Journal of Hydraulic Engineering

In order to avoid operational issues related to entrapped air in water transmission mains, water refilling procedures are often performed carefully to ensure no pockets remain in the conduits. Numerical models may be a useful tool to simulate filling events and assess whether air pockets are adequately ventilated. However, this flow simulation is not straightforward mainly because of the transition between free surface and pressurized flow regimes and the air pressurization that develops during the filling event. This paper presents a numerical and experimental investigation on the filling of water mains considering air pressurization aiming toward the development of a modeling framework. Two modeling alternatives to simulate the air phase were implemented, either assuming uniform air pressure in the air pocket or applying the Euler equations for discretized air phase calculations. Results compare fairly well to experimental data collected during this investigation and to an actual pipeline filling event.

An offline approximation of the Saint Venant equations is proposed for combined sewer overflow (CSO) prediction. Precalculated tabulations of the mass and momentum equations allow online interpolation, accelerating run-time hydraulic computations above those yielded by standard industry software, while preserving accuracy. Previous methods of backwater profile compilation account only for subcritical, open-channel flows. This work further extends the backwater profiles to include pressurized flows, which are key for CSO conditions. Incorporation of the Darcy-Weisbach equation eliminates potential lookup table discontinuities between open-channel and pressurized conditions. Graphical depiction of supercritical flows causes iteration errors as nonunique flow rates appear for equivalent water surface elevations. Iteration errors can be eliminated by allowing the solution to proceed separately upstream and downstream from the governing critical water surface. This precalculated curve approach addresses discrepancies in supercritical flow and submerged weir calculations in the EPA SWMM model. The implicit computational approach surpasses SWMM run times by as much as 57% and proves an accurate tool for evaluation of sewer water levels for real-time optimization model incorporation.

Transient pressure associated with a rapidly filling pipeline containing two entrapped air pockets is investigated experimentally and numerically. A multiple-air-pocket elastic-water model considering multiple moving boundaries of water columns is developed by neglecting inertia and head loss of a short water column near air–water interfaces. The proposed model is validated by experimental data. Results show that when two air pockets in length are much different, the maximum pressure always arises in the smaller air pocket regardless of the blocking column’s length. The case of the upstream air pocket with a similar length to the downstream is the most complicated and dangerous because (1) the maximum pressure may alternately arise in two air pockets as the blocking column increases, and (2) interaction of two air pockets could cause a huge pressure surge, which is likely much higher than with only one air pocket. The existing single-air-pocket model cannot effectively simulate pressure surge of the two air pockets.

Breached levees often cause extensive flood damage to urban and residential areas; there has been considerable interest in the numerical modeling of flooding of urban areas in recent years. However, comprehensive data that describe the complex flow field in residential areas are not available for validating the numerical models or for determining the extent of such damage. To fill this gap, results are presented of experiments on a scale model used as a test setup that examined the flow field in an urban area attributable to flooding caused by a levee breach. Steady state flow depths and flow velocity were measured by a point gauge and by an array of ultrasonic velocity profilers, respectively. In addition, the surface velocities were recorded in the entire flooded area by using a digital particle tracking velocimetry technique. The details of these measurements, limited flow data, and the observed flow characteristics are presented in this study. Full data sets are provided on the Internet. The results of these investigations show that closely spaced buildings act like a single obstacle with a common wake zone, and there is little flow between these buildings. The results also show that the strongest flow is in the prebreach direction and that the local topography plays an important role in distributing the flow in the flooded area. The complete data set may be utilized to validate numerical models of free surface flow.

Diffusion-wave and kinematic-wave approximations of the St. Venant equations are commonly used in physically based, regional hydrologic models because they have high computational efficiency and use fewer equations. Increasingly, models based on these equations are being applied to cover larger areas of land with different surface and groundwater regimes and complicated topography. Existing numerical methods are not well suited for multiyear simulation of detailed flow behavior unless they can be run efficiently with large time steps and control numerical error. A numerical method also should be able to solve both diffusive and kinematic wave models. A total variation diminishing Lax-Friedrichs type method (TVDLF) that is stable and accurate with both diffusive- and kinematic-wave models and large time steps is presented as a means to address this problem. It uses a linearized conservative implicit formulation that makes it possible to avoid nonlinear iterations. The numerical method was tested successfully using steady flow profiles, analytical solutions for wave propagation, and observed data from a field experiment in a mountain stream of Sri Lanka. A grid convergence test and an error analysis are carried out to determine how the model errors of the numerical schemes behave with the discretization.

Previous research into surcharged manholes has highlighted the existence of a threshold surcharge level that separates two distinctly-different hydraulic regimes. Sharp changes in manhole energy loss and solute transport characteristics occur when the surcharge depth passes through the threshold level. With respect to solute transport, two scale-independent cumulative residence time distributions (CRTDs) have been identified, corresponding to the below-threshold and above-threshold hydraulic regimes. However, previous studies focused on large diameter manholes, in which the manhole diameter (${\mathrm{\Phi }}_m$) was at least 4.4 times greater than the pipe diameter (${\mathrm{\Phi }}_p$). This paper utilizes a validated computational fluid dynamics (CFD) modeling approach to explore the hydraulic behavior and mixing processes in small-diameter surcharged manholes ($1.5<{\mathrm{\Phi }}_m/{\mathrm{\Phi }}_p<4.4$). It is shown that the hydraulic threshold does not exist in the small diameter manholes; instead, the flow field is characterized by short-circuiting throughout the full range of surcharge depths. Data generated at low surcharge levels suggest that the mixing effects in the below-threshold region are not independent of surcharge level, as had previously been suggested. The absence of the threshold in small-diameter manholes is explained with reference to jet theory. Several previous studies that have characterized mixing and/or energy losses in surcharged manholes are revisited, allowing findings to be generalized across a broader range of manhole configurations, including effects attributable to benching and change in outlet angle.

A baffle-drop structure is a flow conveyance structure that can be used for transport of urban storm water down to underground storage tunnels. The water cascades down the structure from baffle to baffle and plunges into a pool at the bottom from where it is conveyed to the tunnel through an adit. The structure has been used successfully in a limited number of urban drainage schemes. However, its hydraulics and air entrainment characteristics are not fully understood. Using a series of laboratory experiments, an analysis has been tested and validated that may be used for design. The analysis provides a dimensionless relationship between key variables, including design discharge, shaft diameter, baffle spacing, and position of a vertical wall dividing the shaft in a dry and a wet portion. Using this relationship the shaft can be designed to maintain atmospheric pressure throughout its height with little or no air being entrained into the tunnel.

The scour hole induced by a propeller wash is experimentally investigated with noncohesive sediments. The associated scour profile at the asymptotic condition, which can be expressed by a combination of three polynomials, comprises (1) a small scour hole beneath the propeller, (2) a primary scour hole downstream of the small scour hole, and (3) a deposition mound farther downstream of the primary scour hole. The time-dependent maximum scour depth induced by propeller jets is closely related to the densimetric Froude number, a reference time scale, offset height relative to the propeller diameter, and sediment size. A semiempirical formula, which shows good agreement with the experimental data, is proposed to describe evolution of the maximum scour depth. An equation for the determination of the time to initiate scour is also proposed. Moreover, empirical equations for the maximum asymptotic scour depth induced by a propeller and three-dimensional submerged offset jets agree well with the experimental data. The critical condition for the initiation of scour caused by propeller and offset jets is also presented.

Scour at piers embedded in a bed of cohesive sediment mixture is investigated to quantify the effects of bed compaction. Experiments were conducted in a large rectangular laboratory flume with natural clay sediment collected from a stream located at the Chilean coastal range. Nonintrusive and high-resolution topographic measurements of scoured bed in the vicinity of piers were performed during the experimental runs with an installation of a laser distance sensor (LDS) that was traversed by the precision step motors. Measurements were taken by the LDS in different azimuthal half-planes to study the spatiotemporal variation of the topography of evolving scour holes. Experiments were conducted over 40 h, until an advanced stage of scour was reached, with the approaching flow having a bed shear stress equaling 95% of the Shields critical condition for the initiation of motion of sand in a sand-clay mixture. Results show that the bed is successively scoured in three different forms, chunks of aggregates, aggregate by aggregate, and particle by particle, when increasing the ratio of actual to Proctor’s optimum molding water content, $\hat{w}$. Maximum scour depth was mainly observed at the wake region (downstream of the pier) and ranged from 10 to 58% of that in the sand alone case for mixtures with the same size of the sand fraction. For a given compaction energy, scour depths first decrease with $\hat{w}$, up to a value of $\hat{w}=2.5$, as fine clay particles are flocculated and increasingly dispersed with water content. For values of $\hat{w}>2.5$, the sediment matrix behaves as a fluid mud. In the latter case, scour depths increase with water content as the sediment bed becomes increasingly liquid.