Hsien-Kuo Chang

A two-phase flow model of wave-induced sheet flow

This paper presents a two-phase flow model that simulates the fluid and sediment motions in the sheet flow region under oscillatory conditions. Some major forcing terms such as the fluid/particle and particle/particle interactions and turbulent stresses are included in the model. By improving some assumptions of most existing models, the present model specifies the equivalent sand roughness and bed concentration as a function of the Shields parameter, which is variable with time and is physically more realistic over a mobile flat bed. A wave friction factor, which is governed by a new parameter, is obtained from the present model formulation. The present model is shown to provide a more accurate estimate of sediment concentrations than those models using a constant equivalent sand roughness. Numerical analyses also show that the suspended sediment retards the mean velocity and suppresses turbulence.

A two-point method for estimating wave reflection over a sloping beach

Abstract This work presents a frequency-domain method for estimating incident and reflected waves when normally incident waves’ propagating over a sloping beach in a wave flume is considered. Linear wave shoaling is applied to determine changes of the wave amplitude and phase due to variations of the bathymetry. The wave reflection coefficient is estimated using wave heights measured at two fixed wave gauges with a distance. The present model demonstrates a high capacity of estimating reflection and shoaling coefficients from synthetic wave-amplitude data. Sensitivity tests for the present model due to measurement errors of wave amplitudes and distance of two probes can more accurately predict the reflection coefficients. The measurement error of wave amplitude affects more significantly than measurement error of distance of two probes on calculating reflection coefficient of waves over a sloping bed.

Protection Priority in the Coastal Environment Using a Hybrid AHP-TOPSIS Method on the Miaoli Coast, Taiwan

Abstract CHANG, H.-K.; LIOU, J.-C., and CHEN, W.-W., 2012. Protection priority in the coastal environment using a hybrid AHP-TOPSIS method on the Miaoli Coast, Taiwan. Coastal erosion often happens around the island of Taiwan because of strong waves from typhoons in the summer. The problem of beach erosion cannot be completely solved at this time due to insufficient government budgets. Prioritization of coastal protection should be made yearly to match engineering requirements and annual official budgets. This paper proposes both analytic hierarchy process (AHP) and technique for order preference by similarity to ideal solution (TOPSIS) methods to prioritize the protection of the coastal environment on the Miaoli Coast of Taiwan. The weights of three main criteria (engineering safety, ecology, and coastal landscape) and their subcriteria are determined through the AHP method. Twenty-two segments of the Miaoli Coast are ranked according protection priority by the TOPSIS method. This multipart methodology can help decision makers prioritize coastal engineering and environmental efforts. This procedure also enables researchers to put more expert knowledge together, allowing more precise decisions and moderating personal judgments.

Neural network with multi-trend simulating transfer function for forecasting typhoon wave

This study develops an NN typhoon wave model to accurately and efficiently calculate wave heights at a point of interest. Multitrend simulating transfer functions were first introduced to exemplify the relationship between wave heights and each conceivable input factor by regressive fitting. The proposed NN-MT model can accurately forecast wave peak with an error of less 1.2 m and with time delay within 3h and can be extended to cover the station besides the original station of interest.

Simulated wave-driven ANN model for typhoon waves

This paper investigates an artificial neural network (ANN) model for typhoon waves used to modify poor calculations of the numerical model in special cases. Two key factors, local winds and simulated waves produced by the numerical model, were used as input parameters of the proposed ANN model. The waves were simulated by the numerical model from a wave action equation indicating the physical processes of energy transfer and wave propagation. Simulated wave input is a very important parameter for the proposed ANN model, allowing for the accurate calculation of water waves in the sea. The applicable Mike21_SW model was chosen to provide an accurate calculation. Through model verification, the proposed ANN model has a particularly accurate calculation at the peak of each typhoon and at its occurrence time. The computed waves of each typhoon were examined to be consistent with the observed waves.

Physical impact on the reclamation area resulting from offshore dredging at the Changhwa coast, Taiwan

The physical impact of offshore dredging on the reclamation area at the Changhwa coast, Taiwan, is investigated using a three-dimensional movable-bed model test. A distorted modeling law consisting of maintaining similarity of the equilibrium beach profile between the model and prototype is proposed. The geometric distortion was verified through a series of preliminary experiments conducted in a wave flume. Experimental results show that the distorted modeling is able to reproduce the beach-face slope in nature. An appropriate long-term morphological time-scale was determined based on a comparison of model longshore littoral transport rates and equivalent prototype values. Seabed topographical changes before and after offshore dredging are evaluated in model tests. A suitable countermeasure to prevent beach erosion from wave attack along the reclamation area is suggested from the experimental results.

Beach Erosion and Preventive Countermeasure at Kangnan Coast, Taiwan

Abstract Kangnan Coast has suffered from beach erosion since the extension of breakwaters and two groins of Hsinchu Fishing Harbor was completed. This is a typical example of human impact on coastal erosion in Taiwan. The aim of this article is to explore the erosion at Kangnan Coast using several analyses, such as shoreline revolution detected from satellite images, planform description of depth variation, volumetric change of bathymetry, and variations in the trends of volumetric changes of the sea bottom. Beach erosion at Kangnan Coast in response to structural effects was quantitatively determined. A suggested countermeasure using submerged detached breakwaters, developed through a 3-year study, is proposed to mitigate beach erosion.

An explicit approximation to the wavelength of nonlinear waves

An explicit and concise approximation to the wavelength in which the effect of nonlinearity is involved and presented in terms of wave height, wave period, water depth and gravitational acceleration. The present approximation is in a rational form of which Fenton and Mckee’s (1990, Coastal Engng 14, 499‐513) approximation is reserved in the numerator and the wave steepness is involved in the denominator. The rational form of this approximation can be converted to an alternative form of a power-series polynomial which indicates that the wavelength increases with wave height and decreases with water depth. If the determined coefficients in the present approximation are fixed, the approximating formula can provide a good agreement with the wavelengths numerically obtained by Rienecker and Fenton’s (1981, J. Fluid Mech. 104, 119‐137) Fourier series method, but has large deviations when waves of small amplitude are in deep water or all waves are in shallow water. The present approximation with variable coefficients can provide excellent predictions of the wavelengths for both long and short waves even, for high waves.

Shifting the waterlines of satellite images to the mean water shorelines considering wave runup, setup, and tidal variation

Abstract. Shoreline evolution is a simple and common method to illustrate beach erosion or accretion in coastal engineering. Extracted waterlines on different satellite images are sometimes used for shoreline evolution. However, time-varying waterlines for tidal variation and wave runup are different from the shoreline at the mean water level. Waterline evolution may bring about misunderstanding of beach erosion or accretion. In a former study, the one-line shift method was proposed to determine the waterlines on a satellite image and to shift the waterlines to shorelines while only considering the tidal variation. The upward shift extension of the waterline due to wave runup and wave setup is considered. Some acceptable equations of wave runup were examined to correct the waterlines on three satellite images in one month when the foreshore beach slopes were specified. The suggested equation for wave runup includes both wave conditions and an average beachface slope at two points located shoreward and seaward away from mean water level by an equal distance of 62.5 m. When on-site shore bathymetrical measurement is sometimes unavailable, a method of minimizing the difference between the initially guessed and estimated foreshore beach slopes by the one-line shift method is proposed.

An Analytical Linear Solution of Transient Waves Generated by a Piston-Type Wave-Maker in a Finite Flume

The paper presents an analytical linear solution, which is more practical than the previous solutions for an infinite flume in hydrodynamic model tests, for the transient wave generated by a piston-type paddle in a finite flume. The proposed solution is expressed in a Fourier series which was examined remarkably accurate for computation when the series exceeds 200 terms. The proposed solution includes two parts, indicating a variation of mean water level and a combination of different harmonics, respectively. The computed wave elevation can realistically illustrate the transient motion of wave spreading from a disturbance generated by a wavemaker. The proposed solution can be converted to the previous solutions for semi-infinite wave tank by transforming the Fourier series to an integral for a very long flume. The computed surface elevations for a periodic paddle movement show that a maximum elevation occurs when the 4-5 transient waves go ahead at a position. Good agreement between the proposed solution and the previous solution as well as experimental results show the validity of the proposed solution for small waves rather than large waves.