water distribution networks. Capacity reliability is defined as the probability that the nodal demand is met \Iat or over\N the prescribed minimum pressure for a fixed network configuration under random nodal demands and random pipe roughnesses. The model also incorporates a strategy for identifying the critical nodes on which the reliability constraints are imposed in the cost minimizing step. The computational efficiency of the optimization is shown to be enhanced by deriving the first-order derivatives analytically using a sensitivity-analysis-based technique. The efficiency and capacity of the proposed algorithm are illustrated by application to two sample networks. |
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| publications-4582 |
article |
2003 |
Kapelan, Zoran and Kapelan, Zoran and Savić, Dragan and Savic, Dragan and Walters, Godfrey A. and Walters, Godfrey A. |
Multiobjective sampling design for water distribution model calibration |
Journal of Water Resources Planning and Management |
10.1061/(asce)0733-9496(2003)129:6(466) |
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Sampling design (SD) for water distribution systems (WDS) is undoubtedly an important issue, and has been addressed in the past by a number of scientists and practitioners. The aim of the SD methodology developed here is to find a set of optimal network locations at which to place measurement devices. Optimal locations are determined with the aim of collecting data that will be used later on in the calibration of the analyzed WDS hydraulic model. First, existing calibration and SD approaches in the case of WDS are reviewed. After that, SD is formulated as a two-objective optimization problem. The objectives are maximization of the calibrated model accuracy by minimization of the relevant uncertainties, and minimization of total SD costs. The optimal SD problem is then solved using a multiobjective genetic algorithm based on Pareto ranking, niching, and restricted mating. The methodology developed is applied and verified on a case study. At the end, a summary is made and relevant conclusions are drawn. |
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| publications-4583 |
article |
2002 |
Holme, Petter and Holme, Petter and Kim, Beom Jun and Kim, Beom Jun and Yoon, Cheol Yong and Yoon, Chang No and Han, Seung Kee and Han, Seung Kee |
Attack vulnerability of complex networks |
Physical Review E |
10.1103/physreve.65.056109 |
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We study the response of complex networks subject to attacks on vertices and edges. Several existing complex network models as well as real-world networks of scientific collaborations and Internet traffic are numerically investigated, and the network performance is quantitatively measured by the average inverse geodesic length and the size of the largest connected subgraph. For each case of attacks on vertices and edges, four different attacking strategies are used: removals by the descending order of the degree and the betweenness centrality, calculated for either the initial network or the current network during the removal procedure. It is found that the removals by the recalculated degrees and betweenness centralities are often more harmful than the attack strategies based on the initial network, suggesting that the network structure changes as important vertices or edges are removed. Furthermore, the correlation between the betweenness centrality and the degree in complex networks is studied. |
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| publications-4584 |
article |
2003 |
Munavalli, G. R. and Munavalli, G. R. and Kumar, M. S. Mohan and Kumar, Manoj and Kumar, Manoj and Kumar, Mohan and Kumar, M. S. Mohan |
Optimal Scheduling of Multiple Chlorine Sources in Water Distribution Systems |
Journal of Water Resources Planning and Management |
10.1061/(asce)0733-9496(2003)129:6(493) |
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The specified range of free chlorine residual (between minimum and maximum) in water distribution systems needs to be maintained to avoid deterioration of the microbial quality of water, control taste and/or odor problems, and hinder formation of carcinogenic disinfection by-products. Multiple water quality sources for providing chlorine input are needed to maintain the chlorine residuals within a specified range throughout the distribution system. The determination of source dosage (i.e., chlorine concentrations/chlorine mass rates) at water quality sources to satisfy the above objective under dynamic conditions is a complex process. A nonlinear optimization problem is formulated to determine the chlorine dosage at the water quality sources subjected to minimum and maximum constraints on chlorine concentrations at all monitoring nodes. A genetic algorithm (GA) approach in which decision variables (chlorine dosage) are coded as binary strings is used to solve this highly nonlinear optimization problem, with nonlinearities arising due to set-point sources and non-first-order reactions. Application of the model is illustrated using three sample water distribution systems, and it indicates that the GA is a useful tool for evaluating optimal water quality source chlorine schedules. |
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| publications-4585 |
article |
2004 |
Tolson, Bryan A. and Tolson, Bryan A. and Maier, Holger R. and Maier, Holger R. and Simpson, Angus R. and Simpson, Angus R. and Lence, Barbara J. and Lence, Barbara J. |
Genetic Algorithms for Reliability-Based Optimization of Water Distribution Systems |
Journal of Water Resources Planning and Management |
10.1061/(asce)0733-9496(2004)130:1(63) |
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A new approach for reliability-based optimization of water distribution networks is presented. The approach links a genetic algorithm ~GA! as the optimization tool with the first-order reliability method ~FORM! for estimating network capacity reliability. Network capacity reliability in this case study refers to the probability of meeting minimum allowable pressure constraints across the network under uncertain nodal demands and uncertain pipe roughness conditions. The critical node capacity reliability approximation for network capacity reliability is closely examined and new methods for estimating the critical nodal and overall network capacity reliability using FORM are presented. FORM approximates Monte Carlo simulation reliabilities accurately and efficiently. In addition, FORM can be used to automatically determine the critical node location and corresponding capacity reliability. Network capacity reliability approxi- mations using FORM are improved by considering two failure modes. This research demonstrates the novel combination of a GA with FORM as an effective approach for reliability-based optimization of water distribution networks. Correlations between random variables are shown to significantly increase optimal network costs. |
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| publications-4586 |
article |
2003 |
Bahadur, Rakesh and Samuels, William B. and Grayman, Walter M. and Amstutz, David E. and Pickus, Jonathan M. |
PipelineNet: A Model for Monitoring Introduced Contaminants in a Distribution System |
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10.1061/40685(2003)128 |
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Water systems are responsible for conducting monitoring of drinking water to ensure that it meets all drinking water standards. EPA has established pollutant-specific minimum testing schedules for public water systems. Typical sampling and monitoring sites in distribution systems may include points close to water treatment systems, core business locations, secondary water storage reservoirs, repumping or retreatment facilities and change in water quality. Sampling location is also dependent on investment in labor, transportation requirements, and supplies to carry out sampling commitments. This adds to a water utilities financial commitment. Monitoring and/or predicting the fate and transport of introduced contaminants in water distribution systems is therefore a challenging proposition, involving the identification and operation of numerous hydrological and water quality-related factors. A common and important question in the design of a monitoring network is how many samples should be collected and where? The answer is often based upon best professional judgment and financial considerations. This paper describes a methodology to select nodes for monitoring in case of an intrusion. The monitoring location selection methodology uses a combination of extended period simulation (EPS) model and GIS data. |
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| publications-4587 |
article |
2004 |
Resende, Mauricio G. C. and Werneck, Renato F. |
A Hybrid Heuristic for the p -Median Problem |
Journal of Heuristics |
10.1023/b:heur.0000019986.96257.50 |
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Given n customers and a set F of m potential facilities, the p-median problem consists in finding a subset of F with p facilities such that the cost of serving all customers is minimized. This is a well-known NP-complete problem with important applications in location science and classification (clustering). We present a multistart hybrid heuristic that combines elements of several traditional metaheuristics to find near-optimal solutions to this problem. Empirical results on instances from the literature attest the robustness of the algorithm, which performs at least as well as other methods, and often better in terms of both running time and solution quality. In all cases the solutions obtained by our method were within 0.1\% of the best known upper bounds. |
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| publications-4588 |
article |
2004 |
Jacobs, Heinz Erasmus and Jacobs, Heinz Erasmus and Haarhoff, Johannes and Jacobs, HE and Haarhoff, Johannes and Jacobs, HE and Haarhoff, Johannes and Haarhoff, Johannes |
Application of a residential end-use model for estimating cold and hot water demand, wastewater flow and salinity |
Water SA |
10.4314/wsa.v30i3.5078 |
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The structure and data requirements of an end-use model for residential water demand and return flow are presented in a companion paper. This paper focuses on the practical application of the model. The model is first applied to confirm a few commonly observed water demand patterns: Seasonal variation in demand, the positive correlation between average annual daily water demand and stand size, and the increase in water demand, hot water demand and wastewater flow with an increase in household size. The convergence between the predicted model results and independently observed values by others encourages practical use of the model. Secondly, the effects of some specific water demand management measures are evaluated by adjusting selected model parameters. The measures include xeriscaping, the installation of dual-flush toilets and low-flow showerheads, pool ownership and pool cover use. The model provides a rapid means to obtain first estimates of the likely effects of different water demand management measures. |
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| publications-4589 |
article |
2004 |
GarcΓÂa, Vicente J. Serradell and Garcia, Vicente Juan and GarcΓÂa-Bartual, Rafael and GarcΓÂa-Bartual, R. and Cabrera, Enrique and Cabrera, Enrique and Arregui, Francisco J. and Arregui, F. J. and Arregui, Francisco J. and GarcΓÂa-Serra, Jorge and GarcΓÂa-Serra, Jorge |
Stochastic Model to Evaluate Residential Water Demands |
Journal of Water Resources Planning and Management |
10.1061/(asce)0733-9496(2004)130:5(386) |
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The analysis and modeling of water distribution networks has been a well established engineering field for many years. However, important questions remain concerning the correct assessment of the spatial and temporal distribution of network user demands. To contribute to better knowledge and understanding of consumption patterns in an urban network, a stochastic model for residential water demand simulation is developed. The model is based on a rectangular pulse point process of residential consumption of given duration and intensity. Both variables are considered as statistically independent variables, with a nonhomogeneous point process used to describe pulse occurrences over time. The model includes a total of nine free parameters that define five different statistical functions. The parameters were calibrated from known demands in residential areas located in Milford, Ohio, and in Valencia, Spain. The model is also applied to the simulation of longer periods, with satisfactory agreement generally found between synthetic and historical series for most significant variables used in practical applications. |
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| publications-4590 |
article |
2005 |
Zhang, Henry H. and Zhang, Henry H. and Brown, David F.M. and Brown, David F. |
Understanding urban residential water use in Beijing and Tianjin, China |
Habitat International |
10.1016/j.habitatint.2004.04.002 |
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