Ping Dong

Two-phase flow modelling of sediment motions in oscillatory sheet flow

In this paper, a two-phase flow model is presented which simulates the fluid and sediment motions in the sheet flow regime on a flat bed under oscillatory flow conditions. The model is developed based on the continuity equations and linearised momentum equations for the fluid and sediment phases, respectively. All major forcing terms such as the intergranular stresses and the turbulent stress are included in the model. From the detailed computations and comparison with the available laboratory data it has been demonstrated that the model is capable of predicting fairly accurately both flow kinematics and sediment concentrations. In particular, the model predicts that the well known phenomenon of fluid velocity over-shoot that exists in clear water also appears in the case of lighter sediments but vanishes when the materials are heavier, which is in perfect accord with the experimental observations considered.

Intense near-bed sediment motions in waves and currents

Abstract In the past few years, two-phase models have been developed to describe the detail behaviour of fluid/sediment interactions and transport under the sheet flow conditions. Due to the complexity of the governing equations and uncertainties in the formulations of various stress terms, few complete solutions of these equations are known and the validations are thus far limited to only a few experimental data. In this paper, the numerical predictions of the behaviour of sheet flows using an improved version of an earlier two-phase flow model [Coastal Eng. 36(2) (1999) 87] are described. Although the general structure of the model was retained, a number of improvements had been made to give better account the underlying physics of the flow in areas very close to the stationary bed. All key flow parameters have been predicted and analysed in order to gain insight into the processes. Calculated time-dependent as well as time-averaged concentrations are compared with experimental data from purely oscillatory flows and oscillatory flow plus a current. Good qualitative agreements between predictions and measurements were achieved for the time-dependent concentrations while the time-averaged concentrations are quantitatively accurate as well.

A probability method for predicting time-dependent long-term shoreline erosion

This paper describes a practical procedure for predicting the probability distributions of long-term shoreline erosion. It treats the dynamical response of the shoreline over time as that of a time-dependent stochastic system. The input to the system are the long-term wave climate and shoreline properties while the output of the system are the probability distributions of the maximum shoreline recession within any prescribed time period. The procedure enables the combined effect of the longshore and cross-shore sediment transport processes on the shoreline erosion to be taken into account as both processes are generally responsible for the maximum shoreline recession. A series of simulations using idealised long-term wave distributions which have relatively narrow directional spread was carried out to evaluate the capability of the method. Based on these simulations it was found that both the distributions and variability of wave climates have significant influence on the predicted shoreline erosion probability distributions while the chronology effect is less significant, especially for long simulation time.

Spatial and temporal variability of water discharge in the Yellow River Basin over the past 60 years

Water discharge data of the Yellow River over the past 60 years was analyzed using the continuous wavelet transform (CWT) and Mann-Kendall (MK) test methods to identify spatial and temporal variation patterns. Potential connections between water discharge in the Yellow River Basin and El Niño/Southern Oscillation (ENSO) were also examined by the cross wavelet and wavelet coherence methods. CWT results show that the periodic oscillations in water discharges had occurred at the temporal scales of 1-, 2- to 4-, 6- to 8- and 10- to 22-year. It was also found that at the annual timescale (1-year) the phase relations between water discharge and ENSO were indistinct probably due to the strong influence by human disturbances. However, over the longer time scales, the phase relation becomes much clearer with an anti-phase relation being found mainly at inter-annual scale (2- to 8-year) and in-phase relation at decadal scale (16- to 22-year). According to the MK test results water discharge at most stations except Tangnaihai have decreased significantly and the abrupt change occurred in the mid-1980s or the early 1990s. The changes in water discharge were found to be influenced by both climate changes and human activities. Before 1970 the change in water discharge was positively related to precipitation variations in the river basin, but after 1970 the decrease in water discharge has been largely caused by various human activities including constructions of reservoirs, water abstraction and water-soil conservation with water abstraction being the main cause.

Wave Impact Simulations by an Improved ISPH Model

AbstractThis paper presents an improved incompressible smoothed particle hydrodynamics (ISPH) method for wave impact applications. In most conventional ISPH techniques the source term of the pressure Poisson equation (PPE) is usually treated by either a density invariant or a velocity divergence-free formulation. In this work, both the density invariant and velocity divergence free formulations are combined in a weighted average form to determine the source term. The model is then applied to two problems: (1) dam-breaking wave impact on a vertical wall and (2) solitary wave run-up and impact on a coastal structure. The computational results have indicated that the combined source term treatment can predict the wave impact pressure and force more accurately compared with using either formulation alone. It was further found that depending on the application case, the influence of the density invariant and divergence-free parts could be quite different. For the more violent wave impact case, the divergence-f...

A numerical model of the vertical distribution of longshore currents on a plane beach

Abstract A numerical method is developed to calculate longshore currents generated by monochromatic long-crested waves breaking on a plane beach. The method is based on the linearized time-averaged momentum equation in the longshore direction. The equation is solved numerically using a Galerkin method with a series of cosine functions as basis functions. Longshore current profiles across the surf zone at a range of elevations over vertical direction are presented and compared with experimental data. Although the turbulence model used at this stage is only a simple eddy viscosity model, reasonable agreement between computed and experimental results has been achieved.

Spatial and Temporal Variations in Grain Size of Surface Sediments in the Littoral Area of Yellow River Delta

Abstract The impacts of human activities on the sediment load of the Yellow River and the long-term evolution of the delta have been extensively investigated, but less is known of the variation in sediment grain size in the littoral zone. In this study, the data of surface sediment samples collected from the littoral area of the Yellow River Delta in 2000 and 2007 are used to investigate the spatial distribution and transport pathway of the sediment, as well as the grain size response to the drastic decrease in riverine sediment discharge. By applying the geostatistics analysis method to grain size parameters in determining the characteristic distance in the sediment trend analysis method, an effective way to understand the movement and gradation of surface sediments around the delta has been proposed. The results show that as a whole the sediment in the inshore erosion area is coarser than that in the offshore accretion area. The grain size trends obtained reveal two converging zones of sediment movement. We find a coarsening trend in sediment from 2000 to 2007. Possible mechanisms for this trend are discussed.

An Assessment of Groyne Performance in the United Kingdom

Groynes have been widely used for generations for preventing coastal erosion throughout the world. As coastal processes in a groyne system are complex and the groyne design is largely based on crude empirical methods, it is not always possible to ensure groyne performance at the design stage. Postproject appraisal is therefore of vital importance in assessing the actual performance and adequacy of the design methodology. This article presents the findings from an extensive questionnaire survey that has recently been carried out to assess the performance of existing groynes at many sites in the UK. Factors such as groyne configurations, the types of beach materials, and types of groynes on the performance of these groyne systems have been evaluated in detail. It has been found that the majority of groynes analysed are effective in preventing erosion of the groyned shoreline and that rock groynes perform particularly well compared with the traditional timber groynes.Groynes have been widely used for generations for preventing coastal erosion throughout the world. As coastal processes in a groyne system are complex and the groyne design is largely based on crude empirical methods, it is not always possible to ensure groyne performance at the design stage. Postproject appraisal is therefore of vital importance in assessing the actual performance and adequacy of the design methodology. This article presents the findings from an extensive questionnaire survey that has recently been carried out to assess the performance of existing groynes at many sites in the UK. Factors such as groyne configurations, the types of beach materials, and types of groynes on the performance of these groyne systems have been evaluated in detail. It has been found that the majority of groynes analysed are effective in preventing erosion of the groyned shoreline and that rock groynes perform particularly well compared with the traditional timber groynes.

Natural Variability and Anthropogenic Effects on the Morphodynamics of a Beach–Dune System at Montrose Bay, Scotland

Abstract MILNE, F.D.; DONG, P., and DAVIDSON, M., 2012. Natural variability and anthropogenic effects on the morphodynamics of a beach–dune system at Montrose Bay, Scotland. Beach–dune systems are valuable geomorphic assets providing a buffer between the coastal hinterland and the sea. Sustained coastal erosion can diminish the protective capability of the beach–dune system with potentially negative implications for coastal amenities, infrastructure, ecology, and archaeology. To develop suitable management solutions in response to erosion, it is necessary to ascertain spatiotemporal variations in the processes that affect site-specific coastal morphology. This study examines a beach–dune system at Montrose Bay in eastern Scotland that is undergoing severe erosion, threatening the current layout of the Montrose Medal Golf Course. Sediment eroded from the golf course frontage is deposited at the northern end of the bay where contemporaneous accretion has occurred. Analysis of sediment transport rates, time-series aerial photography, and historical maps indicate that the current spatial distribution of erosion and accretion at Montrose Bay is likely due to a strengthening of northerly drift, driven by wave–climate change during the 1980s. This natural variability was examined further through assessment of earlier changes in coastal morphology apparent in historical mapping and from archaeological evidence of shoreline change reflecting the temporal dynamism of soft sedimentary coasts. The removal of sediment dredged from the South Esk estuary in the closed coastal system is shown to be a major anthropogenic influence on the beach–dune system in recent decades based on a simple quantitative analysis of dune erosion due to removal of beach sediments during dredging operations.

Application of a stochastic differential equation to the prediction of shoreline evolution

Shoreline evolution due to longshore sediment transport is one of the most important problems in coastal engineering and management. This paper describes a method to predict the probability distributions of long-term shoreline positions in which the evolution process is based on the standard one-line model recast into a stochastic differential equation. The time-dependent and spatially varying probability density function of the shoreline position leads to a Fokker–Planck equation model. The behaviour of the model is evaluated by applying it to two simple shoreline configurations: a single long jetty perpendicular to a straight shoreline and a rectangular beach nourishment case. The sensitivity of the model predictions to variations in the wave climate parameters is shown. The results indicate that the proposed model is robust and computationally efficient compared with the conventional Monte Carlo simulations.