Drop in Combined Sewer Manhole for Supercritical Flow

Manholes often contain small drops for various reasons, the most important being submergence. While this may be appropriate for subcritical flow, its effect was considered doubtful for supercritical flow. This note aims at investigating the effect of a manhole drop on the hydraulics of sewer flow. Based on systematic experimental observations, the flow pattern associated with a manhole drop was established. A distinction was also made between small and intermediate drops. Then, the main wave features were analyzed to result in expressions that contain both the upstream filling ratio and the Froude number of the approach flow. In addition, the discharge capacity was also investigated, and selected photographs show typical drop flow in combined sewer manholes. The result of the present study is evident, based on these observations, and recommendations towards future design of combined sewer manholes are also made.     

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.     

Experimental Study of Surcharged Flow at Combining Sewer Junctions

The results of a laboratory investigation on surcharged flow in combining sewer junctions are presented in this paper. Experiments were conducted in a 90° model junction and the 25.8° Edworthy model junction. The study confirmed the existence of three flow regimes in sewer junctions with a steep outgoing pipe: Regime I denotes the open-channel flow through the junction chamber; Regime II flow is partially surcharged flow featured by orifice flow into the outlet pipe; and Regime III flow is fully surcharged flow with all connecting pipes running full. The transition flow from Regimes II to III was investigated, and it may be related to the inlet waves at the entrance of the outlet pipe. Criteria for the transition were provided. Theoretical analyses were conducted based on one-dimensional momentum equation. The derived equations are able to estimate the water depth in the junction chamber. Energy loss in Regime III flow was studied and predictions based on the momentum equation were evaluated.     

Supercritical Exchange Flow Down a Sill

A detailed experimental study was conducted to investigate the hydraulics, interfacial instability, and mixing in the supercritical region of exchange flows downstream of a sill crest. Measurements of the velocity field and the interface position were obtained using flow visualization and particle image velocimetry. Large periodic fluctuations in the measurements of the flow rate and interface position were caused by the Kelvin–Helmholtz (KH) instabilities at the interface as well as the internal seiche. These KH instabilities caused entrainment of fluid from the upper layer into the lower layer, with the entrainment coefficient considerably larger than the values for gravity currents.     

Dimensionless Method to Characterize the Mixing Effects of Surcharged Manholes

Solute transport processes affect the performance of a wide range of water engineering structures. In the context of urban drainage, the effects of dispersion may act to reduce peak concentrations associated with intermittent discharges or cause pollutants to be retained for longer or shorter durations than mean travel times would predict. With respect to surcharged manholes, previous research employed laboratory experiments to identify best-fit parameter values for the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) routing models. This paper presents data from a new set of smaller-scale laboratory measurements and demonstrates that the threshold depth separating two distinct hydraulic regimes can be identified independently of scale. However, the fitted ADE and ADZ routing model parameters are not generally amenable to conventional hydraulic scaling, because the models do not provide good fits to the observed data. An alternative approach is proposed based on the cumulative residence time distribution (CRTD). This approach is shown to be scalable and practical. The solute transport characteristics of a specific configuration of a surcharged manhole are shown to be characterized by just two dimensionless CRTDs corresponding to prethreshold and postthreshold surcharge depths.     

Dynamics of Air Flow in Sewer Conduit Headspace

Pressurization in sanitary sewer conduit atmosphere is modeled using computational fluid dynamics techniques. The modeling approach considers both turbulent and laminar flow regimes. The turbulent model takes into consideration the turbulence-driven secondary currents associated with the sewer headspace and hence the Reynolds equations governing the air flow field are closed with an anisotropic closure model which comprises the use of the eddy viscosity concept for the turbulent shear stresses and semiempirical relations for the turbulent normal stresses. The resulting formulations are numerically integrated. The turbulent model outputs are verified with experimental data reported in the literature. Satisfactory agreement is obtained between numerical simulations and experimental data. Mathematical formulas and curves as functions of longitudinal pressure gradient, wastewater velocity, and sewer headspace geometry are developed for the cross-sectional average streamwise velocity.     

Supercritical Flow Measurement Using a Small Parshall Flume

An experimental program was conducted to determine if a Parshall flume, developed to accurately measure open-channel subcritical flow, could also be used to measure discharge in a supercritical flow regime. Fifteen experimental configurations were tested using two small Parshall flumes [6-in. (15.2-cm) and 9-in. (22.9-cm) crest width] with varying approach channel slopes, approach channel roughness, and flume convergence. It was determined that a single Parshall flume can be used to measure flow (within ±5%) for both supercritical and subcritical flow regimes for a specified range of flows. The original Parshall flume equation was then modified to incorporate crest width, channel slope, channel roughness, and convergence in the prediction algorithm. Unique expressions were developed for both supercritical and subcritical flow regimes to estimate the discharge. A single expression does not appear feasible for accurate discharge measurement for both flow regimes in a Parshall flume at this time.     

Diameter and Surcharge Effects on Solute Transport across Surcharged Manholes

New data are presented describing the retention time and longitudinal dispersion of a solute tracer across circular surcharged manhole structures of different diameters. The variations with both discharge and surcharge level are described and the relationships quantified. The variation of the longitudinal dispersion coefficient exhibits poorly defined trends, however using an aggregated dead zone technique both the reach time delay and travel time show clear variations. A surcharge threshold level for these parameters is evident at the larger manhole diameters and this is explained in relation to jet theory. The variation of the surcharge mean time delay and postthreshold mean travel time are quantified, while the prethreshold travel times are shown to be dependent on both discharge and surcharge. The relationships allow for inclusion in sewer water quality modeling and provide a method for improving predictive techniques.     

Modeling In-Sewer Deposit Erosion to Predict Sewer Flow Quality

High levels of suspended solids are typically observed during the initial part of storms. Field evidence suggests that these suspended solids derive from the erosion of in-sewer sediment beds accumulated during dry and previous wet weather periods. Suspended sediment transport rate models within existing sewer network modeling tools have utilized inappropriate transport rate relationships developed mainly in fluvial environments. A process model that can account for the erosion of fine-grained highly organic in-sewer sediment deposits has been formulated. Values of parameters describing the increase in deposit strength with depth are required. These values are obtained using a genetic algorithm based calibration routine that ensures model simulations of suspended sediment concentrations that correspond to field data collected in a discrete length of sewer in Paris under known hydraulic event conditions. These results demonstrate the applicability of this modeling approach in simulating the magnitude and temporal distribution of suspended in-sewer sediment eroded by time varying flow. Further work is developing techniques to enable the application of this type of model at the network level.     

Hydraulics of Stacked Drop Manholes

This paper presents the results of an experimental investigation on flows inside stacked drop manholes (SDM). An SDM consists of two identical rectangular or square manhole chambers stacked together at an elevation difference. SDMs for different conditions were assessed on their ability to dissipate the energy of the approaching flow and their suitability to perform adequately under different flow conditions. Flow regimes were classified based on the inflow conditions and geometry of the structure in the first chamber and downstream outflows in the second chamber. An analysis based on the integral momentum equation was developed to estimate pool depths and energy losses under critical flow conditions. A fully surcharged stage with inflow and outflow pipes running full was also tested and velocity profiles were measured at a horizontal center plane to the opening connecting both shafts. Additionally, air flow rates were measured to assess the air demand into a large-height SDM.     

One-Dimensional Mixing Model for Surcharged Manholes

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterizing the mixing effects of specific hydraulic structures are of interest to drainage network modelers. Previous research, focusing on surcharged manholes, used the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) models to characterize dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth were identified, the ADE and ADZ models did not provide particularly good fits to observed manhole mixing data, which meant that the derived parameter values were not independent of the upstream temporal concentration profile, and no rules for predicting parameter values based on manhole size and configuration were provided. An alternative, more robust, method is described by using the system’s cumulative residence time distribution (CRTD). This paper shows how a deconvolution approach derived from systems theory may be applied to identify, from laboratory data, the CRTDs associated with surcharged manholes. Archive laboratory data are reanalyzed to demonstrate that the solute transport characteristics of a surcharged manhole with straight-through inflow and outlet pipes over a range of flow rates and surcharge depths may be modeled using just two dimensionless CRTDs, one for prethreshold and the other for postthreshold surcharge depths. The model combines the derived manhole CRTDs with a standard (Gaussian) pipe dispersion model to provide temporal solute concentration profiles that are independent of both scale and the ratio of the pipe and manhole diameters.     

Side Outflow from Supercritical Channel Flow

Side flow on a flood plain from a side outlet of the main channel is investigated both theoretically and experimentally for supercritical main flow. The side outlet as a model simulates a failure of a river bank in a prototype. The discharge ratios of the side outflow to that of the main channel flow, the water depth, and the velocity at the side outlet are obtained. The theoretical discharge ratio is a function of the Froude number of the main channel flow. The theoretical spreading angle of the side flow and the theoretical relationship between the velocities and the distance from an upstream point of the side outlet are also compared and predicted. All the theoretical results are found to be in good agreement with the experimental data.     

Curvilinear Flow Profiles Based on Reynolds Averaging

A new theoretical approach is presented for the derivation of free surface profiles of two-dimensional steady and unsteady flows by solving the Reynolds-averaged Navier-Stokes equations applied to the turbulent flow regime. This approach enables us to compute the steady and unsteady curvilinear flows having small curvatures, such as free overfall and constant velocity surge. In addition, the applications of the theory to the second-order waves are illustrated through the problems of small height bore and second-order tide.     

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.     

Shallow Water Wave Propagation in Convectively Accelerating Open-Channel Flow Induced by the Tailwater Effect

Propagation of shallow water waves in viscous open-channel flows that are convectively accelerating or decelerating under gradually varying water surface profiles is theoretically investigated. Issues related to the hydrodynamics of wave propagation in a rectangular open channel are studied: the effect of viscosity in terms of the Manning coefficient; the effect of gravity in terms of the Froude number; wave translation and attenuation characteristics; nonlinearity and wave shock; the role of tailwater in wave propagation; and free surface instability. A uniformly valid nonlinear solution to describe the unsteady gradually varying flow throughout the complete wave propagation domain at and away from the kinematic wave shock as well as near the downstream boundary that exhibits the tailwater effect is derived by employing the matched asymptotic method. Different scenarios of hydraulically spatially varying surface profiles such as M1, M2, and S1 type profiles are discussed. Results from the nonlinear wave analysis are further interpreted and the influence of the tailwater effect is identified. In addition to the nonlinear wave analysis, a linear stability analysis is introduced to quantify the impact from such water surface profiles on the free surface instability. It is shown that the asymptotic flow structure is composed of three distinct regions: an outer region that is driven by gravity and channel resistance; a near wave shock region dominated by the convective inertia, pressure gradient, gravity and channel resistance; and a downstream boundary impact region where the convective inertia, pressure gradient, gravity and channel resistance terms are of importance. The tailwater effect is demonstrated influential to the flow structure, free surface stability, wave transmission mechanism, and hydrostatic pressure gradient in flow.     

Asymmetric Flow in Symmetric Supercritical Expansions

A theory presented previously by the writer explained the asymmetric flow patterns occurring in symmetric rectangular expansions with subcritical flow. The theory is extended in the present paper to supercritical flows, in which the hydraulic jump is one of the flow features. The theory is shown to agree with available observations. Transverse oscillatory conditions are discussed in terms of a dynamic instability that can accompany static stability. The ranges for which experimental data are available with which to test the theory are identified, making it apparent where additional data would be useful for the full range of the problem. The phenomenological understanding provided by the present paper is important to the development of stabilization measures. In addition to stabilization by submergence, the hydraulic jump should be amenable to stabilization by other referenced methods, including various vane configurations and pipe communication between the eddy regions.     

Preventing Sediment Deposition in Inverted Sewer Siphons

Laboratory experiments were carried out to determine minimum self-cleansing velocities for preventing deposition of sediment in upward sloping and vertical pipes of inverted siphons. Tests were made using sediment sizes of 0.78 and 4.3 mm in a pipe of 150 mm diameter for eight angles of inclination between 0 and 90°. The criterion for self-cleansing conditions was defined as the minimum velocity needed to prevent the formation of deposits on the invert of the pipe. For a given sediment concentration the self-cleansing velocity was found to be a maximum at pipe slopes between about 22.5 and 45°. Analysis of the forces acting on sediment particles in inclined pipes led to the development of a formula for predicting self-cleansing velocities taking account of pipe size, sediment size, sediment concentration, and pipe slope.     

Use of Stereoscopy for Dam Break Flow Measurement

This investigation explored the applicability of video stereoscopy for the measurement of unsteady open channel flows. Specifically, the three-dimensional water surface profile and flow velocities associated with scale model dam break events were considered. Stereo images of the unsteady flow event were obtained using three, time-synchronized, video cameras situated above the tank such that, at all times, the area of interest was captured by at least two of the three cameras. To establish a point of reference from image to image, floating plastic tracking particles were placed on the water surface. The three-dimensional coordinates of the particles were then calculated using the camera positions and the locations of the individual plastic particles in the stereo images. Particle velocities were also deduced from the analysis of consecutive images. Based on this preliminary investigation we conclude that video stereoscopy is a promising method for measuring highly dynamic flows.     

Discharge Ratio of Side Outflow to Supercritical Channel Flow

This study presents the effects of width changes of a side breach, Froude numbers, and bottom slopes on discharge ratios of a side outflow to the main channel flow when a flow is supercritical. The results compare the differences between theoretical discharge ratios and experimental ones. The differences increase in one of three conditions: increase of the side breach, increase of the bottom slope, and decrease of the Froude number.