Journal of Hydraulic Engineering

The number of articles published in the ASCE Journal of Hydraulic Engineering reached 151 this past year. There could not be a better way to celebrate the sesquicentennial year of ASCE. However, the increasing pressure on the Associate Editors has prompted me to enlarge the Editorial Board in preparation for the upcoming challenges. This editorial acknowledges the Associate Editors who have renewed their commitment to the Journal and those who are willing to dedicate anew their valuable time to its future development. Adding more brainpower to the Editorial Board is viewed as the best way to ensure progress toward our goals.

The Brent Reservoir was constructed in the mid-1830s and its siphon spillways were completed in 1936 to protect the dam from overtopping in the event of an extreme flood. Since completion, there have been problems with the hydraulic performance of the siphons, some of which primed simultaneously, causing flooding downstream. A physical hydraulic model study has been conducted to investigate the hydraulic performance of the siphons in order to establish reliable stage discharge relationships. The existing bellmouth siphon system was found to be unsuitable, causing the siphons to prime suddenly at discharges of around 3 m³/s. This was due to the sudden removal of an air pocket from the siphon’s crown. The model tests were carried out in two stages. In the first stage, the existing geometry was examined. Based on the results from stage 1 of the experiments, it was concluded that the air inlet requires redesign and various options to improve the air regulation should be considered. In the second stage, various options to regulate the inlet of air and establish stable siphon performance over the entire range of discharges were considered. It was found that the most stable conditions are provided by an air slot being cut into the spillway hood at an appropriate level. This geometry provides excellent air-regulated stability, unimpaired spillway capacity, and is insensitive to tail water level and wave conditions in the reservoir.

The definitions of depth and hydraulic radius become ambiguous when bed roughness is large relative to flow depth. Various statistics are currently used to describe bed roughness and many different flow resistance formulas have been developed. The volumetric hydraulic radius $R_v$ and the standard deviation of bed surface elevations $d_z$ are rational and unambiguous measures suitable for large relative roughness conditions. Their influence on flow resistance is investigated using conceptual models and digital elevation models of natural alluvial beds. The results show that head-losses for large-scale relative roughness beds can be related to $(R_v/d_z);$ the $(R_v/d_z)$ exponent of power-law flow resistance equations increases from 1/6 to more than 1/2 as relative roughness increases, and flow velocity can be determined from boundary topography measures, water level and slope, without any calibrated coefficients. An overlooked form of the log law, using standard deviation $d_z\text{,}$ performs as well as power laws for predicting flow resistance with high relative roughness and it reverts to the conventional log law when relative roughness is low. A field technique for determining $R_v$ and $d_z$ is described.

The solution of the St. Venant–Exner equations as a model for bed evolution is studied under conditions when the Froude number, F, approaches unity, and the quasi-steady model becomes singular. It is confirmed that the strict criterion for critical flow, the vanishing of a water surface disturbance celerity, is not met, yet the direction of propagation of a bed wave apparently changes depending on whether $F>1$ or $F<1\text{.}$ An analysis of the linearized model problem for an infinitesimal bed wave under near-uniform conditions is performed, and qualitative features of the solution are brought out. Under appropriate sediment transport conditions, when $F^2\to 1\text{,}$ two bed waves, one traveling upstream and the other traveling downstream, are found to develop from an initially single localized bed perturbation. Simulations of the full unsteady problem were performed with the Preissmann scheme to confirm the linear analysis and to study the effects of nonlinearity and friction. A transcritical case, in which a region where $F^2<1$ is succeeded by a region where $F^2>1\text{,}$ is also investigated, and the solution exhibits an apparently different behavior than cases where the flow is everywhere sub- or supercritical, but can be understood as a hybrid of the latter cases.

An artificial neural model is used to estimate the natural sediment discharge in rivers in terms of sediment concentration. This is achieved by training the network to extrapolate several natural streams data collected from reliable sources. The selection of water and sediment variables used in the model is based on the prior knowledge of the conventional analyses, based on the dynamic laws of flow and sediment. Choosing an appropriate neural network structure and providing field data to that network for training purpose are addressed by using a constructive back-propagation algorithm. The model parameters, as well as fluvial variables, are extensively investigated in order to get the most accurate results. In verification, the estimated sediment concentration values agree well with the measured ones. The model is evaluated by applying it to other groups of data from different rivers. In general, the new approach gives better results compared to several commonly used formulas of sediment discharge.

Transverse surface waves were produced in a rectangular open channel in the region where the otherwise steady, uniform, flow passed through a cluster of vertical circular cylinders. The cylinders could represent either naturally occurring obstacles such as vegetation or man-made structures such as a group of piles driven into the bed of a river. The waves were produced from the periodic forces created by the vortex shedding from the cylinders. These forces generated and then amplified transverse waves in the channel. In some experiments the waves had amplitudes of 35% of the mean flow depth. These resonance-generated waves produce a seiching that can occur in hydraulic models as well as prototype systems. Both laboratory experiments and a theory-based analysis were used to determine the relationship between the amplitude of the vortex-generated wave, average velocity of the approach flow, viscosity of the fluid, width of the rectangular channel, depth of flow, cylinder diameter, and cylinder placement configuration.

Surge flow phenomena, e.g., as a consequence of a dam failure or a flash flood, represent free boundary problems. The extending computational domain together with the discontinuities involved renders their numerical solution a cumbersome procedure. This contribution proposes an analytical solution to the problem. It is based on the slightly modified zero-inertia (ZI) differential equations for nonprismatic channels and uses exclusively physical parameters. Employing the concept of a momentum-representative cross section of the moving water body together with a specific relationship for describing the cross sectional geometry leads, after considerable mathematical calculus, to the analytical solution. The hydrodynamic analytical model is free of numerical troubles, easy to run, computationally efficient, and fully satisfies the law of volume conservation. In a first test series, the hydrodynamic analytical ZI model compares very favorably with a full hydrodynamic numerical model in respect to published results of surge flow simulations in different types of prismatic channels. In order to extend these considerations to natural rivers, the accuracy of the analytical model in describing an irregular cross section is investigated and tested successfully. A sensitivity and error analysis reveals the important impact of the hydraulic radius on the velocity of the surge, and this underlines the importance of an adequate description of the topography. The new approach is finally applied to simulate a surge propagating down the irregularly shaped Isar Valley in the Bavarian Alps after a hypothetical dam failure. The straightforward and fully stable computation of the flood hydrograph along the Isar Valley clearly reflects the impact of the strongly varying topographic characteristics on the flow phenomenon. Apart from treating surge flow phenomena as a whole, the analytical solution also offers a rigorous alternative to both (a) the approximate Whitham solution, for generating initial values, and (b) the rough volume balance techniques used to model the wave tip in numerical surge flow computations.

To design and operate a distribution system, one must understand how it will perform when subjected to external hydraulic loads and demands. This paper presents a hybrid model that efficiently tracks the full range of hydraulic conditions, from steady state to waterhammer, in a system over an extended period by coupling a transient simulator with a reservoir routing scheme. The model’s procedure consists of running waterhammer simulations at the start and end of an extended time step to track the rate of filling of a system’s reservoirs and then using this information to update reservoir levels at the end of the time step. Beyond conventional level-of-service and capacity-assessment applications, the hybrid model can help the engineer link system unsteadiness to its associated costs in terms of design and operation. Extended period and worst-case simulations presented in a case study suggest that the hybrid model has a high routing accuracy and can be used effectively to identify the critical state which will produce the most severe transients in a system.

The pressure within a trapped air pocket in a rapidly filling horizontal pipe is investigated both experimentally and analytically. The downstream end of the pipe is either sealed to form a dead end or outfitted with an orifice to study the effects of air leakage on the pressure. Three types of pressure oscillation patterns are observed, depending on the size of the orifice. When no air is released or orifice sizes are small, the cushioning effects of the air pocket prevents the water column from impacting on the pipe end and from generating high water hammer pressures. However, the maximum pressure experienced may still be several times the upstream driving pressure. When the orifice size is very large, the air cushioning effect vanishes and the water hammer pressure is dominant. For intermediate orifice sizes, the pressure oscillation pattern consists of both long-period oscillations (while the air pocket persists) followed by short-period pressure oscillations (once water hammer pressures dominate). Air leakage is observed to play a significant role in increasing the magnitude of the observed pressures during rapid filling, resulting in peak pressures up to 15 times the upstream head. An analytical model, capable of calculating the air pocket pressure and the peak pressure when the water column slams into the end of the pipe, is developed and results are compared with those of experiments. The model was successful in determining the amplitude of the peak pressure for the entire orifice range and was able to simulate the pressure oscillation pattern for the case of a negligible water hammer impact effect. Although the model was unable to simulate the pressure oscillation pattern for substantial air release, it was able to predict the type of pressure oscillation behavior and the peak pressure.

This paper presents the results of an observational study related to the behavior of drainage sewers under conditions of hydraulic overloading. Specifically, the investigation focuses on the interaction of water and trapped air in surcharging drainage and pressurized pipeline systems, by studying the air–water flow behavior in a rapidly filling horizontal pipe. Air–water interface patterns, air entrainment, and air release through an orifice at the pipe end are documented photographically. Synchronously recorded pressure traces are also presented to illustrate the relation between the air–water phase evolution and the pressure oscillation patterns. Depending on the air release rate of the orifice, there are three types of pressure oscillation behavior, each corresponding to a particular behavior of the air–water interface in the rapidly filling horizontal pipe.

Deflectors are recognized as one of the best devices for enhancing deep-water habitat in low-gradient channels. Modern design standards recommended the use of low-profile structures intended to function primarily at a lower stage, but research suggests that pools are typically formed by larger magnitude flows. A flume experiment was conducted in a 0.5-m wide, 6-m long recirculating flume to determine the volume of pool habitat and maximum depth of scour created by low versus high deflectors. Although the high deflectors were 1/3 the width of low deflectors, they created larger volume pools for all flows above bankfull levels. Pools were also as deep or deeper for high deflectors under the larger-flow conditions. Existing deflectors along the Blackledge River, Conn. support these two observations. Conversely, low deflectors produced deeper larger pools for discharges below bankfull levels. These results suggest that high deflectors may be better suited for creating pool habitat, especially in coarse-bedded channels.