Rudy Gargano

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.

Air-water flows in circular drop manholes

Drop manholes, a typical element of urban drainage networks in steep catchments and in reaches of supercritical flow, enhance air entrainment and entrapment. The air flow across drop shafts can be remarkably high. This paper discusses the air transport phenomena and the effects of ventilation absence in drop manholes. Based on an extensive experimental study, air entrainment mechanisms have been accurately described and air demand has been evaluated in different flow regimes. In addition, the effects of ventilation absence on the hydraulics of circular drop manholes, with emphasis on sub-atmospheric pressure onset and pool depth raising, have been investigated. The effects of a possible air flow recirculation have also been evaluated. Issues regarding any scale effects have been discussed. The influence of the main hydraulic and geometric parameters on drop manhole performance was contemplated to provide improved design concepts for sewer systems.

Flow-improving elements in circular drop manholes

ABSTRACT Drop manholes are widely employed in urban drainage systems. They experience operating conditions in which insufficient energy dissipation leads to hydraulic problems that are enhanced by particular configurations of the manhole bottom. Therefore, specific jet-breaker devices were considered to improve the flow conditions. Two different types of jet-breakers have been tested in the present work: plane jet-breaker (PJB) and wedge jet-breaker (WJB). Both have proved to be effective in inducing adequate energy loss, if properly sized, and to improve the overall drop manhole performance. The selection of a jet-breaker element should account for additional features of the drop manhole hydraulics, including the pool depth, the air entrainment phenomena, as well as further practical aspects including cost-effectiveness, simplicity of realization and clogging risk. This study provides the hydraulic basis for the design of the jet-breakers, particularly of the PJB, which has an overall optimum performance, as laboratory experiments have shown.

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.

A novel equation for determining the suction stress of unsaturated soils from the water retention curve based on wetted surface area in pores

A novel equation is proposed for the evaluation of the suction stress of an unsaturated soil. The equation, based on the assumption that suction is transmitted to soil solid particles only through their wet external surface, allows to easily derive the soil suction characteristic curve from the water retention curve. The proposed equation has been verified against published experimental data of suction stress smaller than 1 MPa for soils of various characteristics. In all cases, an excellent agreement between predicted and observed values of suction stress is achieved, showing that the proposed equation performs better than other currently adopted expressions for the evaluation of soil suction stress.

A stochastic approach for the water demand of residential end users

Reliable hydraulic modeling of water distribution networks requires a deep knowledge of water demand. In the past few years, technical literature has been enriched with many contributions aimed at realistically representing the residential water demand of end users. The proposed models generally represent water demand through rectangular pulses, describing demand as the sum of the requests from single domestic appliances. This paper proposes a new stochastic model - Overall Pulse (OP) - which allows the generation of the overall domestic demand as displayed at the house water meter. The proposed model allows the taking into account of the randomness of the arrivals rate and the demand persistence phenomenon. The effectiveness of the proposed model has been tested, comparing the generated data series with those measured for residences with different lifestyles. The generation of the synthetic data series has been made by means of the OP model with the Monte Carlo method.

Novel Approach for Side Weirs in Supercritical Flow

AbstractSide weirs are hydraulic structures widely used in practice and extensively studied in the technical literature. The flow along a side weir is a classic example of spatially varied flow, the study of which is generally performed by means of a momentum or energy approach. The momentum approach is usually more accurate but requires the experimental estimation of correction coefficients leading to a formulation that is difficult to apply to a practical design. The energy approach, instead, is not as accurate when the flow depths on the side weir are significantly higher than the same weir crest. A novel, simplified theoretical model for the study of single side weir in a circular pipe, operating in supercritical flow, is proposed. The model originates from an experimental analysis of the flow power variation along the side weir. The proposed model has shown good performance in free surface representation and is capable of estimating the lateral outflow with good accuracy. The suggested formulation is...

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).

Preliminary Estimate of Detention Basin Efficiency at Watershed Scale

Urban encroachment in natural floodplain areas and infrastructures interfering with watercourses have caused higher flood risks in lowland areas. In this context, detention basins have become a fundamental instrument for stormwater and environmental management at watershed scale. Numerical methods of flood routing are generally coupled with optimization algorithms to investigate the factors that affect the overall efficiency of detention basins in controlling the peak flows throughout a watershed. To overcome the procedure effort due to numerical integration, a simple innovative approach, based on the linear system theory applied to the solution of hydrologic flood routing, is proposed for a preliminary estimate of overall efficiency. First a numerical analysis is performed to ensure that the schematization of the detention basin as a linear system leads to technically acceptable approximation. Then, a simple analytical equation is provided that allows a preliminary estimate of detention basin efficiency in downstream river reaches. Sensitivity analysis of the above equation provides information about the factors that most contribute to the downstream flow reduction variability. Finally, the proposed methodology, adequately extended to a parallel system of stormwater detention basins within a watershed, can be easily integrated in optimization algorithms.

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.