Journal of Waterway, Port, Coastal, and Ocean Engineering

Abstract

Seawater intake caissons are a kind of structure widely used in seawater extraction. Many existing seawater intake caissons have problems with poor water quality and large power consumption. In this paper, a new type of seawater intake caisson with an oscillating water column wave-energy conversion device is proposed to improve water quality and electricity compensation. A wave-energy conversion device at the front end of the structure can reduce the disturbance of wave energy to the rear water intake area, which is conducive to sediment deposition. In the meantime, it can also realize a certain compensation for the electricity consumption required for pumping water. Based on the two-phase flow solver (OpenFOAM-4.x), through a comparison and analysis of streamlining and flow velocity in the rear water intake area, it was found that the water purification effects of the integrated structure proved more effective than those of the traditional structure, and the overall average velocity of the flow field reduced by 10%–40%. The optimal energy conversion efficiency of the integrated structure was obtained by optimizing different parameters such as the ratio of the chamber width to the wavelength, the orifice size, the draft of the front wall, and the draft of the intermediate baffle.

Practical Applications

Seawater intake systems are widely used for seawater extraction. Many seawater intake structures suffer from poor water quality and large power consumption due to the large volume of water extraction. A new structure is proposed, which sets an oscillating water column wave energy–converting device on the front side of the traditional seawater intake caisson to capture wave energy. The working process is divided into three stages: firstly, waves enter the device. Wave surface oscillation causes air exhalation and suction to drive the blade rotation, and the generator rotor also rotates to generate electricity; secondly, the flow enters the water intake area and wave energy is significantly weakened by the reflection from the front and back walls of the chamber; finally, the relatively pure seawater is lifted through the pump to the water intake pipeline. The performance of the proposed device is evaluated by comparing the flow field changes of the proposed device and the traditional one and analyzing the energy conversion efficiency of the proposed device. The new structure is verified to have the dual effects of water purification and wave-energy conversion.

River training structures, such as groynes or spur-dikes, are subject to intensifying ship-induced loads, owing to increasing ship dimensions and traffic density on waterways. In particular, the long-period primary wave system differs from other loading components, such as short-period wind waves, owing to the long wave length. To date, this loading scenario is not reflected within the empirical based design approaches for groynes. In this study, numerical approaches based on shallow water theory and computational fluid dynamics (CFD) are employed for deriving groyne design parameters, particularly by assuming stationary load conditions. Firstly, the numerical tools REEF3D::CFD and REEF3D::SFLOW are validated for the specific parameter range of ship-induced groyne overtopping based on an experimental data set. Secondly, numerical simulations at the prototype scale are connected with empirical equations for armor layer design. Comparing the results with field data from groyne prototypes indicates that the combined approach yields plausible required armor layer dimensions. Further, the examination of geometric variations of groynes confirms that a reduction in groyne slope can reduce the required armor layer dimensions by approximately 10%.

A laboratory experiment is carried out to study the generation mechanism and the evolution process of an infragravity wave (IG wave) on a typical permeable fringing reef. Fifteen capacitance wave gauges are installed along the reef model to record the water surface elevation under irregular wave action. Based on the experimental data, both IG wave generation and strong IG wave interaction are systematically analyzed. The experimental results indicate that the IG waves are generated by the time-varying breakpoint mechanism at the reef edge, and the tidal modulations on the IG wave height are mainly influenced by wave breaking and wave dissipation. Around the reef edge, the tidal modulation is controlled by wave breaking and the IG wave height monotonously decreases with the increase of the tide level. At the shoreline, the tidal modulation is controlled by wave dissipation at the low tide level and wave breaking at the high tide level. Therefore, the IG wave height first increases and then decreases with the increase of the tide level, and the maximum IG wave height occurs at the middle tide level.

A series of laboratory experiments for wave interacting with suspended vegetation was conducted. The vegetation was modeled by a group of rigid vertical cylinders with three different configurations. The aim was to study the characteristics of turbulence kinetic energy (TKE) inside the suspended vegetation domain. The velocities at different elevations inside and outside the vegetation domain were measured by acoustic Doppler velocimeter. The forces acting on cylinders were also directly measured using a force transducer. The drag force coefficients were then calculated using the velocity and force data in the Morrison’s formula. A clear negative current was found near the bottom of the suspended vegetation domain. The strength of the current grew with the increase of vegetation volume density and incident wave height and the decrease of relative water depth kh. The time-averaged TKE followed the same trend. Two peaks were found in the phase-averaged TKE. The variations of measured force with incident wave height, relative water depth, and vegetation volume density were also studied. A model previously proposed in the literature was modified to predict the TKE under water waves by introducing the Keulegan–Carpenter (KC) number to quantify the oscillatory motions. The drag force coefficient in the model was also replaced by a new empirical formula in terms of KC number. The results of the new model showed better agreement with the experimental data obtained in this paper. The new model was further validated using other published experimental data, showing reasonable agreement as well.

Abstract

A new method to determine design scour levels for dune revetments and seawalls is presented. The method is based on the thesis that toe scour is a function of the nearshore incident wave energy and was calibrated with results from several moveable bed model studies covering a large range of scales. The method determines scour levels to a still water level datum defined at the incident wave-breaking location. Therefore, the method incorporates many more parameters than those that pertain to existing methods including tidal stage, storm surge, wave setup, wave period, and seabed slope. However, application of the method is not difficult with a methodology being presented. The sensitivity of the method to variations in the values for the input parameters is examined and the limits of their applicability are presented. Recommendations for further research are given.

Practical Applications

Seawalls are prone to collapse during storms if their footings are undermined by breaking waves. When designing seawalls, it is important to determine the level to which such scour can occur. However, existing methods for determining scour are simplistic and do not provide a scour level. This paper presents a new equation to predict scour levels for the design of seawall footings. The equation includes many of the important factors relating to storm scour including storm tide level, wave, and beach conditions. The equation was developed on the results of several scale model studies carried out in various laboratories over several decades with some comparisons with real-world experiences. The paper comprises all of the information used with clear diagrams. A simple application method is presented, the various factors that need to be considered for design are discussed, and recommendations for further studies are made.