Corrado Gisonni

Supercritical flow in the 90° junction manhole

Abstract Based on previous observations of the 45° junction manhole for supercritical flow in the main and lateral branches, the hydraulics of the more common 90° junction manhole were explored. Using a selected manhole geometry involving: (1) a short straight piece in the lateral branch to inhibit full development of the bend wave, and (2) the addition of the junction extension as used in previous designs for the bend manhole, the present study gives results that are in basic agreement with those collected in the 45° junction manhole. This surprising result thus allows for a design basis independent of the junction angle. The present paper defines three waves that may occur in a junction manhole, i.e. bend wave, junction wave and the swell at the manhole outlet into the downstream pipe. In addition, the position of the determining junction wave was established. Important for the junction design is the discharge capacity for which supercritical flow can be maintained across the manhole. It was found that the lateral branch flow depth and the pipe diameter have an important effect on this capacity, for both branches or only one of the branches in operation.

Head losses in junction manholes for free surface flows in circular conduits

Former studies on combining flows resulted in an efficient layout of sewer junctions operated under supercritical approach flow conditions. Straight extensions allowed a reduction in the shock wave heights generated by the merging flows, so that the global discharge capacity was significantly increased. Herein, an extensive experimental campaign is presented on a physical model with the aforementioned layout, although with generalized geometrical conditions now including various conduit diameters. The effects of the main parameters governing the energy losses for combining flows were ascertained to enhance the information available from the literature. The results and their analysis provide a basis for the prediction of energy losses at junction manholes with different upstream and lateral conduit diameters and various flow conditions.

Supercritical flow in sewer manholes

The breakdown of supercritical flow in sewer manholes may be dangerous when designed according to current practice. This issue is particularly relevant for drainage systems conveying discharge in both stormwater and combined sewers. Based on detailed hydraulic experimentation, the main features of three manhole types were observed, including the through-flow, the bend and the junction manholes. Using systematic observations and a hydraulic approach, the large data sets allowed presentation of a modified and generalized design for such hydraulic structures, thereby accounting for Froude similitude. Both the hydraulic flow patterns along with design guidelines are presented to allow for a safe and straightforward design of manholes in these sewer networks. The design relates particularly to freeboard requirements and to the discharge capacity of sewer manholes. It is demonstrated that the dimensionless discharge capacity of the junction and the through-flow manholes is roughly twice and three times larger than for bend manholes, respectively.

Vortex dropshaft retrofitting: case of Naples city (Italy)

This work describes the main hydraulic features of vortex dropshafts along with some examples of their mis-designs and consequent malfunction. Laboratory experiments were conducted to test the hydraulic performance of subcritical vortex intakes using supercritical approach flow design basis. The purposes of these drop structures are to: (i) convey the storm runoff towards the sea through the existing sewer system located at lower elevations and (ii) by-pass insufficient sewers. The proper operation of these structures is crucial to prevent flooding and to guarantee the urban safety. This issue is particularly relevant for the sewer system of Naples (Italy), which is strongly constrained by its hilly landscape. Several drop structures were realized to connect the modern urban drainage system to the early sewer mains located at the sea level. This work intends to highlight practical issues to enhance the existing vortex dropshafts, with possible applications to other cases similar to the Neapolitan context.

Design of a Scroll Vortex Inlet for Supercritical Approach Flow

Vortex drop shafts are used in urban drainage systems to connect two sewers located at considerably different elevations. After their introduction in 1947, these were studied with particular reference to subcritical approach flow. Vortex shafts for supercritical approach flow can also be used, but the intake structure may have relatively high cost due to the complex geometry. The present study includes experimental results of a specific investigation on the changes to be made in the supercritical approach channel if a subcritical vortex intake is used. The experimental investigation analyzes the effect of a hydraulic jump on the performance of vortex intake structure to define appropriate technical solutions, essentially consisting in a negative step to be located along the supercritical approach channel. Design criteria are finally presented for the evaluation of the step height and its distance from the vortex intake structure.

Vortex Drop Shaft for Supercritical Flow

Vortex drop shafts are used in urban drainage systems to connect two sewers located at considerably different elevations by means of a vertical conduit. The vortex drop shaft was first designed by Drioli (1947). It was then studied by other authors with reference to subcritical approach flow. Vortex shafts specifically conceived for supercritical flow can also used, but at very high costs due to the specially features required for the intake structure. The present study shows the experimental results of a specific investigation into the changes to be made in the approach channel for supercritical flow, when a subcritical vortex intake is used. The proposals concern the definition of the height of the step to be located in the approach channel, and the length of the lower-bottomed section in the approach channel, while maximizing the hydraulic efficiency of the system. Proper step height will cause the hydraulic jump to conveniently occur downstream of the step, whereas a regular subcritical flow in the intake structure of vortex shaft will result from lowering the bottom of the approach channel for the appropriate length.

Vortex shaft outlet

Vortex drop shafts are widely used in practice to connect sewer mains characterized by large elevation difference. These structures conventionally include three key elements: intake structure, vertical shaft and outlet structure, also named dissipation chamber. The latter has not received much attention as compared to the first two parts, and only few experimental investigations are currently available from the literature (Viparelli, 1950; Kellenberger, 1988). Actually some rules of thumb are available as design criteria (ATV, 1998; Hager, 1999), but no systematic hydraulic investigation is available so far. The aim of the present study is to present preliminary results of an experimental campaign conducted at the Department of Hydraulic and Environmental Engineering, University of Naples, Italy. The physical model of a vortex drop shaft allowed the Authors to investigate the main hydraulic features of the dissipation chamber, in order to characterize the performance of various types of outlet structures.

Henry Bazin: Hydraulician

The recognition of Henry Bazin (1829–1917) as being one of the outstanding experimenters in hydraulics of the 19th century was founded by his 1865 report Recherches Hydrauliques . This paper presents the main findings of that historically important work including the fundamental results on uniform flow, open channel velocity distributions, gradually varied free surface profiles and waves. The latter are of particular interest because the solitary wave, previously observed by the Englishman Russell, was reexamined as well as wave breaking and surge formation due to a sudden blockage of flow. Other topics addressed are the story about the Darcy and Bazin uniform flow formula, and the reason why others that used the Bazin data sets produce formulas that are still in use. Engravings from the original 1865 report and photos from a recent visit to the canal site illustrate this note on a historically outstanding contribution to open channel hydraulics.

Impact Forces of a Supercritical Flow of a Shear Thinning Slurry Against an Obstacle

In mountainous areas after long or intense rains, landslides may evolve into debris- or mud-flows. Their impact against obstacles may produce huge damages, sometimes with loss of lives. Prediction of the impact forces is required for a proper design of the flow barriers protecting risk prone areas. To this aim, both the effective characterization of the mud rheology and a suitable mathematical model of the flow propagation are needed. The present paper proposes a modeling framework in which the mudflow is idealized as the flow of a power-law fluid over an incline with a rigid impervious wall at the downhill end. The flow model employs the von Karman depth-integration of the one-dimensional mass and momentum conservation equations, in the long-wave approximation. The governing equations have been solved through a space/time second-order accurate numerical method. This modeling framework is applied to a test-case, based on the soil collected from Cervinara site (Avellino, South Italy), affected by a catastrophic landslide in 1999. The investigated soil is both the raw one and a washed one, the latter introduced to mimic the effect of an intense rain in terms of removal of the dissolved soil organic carbon. The rheology of both the shear-thinning mixtures has been deeply characterized in the form of a power-law function, and the dynamics of a dam-break wave ad its impact on an obstacle, has been numerically analyzed. It is shown that the removal of the soil organic carbon affects the propagation of the mudflow and at a minor extent the maximum forces and torques acting on the downstream wall. Remarkably, in the investigated conditions, the mudflow action consists of a strong impact occurring few seconds after the landslide triggering, and a subsequent cyclic loading of about three minutes.

Supercritical flow in junction manholes under invert- and obvert-aligned set-ups

ABSTRACT Junction manholes are a part of urban drainage infrastructures. They merge inlet branches into an outlet branch. Former studies focused on junction flow by considering a specific manhole layout: identical diameters were assigned to the branches and the invert was flush. Nevertheless, engineers are often involved in designing junctions under generalized geometries, with different branch diameters and, sometimes, bottom offsets at manhole inlets. For these junction arrangements, the empirical relations documented in literature are not applicable. An experimental campaign was thus conducted to determine the main flow features of junction manholes under generalized set-ups. The results include a diagram of state to be used if junctions operate under mixed flow conditions. Information regarding the main wave patterns, with detailed discussion of wave features and heights, is provided to support the hydraulic design of junction manholes under the supercritical flow regime.