Chaos, fractals and self-organization in coastal geomorphology: simulating dune landscapes in vegetated environments

The purpose of this study is to describe the characteristics of occurrence of coastal freak waves and to investigate their statistical and spectral structures. According to Ochi's definition of fre...

http://www.comc.ncku.edu.tw/chinese/h_paper/期刊論文檔案2012/91年度%20ON%20The%20Characteristics%20of%20Observed%20Coastal%20Freak%20Waves.PDF

On the characteristics of observed coastal freak waves

. Due to the potential disasters induced by rogue waves, research in this field has increased rapidly in the last 2 decades. However, there are still a lot of open questions left, including some classic ones, such as whether the rogues waves are just rare events or not. One of the key reasons is that not enough of the observed rogue waves have been investigated. China has a wide sea area, but none of the research has addressed the observed rogue waves. In the present study, 1 year of observed wave data from Jiangsu coastal area, China, are analyzed. It is found that rogue waves are present, although the wave heights are not very large; furthermore, the probability of their occurrence is similar to the Rayleigh distribution prediction, due to the local silty coastal topography. The characteristics of rouge waves are investigated and the results indicate that a new type of rogue wave may exist.

/pdf/a-preliminary-investigation-of-rogue-waves-off-the-jiangsu-3gydlbzis6.pdf

A preliminary investigation of rogue waves off the Jiangsu coast, China

In this study, the change characteristics of wave dissipation performance of rubble mound revetment under the damage progression of armor layer are investigated using a numerical wave flume and a series of model profiles for damaged armor layer. The reflection coefficient and wave overtopping rate are used as the performance measures. A number of irregular waves with different wave profiles and wave grouping properties are generated in the numerical wave flume to obtain many data of the performance measures. Secondly, an artificial neural network (ANN) model is applied to predict the change of the performance measures with the cumulative damage of armor layer. The three - layered neural network calibrated by the results of the numerical experiments can predict the reflection coefficient with sufficient accuracy and the overtopping rate reasonably well.

/pdf/prediction-model-for-change-in-performance-of-rubble-mound-47oudszvkn.pdf

Prediction model for change in performance of rubble mound revetment under damage progression

The beach drainage can be included among the soft engineering methods aimed to counteract the shoreline retreat related to the sediment redistribution along the beach profile. The idea that the groundwater table plays a role in the mobilization of the sediment grains underlies the application of the beach drainage system (BDS) as a tool able to modify the natural dynamics of groundwater table at sandy beaches. Indeed, BDS consists of a series of alongshore buried drains aimed to lower the groundwater table. Due to the drainage, the thickening of the layer of dry sand makes the solid grains less prone to be mobilized by the action of the waves at the swash zone, where interaction phenomena between the percolating and the incoming water lens take place. This tool had a troubled history. Its first applications were encouraging. Since then, controversial performances of both experimental and prototype scale experiences have been observed around the world. This paper aims to present an up-to-date full review of the studies and experiments carried out so far, in order to provide the reader with a complete perspective on its strength and weakness as well as open challenges to be faced in the near future.

/pdf/beach-drainage-system-a-comprehensive-review-of-a-1p86o3rm.pdf

Beach Drainage System: A Comprehensive Review of a Controversial Soft-Engineering Method

https://icce-ojs-tamu.tdl.org/icce/index.php/icce/article/download/5272/4950

An attempt at applying the chaos theory to wave forecasting

A probability distribution of the simultaneous maximum wave amplitude within a finite width along a virtual vertical plane placed in a 3-dim. irregular sea state is studied theoretically. The probability distribution differs considerably from the Rayleigh distribution even when the width of the structure is an order of 1/10 significant wave length. The difference increases with the interval width. Especially, appearant probability of large amplitude is considerably larger than that expected from the Rayleigh distribution. Coastal structures which were designed with sufficient safety margin have sometimes failes. There are possibilities that the structures were not attacked by "unexpectedly large waves" but large waves which could have been possibly expected if we had taken the width effect into account.

/pdf/probability-distribution-of-the-maximum-wave-height-along-a-b6m87mf282.pdf

Probability distribution of the maximum wave height along a sea wall with finite width

In this study, the appearance probability of the freak waves is theoretically introduced applying the definitions by Klinting and Sand (1987). Their three conditions are formulated theoretically applying the probability distributions for the run of wave heights (Kimura,1980) and the distance of a mean point of the zero-crossing wave crest and trough from mean water level (Kimura and Ohta, 1992b). Its appearance probability in a uni-directional irregular wave condition is studied first, the theory is extended then to the 3-dimensional wave condition, and the definition is discussed in terms of the probability in the last.

/pdf/probability-of-the-freak-wave-appearance-in-a-3-dimensional-3fxeoqmx6m.pdf

Takao Ohta

Papers

An attempt at applying the chaos theory to wave forecasting

Probability distribution of the maximum wave height along a sea wall with finite width

Probability of the Freak Wave Appearance in a 3-Dimensional Sea Condition

Numerical verification experiment on usefulness of measures for a safe evacuation plan in campus

Verification of an effective tool for enhancing disaster prevention consciousness of residents