Journal of Hydraulic Engineering

This paper presents a theoretical approach to the problem of designing the opening in a slit-check dam. The approach is based on the conservation of the mass of water and sediments and on the energy balance under steady conditions. It leads to a relationship among opening width, sediment characteristics, mountain river geometry, and water and sediment discharge. The final relationship can be simplified to make it suitable for practical applications. Also, the problem of unsteadiness of both water and sediment is considered, as well as the possibility of treating the unsteady flow as a sequence of steady states. The results of the theory were checked in a laboratory investigation using a scale model. Different opening widths were tested under conditions of steady and unsteady supplies of water and sediment. The mean grain size of the sediment, as well as the rates of sediment and water discharge, were changed in the experiments. The results of the experiments confirm the theory quite well.

Open-check dams are built in mountain streams to control sediment transport during a flood. Sediment passes through them at the lowest discharges, whereas deposition occurs during the highest discharges. Open-check dams are currently designed based mainly on construction experience. Modeling of hydraulics and bed morphology in check dams involves mixed flows (supercritical and subcritical) as well as discontinuities such as hydraulic jumps. In this paper an unsteady coupled numerical mobile-bed model that can tackle rapid varying flows and discontinuities is applied. The numerical technique is based on the classical staggered grids and implicit integration schemes, together with a proper mass and momentum balance. The 1D numerical model is successfully verified with experimental data of slit-check dams. The applicability of the model in the design of open-check dams is also illustrated.

Using numerical models for the purpose of channel-routing calculation has been well accepted in engineering practice. However, most traditional models fail to predict the transcritical flows because of numerical instability. This paper presents two high-resolution, shock-capturing schemes for the simulation of 1D, rapidly varied open-channel flows. The present schemes incorporate the method of characteristics to deal with the unsteady boundary conditions. Also, the Strang-type splitting operator is used to include the effects of bottom slope and friction terms. To assess the performance of the proposed algorithms, several steady and unsteady problems are simulated to verify the accuracy and robustness in capturing strong shocks in open-channel flows. Furthermore, the results of dynamic flood routing and steady routing are compared to demonstrate the risk of using steady routing for flood mitigation.

In this paper it is suggested that the double-averaged (in temporal and in spatial domains) momentum equations should be used as a natural basis for the hydraulics of rough-bed open-channel flows, especially with small relative submergence. The relationships for the vertical distribution of the total stress for the simplest case of 2D, steady, uniform, spatially averaged flow over a rough bed with flat free surface are derived. These relationships explicitly include the form-induced stresses and form drag as components of the total stress. Using this approach, we define three types of rough-bed flows: (1) Flow with high relative submergence; (2) flow with small relative submergence; and (3) flow over a partially inundated rough bed. The relationships for the double-averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible. The double-averaged turbulent and form-induced intensities and stresses for the case of regular spherical-segment-type roughness show the dominant role of the double-averaged turbulence stresses and form drag in momentum transfer in the near-bed region.

The frequency response method is used to determine the location and rate of leakage in open loop piping systems. A steady-oscillatory flow, produced by the periodic opening and closing of a valve, is analyzed in the frequency domain by using the transfer matrix method, and a frequency response diagram at the valve is developed. For a system with leaks, this diagram has additional resonant pressure amplitude peaks (herein referred to as the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks (herein called primary amplitude peaks) for the system with no leaks. Several piping systems are successfully analyzed for all practical values of the friction factor to detect and locate individual leaks of up to 0.5% of the mean discharge. The method, requiring the measurement of pressure and discharge fluctuations at only one location, has the potential to detect leaks in real-life pipe systems conveying different types of fluids, such as water, petroleum, and so on.

The condition of choke-free flow in ovoidal sewers subject to a rise in bed elevation is considered. For a subcritical approach flow with a smooth and gradual transition zone, the maximum limiting rise in bed elevation for ovoidal sewers is determined. Based on a condition of critical flow downstream of the transition zone, the energy and continuity equations are solved numerically to obtain the maximum permissible limits of bed elevation rises and allowable ranges for upstream flow depths. Design graphs are presented for practical use.

Aggradation at bridges causes the bridge waterway opening to be reduced, possibly resulting in upstream flooding and increased contraction scour. Aggradation results when the sediment load supplied to a reach of river from upstream exceeds its capacity to transport sediment. Solutions to aggradational problems at bridges are often complex and expensive. Solutions include increasing sediment transport through the bridge by modifying the channel, constructing an upstream sediment trap, redesigning the bridge, dredging, and treating the cause of the aggradation. At many bridges, aggradation problems can be severe. As an example, aggradation at a bridge in northern Pennsylvania is described. The benefits, disadvantages, and costs for various possible solutions to the example problem are compared and the most cost-effective solution is presented.