Journal of Hydraulic Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Numerical methods generally need analytical solutions as test cases and validations in simplified problems. This work provides Laplace-domain explicit analytic solutions for fluid–structure interaction (FSI) water hammer waves within a pipe. Rather than applying the transfer matrix method (TMM) to the FSI four equations, it is transposed to the equivalent two-wave propagating problem considered instead. Using the classical wave matrix diagonalization approach permits decoupling the waves’ propagation while at the same time coupling boundary conditions in the diagonal base. This approach permits the transfer matrix for coupled waves boundary conditions to be provided so as to obtain a Laplace-domain solution for the pressure/stress vector solution. This solution is written in a general framework that can be adapted for general applied boundary conditions for a single pipe. Three sets of boundary conditions are considered as examples and illustrations from solving the inverse Laplace transform of the considered explicit solutions. Consistent results with recently proposed time-domain solutions are found, and a one-to-one mapping between Laplace-domain and time-domain approaches is also established. This permits finding the discrete spectrum of FSI water hammer wave mode decomposition from TMM solutions.

This study examined the utilization of transient shear stress for cleaning sediment and biofilm materials adhered to the inner walls of pipes in water distribution systems. This approach is presented as a potential method for material resuspension during pipe flushing. This is done to prevent discoloration and low-water-quality events, which can be a major concern for both water utilities and consumers. This work builds and expands on previous work on biofilm detachment under unsteady flows. This study explored the manipulation of existing valves to induce high transient shear stresses that, in turn, resuspend materials adhering to the pipe walls. Through the use of valves strategically positioned along the water distribution system, the systems are subjected to consecutive controlled transient pressure waves. It is possible to capitalize on the interference properties of the controlled transient waves at various points in the system. It is in this condition that various pressure waves can merge within the system, causing high pressures and relatively high shear stresses. However, it is vital to keep the pressure confined within the allowed range to ensure the integrity of the system. A Lagrangian with unsteady friction method was implemented and applied for modeling the transient behavior in two water networks.

Abstract

A large collection of laboratory measurements of piezometric head ($h$) and discharge ($Q$) were made over hydraulic models of a tilting weir at nine different angles, ranging from 25.7° to 90° within a 0.3-m-wide flume. These measurements were corroborated with additional laboratory data taken within a 1-m-wide flume across four inclination angles. The range of both inclination angle ($\theta$) and flow scale examined in this study elucidate the nature of the head-discharge rating equation beyond previous work. Results show that as $\theta$ decreases under a constant $Q$, the $h$ over the weir decreases in a monotonic fashion due to the shallower angle of attack of the flow, which results in less curvature of the streamlines over the crest and therefore less deviation from the upstream hydrostatic pressure condition. To incorporate this effect into the head-discharge rating equation, a transformation of the $h$ term was applied by multiplying the measured $h$ that occurs over a tilting weir by a correction factor to match the effective $h$ that would occur if the weir were aligned perpendicular to the flow at the same discharge. Thus, a modified form of the classical sharp-crested weir rating equation can be used as a means for determining the value of $Q$ for tilting cases to a high degree of accuracy. The degree of accuracy is dependent on dimensionless Reynolds and Weber numbers describing the flow inertia in the approach to the weir in relation to respective viscous and surface tension scale effects. This approach portends marked flow measurement enhancement for flow conditions above a suggested inertial threshold.

Practical Applications

To achieve reliable and equitable water distribution, it is necessary to accurately estimate the volumetric discharge of water to flow through “rivers and canals.” One such method of accomplishing this is by using hydraulic structures, such as the tilting weir. This structure not only can be used to measure the discharge of water on a continuous basis, but also allows for regulating the water level upstream of the structure. Presently, tilting weirs are often used to control water levels, but the literature on their utilization as flow measurement structures has remained sparse. This study presents a thorough analysis of laboratory experiments on scaled models of tilting weirs, where the flow depth upstream of the structure and the inclination angle are used to calibrate an equation to estimate the discharge to a high degree of accuracy. Operational guidelines to ensure minimum uncertainty in measurement are also given.

Poor mixing of old and new water in municipal water storage vessels is a well-documented basis for potentially harmful water quality degradation in drinking water distribution systems. This numerical study investigates the effects of inflow and operational variables on mixing in the jet-driven filling process, with a particular focus on the transition from inadequate to sufficient mixing levels. An isothermal unsteady reynolds-averaged-navier-stokes volume-of-fluid (RANS-VOF) simulation is used to model the variable-volume filling process, accounting for the moving free surface following a draw-down in the stored water volume. A low diffusivity tracer is used to mark the old-water volume, and a coefficient of variation (CoV) quantifying the departure from a uniform tracer distribution is used to monitor the time-dependent mixing. The results indicate that adequate mixing does not necessarily follow refills from common draw-down levels. Three distinct mixing regimes are identified by unique CoV transients. Introducing consideration of the mean-flow kinetic energy, the observed mixing behaviors are readily explained by the jet inlet power and the distribution of the mean-flow kinetic energy in the tank. Extending the simulations to periods after cessation of the inflow and to partial refills, the role of residual mean-flow kinetic energy is further highlighted, especially its limited vertical reach. For cases in which a sufficiently mixed condition is achieved, the time-to-mix results are well described by a mixing-time correlation closely matching previously published results.

Intense rain events and sprawling urbanization have contributed to more frequent flash flooding in cities, often due to the pressurization of drainage systems. Stormwater collection networks (SCNs) can become pressurized if their conveyance capacity is exceeded, leading to on-street flooding through backflow out of curb inlets. Due to the complexity of SCN geometry and spatiotemporal rainfall variability, studies evaluating pressurization in stormwater systems have previously been conducted for relatively simple geometries and inflow conditions. Thus, to date there have been few network-scale insights into how pressurization develops, making it difficult to understand drivers that influence pressurization: slope, roughness, connectivity, and inflow rate. The present work evaluates the process of SCN pressurization using numerical modeling through a systematic variation of these variables. Herein, three distinct pressurization mechanisms were identified by using EPA SWMM 5.1 to model idealized SCN topology and junction inflows. New nondimensional flow indexes (NDFIs) are proposed to characterize the pressurization conditions after an initially empty stormwater system reaches steady state under application of hydrographs. This study provides a basis for further systematic evaluation of factors influencing drainage system pressurization, guiding future actions to mitigate urban flash flooding.

A bridge versus culvert hydraulics approach may be taken to estimate the headwater of larger-span (or equivalently short) culvert-like structures, and design manuals often argue that the former should be technically preferred. A laboratory study was undertaken to investigate short and long box culverts, including roughness effects, with special attention paid to short-structure cases with entirely open-channel flow. The data were analyzed using discharge coefficients and approximate analytical models. Simulations were conducted with the HEC-RAS bridge model (with mainly default or recommended parameter values and options) and HY-8 to assess the predictive performance and benefits, if any, of a bridge versus culvert hydraulics approach. Observations support the presence of shortness and roughness effects, but do not support any significant advantage of a bridge versus culvert hydraulics approach. On the contrary, the performance of HY-8 was found to be superior to that of the HEC-RAS bridge model for rough-bed cases, perhaps fortuitously, due to its often conservative choice of parameter values.

Abstract

Gas bubble disease (GBD) is a condition that adversely affects aquatic animals exposed to supersaturated water. Fish mortality attributed to GBD has been observed in the tailrace of the Colider Dam on the Teles Pires River in Brazil during spillway releases. Gas supersaturation downstream of dams typically is caused by the dissolution of air bubbles entrained at depth in the stilling basin. The most common strategy to reduce total dissolved gas (TDG) concentration, and the risk of GBD, is to install spillway flow deflectors that redirect plunging flows horizontally and thus minimize the bubble transport to a depth at which dissolution is enhanced. This paper presents a numerical study, based on the open-source code OpenFOAM, assessing the design of spillway deflectors at Colider Dam and determining their effectiveness at reducing TDG. A two-fluid flow model was implemented in a free-surface solver to predict spillway jet regimes and TDG distribution for 900 m of the Colider tailrace. Residence time and TDG exposure in the tailrace were estimated using a particle tracking technique. A rigid-lid model was used to predict TDG distribution in the Teles Pires River 10 km downstream of the dam. Reasonable agreement was found between numerical model results and velocity data measured in a 1:120 reduced-scale hydraulic model. Predicted spillway jet regimes were consistent with the deflector performance curve obtained in a 1:60 reduced-scale sectional model. After calibration and validation against TDG field data, numerical simulations were performed to better understand the effect of the deflector curvature radius, length, and elevation on TDG production. Fish mortality or injury due to GBD was not observed in Colider during spill events after deflectors recommended in this study were installed in 2021. Field measurements indicate that TDG reduction by the deflectors is consistent with values predicted during the design of the deflectors.

Practical Applications

Fish mortality with gas bubble disease symptoms due to elevated total dissolved gas concentrations have been observed downstream of Colider Dam since the first spillway operation in 2018. The most common strategy to reduce TDG production downstream of hydropower dams is to install flow deflectors on the spillway face. This paper presents a numerical study to assess the performance of flow deflectors for Colider Dam. A computational fluid dynamic (CFD) model, comprising the air and water phases, and bubbles in the water phase, was developed to predict spillway jet regimes and TDG concentration. The model included the main features of the dam and a 10-km reach of the Teles Pires River. After calibration and validation, 28 simulations were performed to evaluate and improve the deflector design. A final deflector design was recommended based on TDG production and TDG exposure time. No fish mortality has been observed since the deflector construction was completed in 2021, even under operational conditions fatal to fish in the past.

Under large unit discharges, stepped chutes are susceptible to cavitation damage due to lack of entrained air. Among various preaeration devices, the simple-structured abrupt contraction aerator (ACA) has a good performance on mild-slope ($\mathrm{V}:\mathrm{H}=1:5$) stepped chutes with a combined flow pattern consisting of a jet and a local hydraulic jump. To advance the understanding of ACA hydraulics, this paper investigates the effects of stepped chute slope and slit location on jet dynamics. A series of physical experiments were carried out on a steep-slope ($\mathrm{V}:\mathrm{H}=4:5$) stepped chute with varied slit locations. It was found that a jet was generated by ACA, and sufficiently aerated flow skimmed over the chute bottom with no black water, indicating an effective reduction of cavitation damage. The slit location had little effect on the flow patterns and aeration but might raise the flow profiles and enlarge the jet. Based on the experimental data, the empirical formulas for maximum height and distance of the jet are finally presented to provide references for sidewall design.