Journal of Hydraulic Engineering

With the development of urban areas, many municipalities are facing issues where the flow velocity in their sewer systems is relatively high at various locations. This is because of either geographical or local hydraulic conditions. Existing design guidelines generally include a velocity limit of $3\mathrm{m}/\mathrm{s}$. However, the rationale for this limit is not presented in these guidelines. This study explores the feasibility of higher allowable sewer velocities on municipal infrastructure. Potential issues that may be considered when increasing the velocity to a higher value of $6\mathrm{m}/\mathrm{s}$ or even around $10\mathrm{m}/\mathrm{s}$ are discussed, including capacity, air demand, and transient flow. Considerations as to system protection are then reviewed including the forces exerted on structures, cavitation, and presence of sediments. Design alternatives are also discussed including drop shafts and energy dissipators. Increasing the existing maximum allowable velocity of $3\mathrm{m}/\mathrm{s}$ to a higher value may result in a decrease in pipe diameter and depth of cover, which would yield financial savings. A value for the maximum allowable velocity in sewer system design is not proposed in this paper given the complicated hydraulic features of high-speed flow and the significant gaps between research and actual engineering requirements.

This work aims to examine the flow dynamics that contribute to the transient wall shear stress ${\tau }_w(t)$ of accelerating and decelerating turbulent pipe flows, using a series of direct numerical simulations (DNSs) of accelerating and decelerating flows between two fully turbulent states. Results show that accelerating and decelerating pipe flows exhibit different time dependence, especially in their turbulence response. It is observed that decelerating flows respond earlier than accelerating flows; however, they also require more extensive periods in which to relax towards their final turbulent state than accelerating flows. An identity that decomposes ${\tau }_w(t)$ into its dynamic contributions was used to determine the dominant flow dynamics involved within the different stages experienced by these flows. It is revealed that one of the existing 1D unsteady friction models accurately predicts one of the components of the dynamic decomposition of ${\tau }_w$. Nonetheless, it is noted that the 1D model does not capture the transient response of the laminar and turbulent contributions. Consequently, the identity mentioned above was utilized as a framework to develop a hybrid model that improves the current 1D unsteady friction approaches.

The primary challenge for river managers in handling open-channel bifurcations is the prediction of the discharge distribution to downstream branches. In this study, we aimed to summarize and complete the available database and empirical correlations for T-shape bifurcations at 90°, both in subcritical and transcritical flow regimes, in equal and reduced side branch width geometries. An experimental campaign generated a database of 668 new configurations in addition to 299 sets collected from the literature. The existing correlation for predicting the discharge distribution in the subcritical regime appears to be of limited agreement with these data, whereas a new empirical relationship significantly increases the accuracy and is useful for a side branch as narrow as a third of the main branch width. Regarding the free-recirculation transcritical regime, the present data validate the correlation proposed in 1967 for downstream branch ratios not tested by the authors. An approach was proposed to predict the flow discharge distribution for configurations without a priori knowledge of the flow regime.

As power systems include more intermittent renewable energy sources, energy storage solutions are needed to support them. Pumped hydro is a reliable alternative for long-term energy storage. A solution for bringing more pumped hydro into the power system is to upgrade existing hydropower plants into pumped hydro. To investigate this possibility, a case study with a complex tunnel system and multiple surge tanks along both the headrace and the tailrace tunnels is selected. A hydraulic scale model and validation methods were developed. The challenges in developing and validating complex models involving multiple surge tanks, throttles, head loss, and limited prototype information are detailed.

When heavy rainfall, riverine flooding, and coastal storm surge coincide catastrophic flood conditions can occur, a process referred to as compound flooding. Many studies investigating the effects of compound flooding have relied on discrete models that simulate flooding processes separately. Consequently, they fail to mechanistically account for the complex interactions of multiple flood processes and the resulting impact on hazard estimates. We implemented the Adaptive Hydraulics Modeling System (AdH) that fully couples two-dimensional (2-D) shallow-water equations (SWEs) and 2-D diffusive wave equations (DWEs) to conduct monolithic simulations of compound flooding and quantify its impacts on water levels relative to Hurricane Harvey. The SWE simulated coastal storm surge and riverine flow while the DWE simulated overland flow. Spatially and temporally variable rain rates and wind effects within the model resolved heterogenic storm characteristics. A dense network of gage observations and surveyed high-water marks indicated the model’s ability to simulate the magnitude and timing of water levels. A comparison against benchmark flood extents indicated similar spatial inundation patterns. Model results revealed a transition zone, where multiple flood processes interact, that extends for almost 40 km along Buffalo Bayou highlighting the extensive impacts of compound flooding and the need to account for interacting physical phenomena with a fully coupled model. The results demonstrate the ability of this model and methodology to mechanistically account for the interactions of multiple flood processes with a 2-D hydrodynamic model.

Recent studies have indicated that low-slope bedrock reaches are more common in nature (the bedrock surface slope is slightly lower than 0.005). This paper focuses on the hydrodynamic structure and turbulent characteristics of low-slope and critically sharp bedrock bends under flood conditions. Three experiments under different flood frequencies were conducted to investigate the influence of the ratio of centerline curvature radius to depth ($R_c/H$) on the distribution and characteristics of the secondary flow, turbulence, and momentum transport measured by acoustic Doppler velocimeter (ADV). The results were then compared with the flow structure of a sharp bend. Our results indicated that the distribution area and the intensity of the secondary flow decreased with $R_c/H$, especially in the outer-bank cell and the center-region cell. The lower $R_c/H$ was, the more obvious the flow separation at the inner bank was. The core area of the turbulent kinetic energy moved upstream gradually, and its intensity increased with $R_c/H$. The friction factor increased in the upstream bend and did not move further upstream when $R_c/H$ was below 13.6. The intensity of the transverse bed shear stress decreased with $R_c/H$ in a critically sharp bend, and the maximum value near the upstream centerline implied the potential erosion in the bedrock bend. These experimental results are conducive to understanding the flow process of erosion and deposition in the bedrock bend reach.

Hydrodynamic forces, especially uplift forces, during a flood event are critical to the stability of bridge superstructures. This study focuses on the interaction between the migrating dunes, the scour depth beneath the deck, and the uplift force that fluctuates during bedform migration. Experimental results show that the fluctuation of uplift forces is closely related to the arrival of dune crests and troughs, which periodically alter the approach flow behavior. The fluctuation becomes particularly apparent for partially submerged decks, for which the intermittent overtopping phenomenon plays an essential role in producing vertical forces with varying directions. In contrast to previous studies using fixed plane beds, positive uplift forces may occur in this study for some partially submerged decks, which poses an additional risk to the superstructure. New lift coefficient equations are proposed to take the effect of migrating dunes into consideration and enhance the safety margin in design for highly dynamic fluvial environments.

The effect of wave skewness and sediment particle size on near-bed sediment dynamics and transport owing to wave-induced streaming is examined in this study. Here, wave-dominated flow over a flat rough bed is studied, with wave propagation forming a nonzero angle with the current. It was observed that the increase in wave skewness increases the mean sediment transport, which is consistent with prior findings for the particular situation of horizontally uniform Stokes forcing. The mean sediment transport beneath combined second-order Stokes waves and current has been investigated for fine, medium, and coarse sand, respectively. Due to inertia, both the mean bed-load vector and the depth-integrated suspended flux vector are less rotated (relative to the wave propagation direction) for coarse sand than for fine sand. The mean bed-load transport is largest for coarse sand, whereas the mean suspended sediment transport is largest for fine sand.