Giovanni Battista Ferreri

Flow resistance of Posidonia oceanica in shallow water

Management of coastal waters and lagoons by mathematical circulation models requires determination of the hydraulic resistance of submerged vegetation. A plant typical of sandy coastal bottoms in the Mediterranean Sea is Posidonia oceanica, which is constituted by very thin and flexible ribbon-like leaves, about 1 cm wide and up to 1.5m long, and usually covers the bottom with a density of 500–1000 plants/m2. From the hydraulic viewpoint, P. oceanica constitutes a particular roughness, because, as the velocity increases, the leaves bend more and more until they lie down on the bottom. Although P. oceanica is widespread, in the technical literature it is difficult to find indications about flow resistance due to this plant. In this paper, the results of specific experimental research are reported. The runs were carried out in a laboratory flume, where the plants were reproduced assembling plastic strips. In these experiments, the leaf length was larger than the flow depth, reproducing a shallow water situation which is very frequent in lagoons. The results allow one to recognize the hydraulic behaviour of the plants with variation in the Reynolds number of flow and the ratio between the leaf length and the flow depth. Velocity distribution in the section is also examined and a simple flow resistance law is achieved, which expresses Darcy–Weisbachs friction factor as a function only of a particular Reynolds number.

Hydraulic jumps at drop and abrupt enlargement in rectangular channel

The different types of hydraulic jumps that occur in a rectangular channel at an abrupt increase in section are experimentally studied. The abrupt section increase is due to both a drop and an increase in the channel width. Experiments were carried out with three different values of the ratio L/l between the channel widths respectively downstream and upstream of the abrupt section increase. For each L/l value five values of Froude number F1, of the supercritical flow upstream of the section increase were considered, and for each of them live values of the depth y1 of the same flow. The experiments showed that, as the depth y2 of the downstream subcritical flow increases, several types of hydraulic jumps occur. The sequence of hydraulic jump types and several characteristics of hydraulic jumps of the same type change with the flow parameters L/l, F, and s/y1, with s the drop height. Physical explanations of these changes are proposed, based on both direct observation of phenomena and comparison with results of other authors relative to the cases of either drop only or enlargement only.

Ability of Preissmann slot scheme to simulate smooth pressurisation transient in sewers.

Urban drainage networks are generally designed to operate in a free-surface flow condition. However, as a consequence of heavy rainfall events or network malfunctions, the filling of sewers (pressurisation) and network overflow may occur. Several modelling software products are commonly used to simulate floods in drainage networks, and their results are usually thought to be reliable and robust. However, no specific studies have been carried out on the behaviour of these modelling products during the pressurisation transition. Mathematical models often use the Preissmann slot concept to handle pressurisation. In this paper, on the basis of laboratory pipe tests, the reliability of such a scheme is studied by means of a popular and open-source software product: SWMM (Storm Water Management Model). Many numerical tests were carried out with SWMM, varying the spatial and time steps and the Preissmann slot width, in order to examine the performance of the modelling software over intervals of these parameters even wider than what is usual in practical applications. The comparison between simulated and experimental surges allows one to draw interesting conclusions regarding the effectiveness of software products analogous to SWMM in simulating pressurisation, as well as the choice of the parameters themselves.

Storm sewer pressurization transient – an experimental investigation

ABSTRACT Pipe pressurization is examined experimentally by 144 laboratory experiments in a circular tilting pipe between two tanks, in which the transient was triggered by sudden closing of the downstream tank outlet. The experiments cover ranges of values of slope, velocity and filling ratio of the open-channel flow not explored in previous studies. Situations involving considerable air quantity and consequent intense pressure oscillations were also reproduced. Two different pressurization patterns, defined as “smooth” and “abrupt”, were observed, but only the abrupt pattern produced intense pressure oscillations. The comparison among all the abrupt pressurization surges showed how the oscillations changed in starting time, intensity and duration as the pipe slope, the flow rate and the free-surface flow filling ratio varied. The experimental results also stressed the major role of entrapped air in determining the oscillation characteristics, showing that oscillations were actually produced by the pulsating of large air pockets during their migration along the pipe and their release through the upstream manhole.

Flow hydraulic characteristics determining the occurrence of either smooth or abrupt sewer pressurization

ABSTRACT Laboratory experiments showed that pipe pressurization consequent on a drastic reduction in the downstream discharge can occur either by a gradual rising of the free-surface (“smooth” pressurization) or by propagation of a front filling the whole cross-section (“abrupt” pressurization). This study examines the free-surface flow characteristics that determine smooth or abrupt pressurization pattern through a theoretical approach using dimensionless variables. A critical flow rate value, which separates the pressurization patterns, exists for any given pipe diameter. For flow rates higher than this specific value, only abrupt pressurization occurs. For lower flow rates, either smooth or abrupt pressurization can take place; smooth pressurization occurs when the free-surface flow depth falls within a specified range, depending on the flow rate itself and the pipe diameter, whereas abrupt pressurization occurs when the depth falls outside this range. The comparison with actual uniform-flow conditions allows one to predict the pressurization pattern and the related pipe surcharge (in the case of abrupt pressurization). The analysis also shows that, in practice, severe surcharges can be expected even in the case of only partial reduction of the downstream discharge.