B S Murty

An existing water distribution network (WDN) may need to be expanded by adding a sub-network for the newly developed areas. The size of the problem becomes larger when the stochastic nature of domestic demands, optimal design and layouts, control, and operation of various hydraulic components are considered. In this study, the single-port Thevenin theorem used in electrical circuits is applied to reduce a large WDN with its equivalent network consisting of a single source and a single pipe. The equivalent network is then attached to a sub-network for focused analysis. The accuracy and robustness of the proposed network reduction procedure are investigated on realistic WDNs for various sub-network demands using steady and extended period simulations. A simplified approach is also presented to achieve the same objective but constrained by the level of accuracy. Hydraulic engineers can use the proposed methodology as an efficient network reduction tool.

A sub-time stepping method is described for computational fluid dynamics problems that utilize implicit-type time marching procedures to resolve transients. In this method, small time-step sizes are used in portions of a domain where interest and activity is high, with larger time-step sizes being applied in other locations. The sub-time step is an integral portion of a larger time step - i.e., multiple sub-time steps over a sub-timed part of a domain add up to the time interval of the full time step used over the remainder of the domain. The technique is particularly suitable for extensive simulations where large portions of a domain are temporally over-discretized. The principles underlying implementation of the implicit sub-timing procedure, the computational effort in relation to conventional implicit time-stepping methods, and an analysis of the effects of sub-timing on the matrix structure are presented. Feasibility and applicability of the implicit sub-timing method is demonstrated through three proof-of-the-concept example problems. These examples include implementation of implicit sub-timing to one-dimensional as well as two-dimensional flow and solute transport problems. © 2003 Elsevier Science Ltd. All rights reserved.

Representations of land use change in hydrologic impact assessment studies mostly rely on static land use information of two points in time, even though the availability of dense time series of land use data allows for the incorporation of dynamic land use changes. We compare the hydrologic impacts of dynamic land use change assessments to those of static land use change assessments. These effects are illustrated with the help of two land use scenarios applied to a hydrologic model of a rapidly developing meso-scale (2036 km2) catchment upstream of Pune, India. The results show that a linear dynamic land use development could be better approximated with the static approach than a non-linear development. An analysis of the impact of the frequency of land use updates indicates that the prediction of non-linear land use change impacts already improves substantially when frequent land use information every five to nine years is used.

The aim of the investigation, reported in this paper, was to study the role of flood plains on flood peak subsidence and the time of occurrence of peak. A two-dimensional storage routing model was formulated for this purpose. The river and the flood plains form a two-dimensional network in the horizontal plane. Flow between adjacent cells is governed by river type links or weir type links . River type links are governed by Manning's formula and weir type links are governed by weir formulae. The various parameters that were considered are (1) the ratio of overall width to the main river width (B,r),(2) the ratio of flood plain roughness to the main river roughness (Nr), and (3) the ratio of peak flow to base flow (Qr).

Dynamic simulation models can be used along with flow and pressure measurements, for on-line leak detection and identification in gas pipeline networks. In this two part paper, a methodology is proposed for detecting and localizing leaks occurring in gas pipelines. The main features of the proposed methodology are: (i) it is applicable to both single pipelines and pipeline networks and (ii) it considers non-ideal gas mixtures. In order to achieve the desired computational efficiency for on-line deployment, an efficient state estimation technique based on a transfer function model, previously developed by the authors, is embedded in a hypothesis testing framework. In Part-I of this paper, a detailed description of the methodology is presented, and its performance is evaluated using simulations on two illustrative pipeline systems. The proposed method is shown to perform satisfactorily even with noisy measurements and during transient conditions, provided there is sufficient redundancy in the measurements. © 2010 Elsevier Ltd.

In this paper, an overlapping control volume method is presented for the numerical solution of transient 2D solute transport problems in ground water. The method is applicable for nonorthogonal grids and uses an isoparametric formulation for computing the dispersion and for second-order upwinding. Time integration is performed using an implicit approach. Three test cases are considered for comparing the numerical with analytical solutions. The scheme is second order in space and, when combined with Crank-Nicholson, is also second order in time. The results using Crank-Nicholson and full implicit (first-order) time integration methods are compared for problems with a variety of boundary conditions. For diffusion-dominated flows (PΔ ≤ 2) Crank-Nicholson works well, but for convection-dominated flows it produces spurious oscillations due to numerical dispersion errors. These oscillations are controlled by a flux limiter but only for Courant numbers below unity. It is shown that for high Courant and Peclet numbers a slight weighting of the time stepping toward fully implicit is effective against spurious oscillations and offers an optimum compromise between numerical dissipation and dispersion errors for a wide range of Courant and Peclet numbers. The scheme is shown to work on mildly nonorthogonal grids.

Groundwater quality in the towns of Namakkal and Erumaipatti in India was studied to understand the nexus between surface sanitation and groundwater quality in hard rock regions. In total, 32 wells, both shallow open and deep bore wells, were monitored over a two-year period. The presence of fecal coliforms (FCs) up to 600 CFU/100 mL in wells as deep as 100 m showed that bacteriological contamination had reached deep aquifers through fractures and fissures. Statistical analyses showed that bore wells located in Namakkal were bacteriologically more contaminated than those in Erumaipatti (p = 0.017 for FC) because of urbanization, the type of top soil and the shallow groundwater table. Wells in densely toileted areas of Namakkal were more contaminated than those located in open defecation areas. After replacing a soak pit with a septic tank, concentrations of FC and chemical oxygen demand (COD) in the leachate at a depth of 2.1 m reduced from 2,500 to 1,000 CFU/100 mL and from 200 to 50 mg/L, respectively, after 150 days of the construction of septic tanks. To improve the hygiene and sanitation, the provision of toilets along with on-site waste management systems, capable of achieving required effluent quality, are essential.

Leakage in water distribution systems is a growing concern throughout the world. Huge amounts of water and energy is wasted through undetected leaks and unauthorized usage in water distribution networks alongside potential health hazards caused by the leaky pipes. As most of the pipelines are buried underground, detecting a leak becomes a complex task. This work focuses on application of graph partitioning technique to Intermittent Water Distribution Networks for identification of leaks without use of a hydraulic model. The network structure is exploited to find the minimum number of valves to be closed or pipes to be isolated, and volume balance is carried out at each stage of partitioning to isolate the leak to a desired level of network resolution. This methodology is applied to a custom built, unique laboratory experimental setup that reflects intermittent water distribution network. The results demonstrate that the leaks can be localized to a smaller part of large network with the application of repeated graph partitioning technique.

Dynamic simulation models of gas pipeline networks can be used for on-line applications such as state estimation, leak detection, etc. A prime requirement for such models is computational efficiency. In this paper, a transfer function model of a gas pipeline is used as a basis for developing a dynamic simulator for gas pipeline networks. The simulator is incorporated in a data reconciliation framework, which is ideally suited for on-line state estimation based on all available measurements of pressures and flow rates. The American Gas Association (AGA) model is used for making realistic computations of the gas compressibility. Accuracy and computational efficiency of the proposed method are evaluated by comparing our results with those obtained using a fully nonlinear second-order accurate finite difference method. The ability of the proposed approach for obtaining accurate state estimation from noisy measurements is demonstrated through simulations on an example network. We also demonstrate the use of the proposed approach for estimating an unknown demand at any node by exploiting the redundancy in measurements. © 2006 American Chemical Society.