scholarly journals COMPARISONS OF WAVE OVERTOPPING DISCHARGES AND DAMAGES OF THE NTOU VERTICAL SEAWALL DUE TO TWO SIMILAR SUPER TYPHOONS ON KEELUNG COAST OF TAIWAN

2011 ◽  
Vol 1 (32) ◽  
pp. 3 ◽  
Author(s):  
Da-Wei Chen ◽  
Shiaw-Yih Tzang ◽  
Shan-Hwei Ou
Keyword(s):  
Water Depth ◽  
Wave Period ◽  
Wave Overtopping ◽  
Swan Model ◽  
Effective Reduction ◽  
Failure Event ◽  
Average Wave ◽  
Wave Conditions ◽  
Typhoon Herb

In this study, SWAN model was first applied to obtain wave conditions during Typhoon Herb (1996) and Krosa (2007). Then the results were used for estimating the wave overtopping discharge with existing empirical formulae selected from EurOtop manual. In the EurOtop formulae, calculations of overtopping discharges can be improved by adapting average wave period (Tm-1,0) for swell conditions. The results show that the peak overtopping discharges during Typhoon Herb (1996) are larger than those during Typhoon Krosa (2007) at the two selected sites. In addition, the water depth at toe of eastern NTOU seawalls (NTOU 2) are shallower than that at northern NTOU seawall (NTOU 1) so that the discharges at NTOU 1 are larger than those at NTOU 2. The calculations show that the peak wave overtopping discharges during both typhoons are greater than the criteria for damages on back slope of seawalls, which agrees with the NTOU seawall failure event during Typhoon Herb. The predicted failure does not again happen to NTOU seawall during Typhoon Krosa implying the effective reduction in overtopping discharges by lifting up 1m of the crest after rebuilding the previously damaged seawall.

Assessment of the wave attenuation capacity of a mangrove forest based on its age and the incident wave conditions

2020 ◽  
Author(s):  
Maria Maza ◽  
Javier L. Lara ◽  
Iñigo J. Losada
Keyword(s):  
Wave Height ◽  
Water Depth ◽  
Mangrove Forest ◽  
Wave Attenuation ◽  
Volume Fraction ◽  
Coastal Protection ◽  
Local Flow ◽  
Ecosystem Properties ◽  
Wave Decay ◽  
Wave Conditions

<p>Although mangroves reduce annual flooding to millions of people there is not a methodology to implement these solutions and it is still difficult to estimate the protection provided by them under different environmental conditions and ecosystem properties. To move forward in the consecution of an engineering approach when implementing these solutions for coastal defense, the first step to make is to better understand and parameterize the basic physical processes involved in flow-mangroves interaction. With the aim of getting a new formulation for wave decay provided by Rhizophora mangrove forests based on flow and ecosystem properties, an experimental campaign was carried out where both wave attenuation and forces on mangrove individuals were measured under different wave conditions. Both, the hydrodynamic conditions and the mangrove forest, were scaled according to field conditions for short waves. The detailed wave attenuation and drag force measurements obtained in these experiments allowed to obtain new formulations of wave decay produced by the forest depending on the flow, i.e.: water depth, wave height and period, and on the forest characteristics, i.e.: individuals submerged solid volume fraction and density. These formulations are used to get attenuation rates under different flow and ecosystem conditions. The resultant curves provide with the wave decay produced by a specific Rhizophora forest subjected to the defined wave conditions. The forest is defined on the basis of its age, considering the differences in individual trees depending on their maturity and the density of the forest as the number of trees per unit area. Wave conditions are defined by the root mean square wave height and the peak period and water depth is also considered. The obtained curves allow to estimate the width of the forest necessary to reach a certain level of protection considering the local flow conditions and the forest age. This can assist in the inclusion of nature-based solutions in the portfolio of coastal protection measures.</p>

scholarly journals Caisson Breakwater for LNG and Bulk Terminals: A Study on Limiting Wave Conditions for Caisson Installation

2020 ◽  
Author(s):  
Yu Lin ◽  
Ghassan El Chahal ◽  
Yanlin Shao
Keyword(s):  
Water Depth ◽  
Oil And Gas ◽  
Numerical Study ◽  
Wave Structure ◽  
Viscous Drag ◽  
Viscous Effects ◽  
Drag Coefficients ◽  
Simplified Approach ◽  
Wave Conditions ◽  
General Guidance

Abstract As the worldwide oil and gas market continues to grow and environmental concerns with respect to in-port offloading of gas have increased, there has been a boom of interest in new liquefied natural gas LNG terminals in the past years. Loading - offloading operations at LNG and bulk terminals are generally protected by a breakwater to ensure high operability. For these terminals, caisson breakwaters are generally a preferred solution in water depth larger than 15 m due to its advantages compared to rubble mound breakwaters. The caisson installation is generally planned to be carried out in the period where sea conditions are relatively calm. However, many of these terminal locations are exposed to swell conditions, making the installation particularly challenging and subject to large downtime. There is no clear guidance on the caisson installation process rather than contractors’ experiences from different projects/sites. Therefore, studies are required in order to provide general guidance on the range of acceptable wave conditions for the installation operations and to have a better understanding of the influence of the caisson geometry. This paper presents a numerical study to determine the limiting wave conditions for caisson installing operations at larger water depth of 30–35 m for a confidential project along the African coast. Three caisson sizes/geometries are considered in order to assess and compare the wave-structure hydrodynamic interaction. The linear frequency-domain hydrodynamic analysis is performed for various seastates to determine the limiting wave conditions. Viscous effects due to flow separation at the sharp edges of the caisson are considered by using a stochastic linearization approach, where empirical drag coefficients are used as inputs. Parametric studies on caisson size and mooring stiffness are also presented, which can be used as a basis for future optimization. The uncertainty in the applied empirical viscous drag coefficients taken from the literature is examined by using a range of different drag coefficients. Further, the use of clearance-independent hydrodynamic coefficients (e.g. added mass and damping) may be questionable when the caisson is very close to the seabed, due to a possible strong interaction between caisson bottom and seabed. This effect is also checked quantitatively by a simplified approach. The findings of the study are presented in the form of curves and generalized to be used by designers and contractors for general guidance in future projects.

scholarly journals Wave Overtopping Discharge for Very Gently Sloping Foreshores

Water ◽  
2020 ◽  
Vol 12 (6) ◽  
pp. 1695
Author(s):  
Thu-Ha Nguyen ◽  
Bas Hofland ◽  
Vu Dan Chinh ◽  
Marcel Stive
Keyword(s):  
Wave Period ◽  
Wave Overtopping ◽  
Slope Method ◽  
Near Shore ◽  
Crucial Parameter ◽  
Sea Dikes

The spectral wave period T m − 1 , 0 at the toe of sea-dikes is a crucial parameter to predict wave overtopping discharge over sea-dikes. It is known from literature that this period quickly increases when waves reach shallow foreshores; however, sometimes the assumption is made that the wave period remains constant from offshore to near-shore, leading to an underestimation of the near-shore wave period. Several formulae have been proposed to resolve the underestimation of wave overtopping discharges for very shallow foreshores. These corrective formulations confirm the tendency of underestimating the overtopping discharges over a very gently sloping foreshore but are not validated for foreshore slopes gentler than 1:500. The “equivalent slope” method based on a recent study is inappropriate for these very gently sloping foreshores due to the breaker parameter being much smaller than seven. This study proposes an extension of the correction and finds that spectral wave periods can reach values two times those offshore.

scholarly journals ANFIS and ANN models for the estimation of wind and wave-induced current velocities at Joeutsu-Ogata coast

2015 ◽  
Vol 18 (2) ◽  
pp. 371-391 ◽  
Author(s):  
Morteza Zanganeh ◽  
Abbas Yeganeh-Bakhtiary ◽  
Takao Yamashita
Keyword(s):  
Wind Speed ◽  
Wave Height ◽  
Water Depth ◽  
Incident Wave ◽  
Wind Direction ◽  
Significant Wave Height ◽  
Wave Period ◽  
Inference System ◽  
Significant Wave ◽  
Wave Induced

In this study, the adaptive network-based fuzzy inference system (ANFIS) and artificial neural network (ANN) were employed to estimate the wind- and wave-induced coastal current velocities. The collected data at the Joeutsu-Ogata coast of the Japan Sea were used to develop the models. In the models, significant wave height, wave period, wind direction, water depth, incident wave angle, and wind speed were considered as the input variables; and longshore and cross-shore current velocities as the output variables. The comparison of the models showed that the ANN model outperforms the ANFIS model. In addition, evaluation of the models versus the multiple linear regression and multiple nonlinear regression with power functions models indicated their acceptable accuracy. A sensitivity test proved the stronger effects of wind speed and wind direction on longshore current velocities. In addition, this test showed great effects of significant wave height on cross-shore currents' velocities. It was concluded that the angle of incident wave, water depth, and significant wave period had weaker influences on the velocity of coastal currents.

Characteristics of Leading Tsunami Waves Generated in Three Recent Tsunami Events

2014 ◽  
Vol 08 (03) ◽  
pp. 1440001 ◽  
Author(s):  
Chao An ◽  
Philip L.-F. Liu
Keyword(s):  
Water Depth ◽  
Wave Amplitude ◽  
Propagation Speed ◽  
Frequency Dispersion ◽  
Wave Period ◽  
Wave Crest ◽  
Haida Gwaii ◽  
Tsunami Waves ◽  
Wave Nonlinearity ◽  
Chile Tsunami

In this paper, the time series of ocean water surface elevation, recorded by Deep-ocean Assessment and Recording of Tsunamis (DART) sensors in the Pacific Ocean, during three recent tsunami events — 2010 Chile tsunami, 2011 Tohoku tsunami, and 2012 Haida Gwaii tsunami — are analyzed. The characteristics of leading tsunami waves are examined in terms of their propagation speed, wave period and wave amplitude so as to determine the importance of wave nonlinearity and frequency dispersion. Using the estimated arrival time of leading waves at each DART station and the distance from each station to the epicenter of the corresponding earthquake, the averaged propagation speed of leading waves for each event is calculated. It is found that the wave propagation speed for 2010 Chile tsunami is roughly 190 m/s, and is slightly slower than that of 2011 Tohoku and 2012 Haida Gwaii tsunamis, 210 m/s for both events. Two time scales associated with the leading waves are introduced: the duration of leading wave crest and the leading wave period obtained from a wavelet analysis. The results show that the leading wave crest duration is roughly 15–20 min and the wave period is roughly 25–30 min at most of DART stations for all the three events. The wave nonlinearity and frequency dispersion parameters, being defined as the wave amplitude to water depth ratio and the square of water depth to wavelength ratio, respectively, are calculated for the leading waves. The parameter for wave nonlinearity is found to be smaller than 4.0 × 10-4, while the parameter for frequency dispersion is smaller than 0.02 at all stations for all the three events. Finally, the cumulative effects of nonlinearity and frequency dispersion for the leading waves are investigated. It is found that the distances between the epicenter and all DART stations in each event are much smaller than those required for the nonlinearity and/or frequency dispersive effects to become significant.

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