G. de Marinis

Hydraulics of Circular Drop Manholes

Circular drop manholes are widely employed in steep urban drainage systems. Drop manholes may lead to poor hydraulic conditions if their energy dissipation is inadequate. The dominant hydraulic features of drop manholes depend on the flow regimes, characterized in terms of the dimensionless impact parameter. Depending on the latter parameter, the energy dissipation can vary within large limits, affecting thereby the downstream flow features. Also, the water pool depth inside the manhole and the air entrainment have been studied in terms of both the hydraulic and geometric parameters. Moreover, the conditions for which a drop manhole generates flow choking at its inlet or outlet have been investigated. Empirical equations for practical manhole design are provided. The importance of suitable manhole aeration is highlighted.

Creep functions for transients in HDPE pipes

New experimental results obtained from a high density polyethylene (HDPE) pipeline for transient conditions are presented and discussed. Transient pressure data at different pipe sections were collected. Peculiar features were observed, which are typical of viscoelastic pipes, in particular, a significant damping of extreme pressures was observed in comparison to what is typically obtained for metal pipes. Physical constraints of the pipe are important to describe some vibrations observed in the system. The viscoelastic transient solver developed by Covas et al. (2005) was used to carry out numerical simulations of experimental tests. The comparison of numerical results with experimental data has shown that the viscoelastic model accurately predicts observed transient pressures. Additional considerations are carried out about the calibrated creep function for this test case in comparison with the ones obtained in previous researches.

Economic level of reliability for the rehabilitation of hydraulic networks

The problem of water distribution system rehabilitation is formulated here as a multiobjective optimisation problem under uncertainty. The two objectives are to minimise the structural rehabilitation cost and to maximise the hydraulic reliability of the system. In this context, reliability is defined as a probability of simultaneously satisfying minimum pressure head constraints at all nodes in the network. An economic analysis has been performed, taking into account not only the structural costs but also lost revenue owing to the volume of water required by users but not supplied to them because of the structural inadequacy of the network. Because of the uncertainty in water demand, a probabilistic approach is used within the optimisation model. The probabilistic distribution and its parameters were estimated through an experimental study conducted on a real water distribution network. The recently developed robust Non-dominated Sorting Genetic Algorithm II optimisation algorithm is used to solve the optimisation problem. The methodology presented allows the identification of the specific optimal solution of the Pareto front that corresponds to minimal structural cost and minimal lost revenue yet corresponding to a high level of reliability – the economic level of reliability. This value could be assumed as the optimal solution or as a lower threshold, which restricts the gamut of technically feasible solutions that could be adopted in a rehabilitation programme.

WATER DEMAND MODELS FOR A SMALL NUMBER OF USERS

A better knowledge of the residential water consumption and the maximum water requirement allows for a more effective design or management of a Water Distribution System (WDS). The analysis of the behaviour of a small number of users in the request of water might be important also to understand the flow demand of numerous clusters of users supplied by a WDS. Indeed, each flow demand node can be seen as the aggregation of several consumers and, consequently, it represents the sum of several water requirements. An empirical study has been developed by monitoring a WDS in a small town, in southern Italy of about 1,200 inhabitants, in order to investigate water demand patterns and which probabilistic model could be better suitable to represent the residential water consumption. In particular, analysis on the experimental data collected from the system under consideration, has allowed to define probabilistic models which characterise the daily residential water demand. This study has highlighted that the goodness of those models depends on the number of users supplied and on the probability that a tap is opened. By means of statistical inferences on a large data sample, it has been shown that — at least for the range of users herein investigated — the Gumbel and Log-Normal distributions best represent the peak water demand; the mean water demand can be considered Normally distributed; the night flow requirement is well characterised with the Poisson model. In addition, in relation to the number of users supplied, the parameters of the probabilistic models have been estimated. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).

RESIDENTIAL WATER DEMAND - DAILY TRENDS

The behavior of residential users in the request of water influences the demand patterns which characterize the water consumption at each node of the network during the time. This stochastic variable influences greatly the performance of hydraulic networks and a better understanding of residential water consumption variability might allow for a more effective design or management of Water Distribution Systems. In order to contribute to this analysis, stochastic models are herein defined and proposed for representing demand variation, especially at the peak condition. This study is based on an experimental approach, comparing empirical data collected in several real water distribution networks which refer to different habits and number of users. Statistical inferences on large data samples have allowed defining probabilistic models and relative parameters which characterize the maximum water consumption at network nodes. The study has been developed considering both the deterministic cyclical component of the demand variability and its uncertain component that accommodates the random nature of the water requirement. Results obtained have shown that Gumbel and Log-Normal distributions best represent the peak water demand for all the monitoring systems considered.

Discussion of “Hydraulic Characteristics of a Drop Square Manhole with a Downstream Control Gate” by Rita F. Carvalho and Jorge Leandro

Vo (m=s) 0.97 1.06 1.12 1.17 1.21 1.28 1.32 1.38 1.4 1.41 1.47 1.48 1.52 1.53 I (-) 0.52 0.56 0.58 0.60 0.62 0.65 0.67 0.69 0.70 0.70 0.73 0.73 0.75 0.75

MULTIOBJECTIVE OPTIMAL REHABILITATION OF HYDRAULIC NETWORKS BASED ON THE COST OF RELIABILITY

The problem of Water Distribution System (WDS) rehabilitation is formulated here as a multiobjective optimisation problem under uncertain demands. The objectives are to minimise the total rehabilitation cost and to maximise the hydraulic reliability of the WDS. In this context, reliability is defined as a probability of simultaneously satisfying the minimum pressure head constraints at all nodes in the network. The total rehabilitation cost takes into account, not just the structural costs, but also the revenue lost due to the volume of water required by users but not supplied to them because of the structural inadequacy of the network. On this basis, safeguarding the hydraulic reliability of a WDS also implies a reasonable rate of return on the water company’s capital investment. A probabilistic approach is used to characterise demand uncertainties within the optimisation model. The relevant probability density functions and their parameters are estimated through an experimental study conducted on a real-life WDS. The rNSGAII optimisation algorithm is used to solve the optimisation problem. The lost revenue estimation is first performed as a part of the post-optimisation procedure, and then as an integral part of the optimisation process. The results obtained in the two approaches are compared showing the better performance of the integrated type approach. The methodology presented allows the identification of the specific optimal solution of the Pareto front that corresponds to the minimal total rehabilitation cost (structural cost plus lost revenue) yet corresponding to an high level of reliability, – the Economic Level of Reliability (ELR). This value could be assumed as the optimal solution or, allowing for socio-economic considerations (if higher WDS performance standards are required), as a lower threshold of reliability which restricts the range of technically feasible solutions that could be adopted in a rehabilitation programme. This way, the proposed methodology contributes to the definition of the reliability threshold values with which studying or designing a WDS.