| Title | The Impact Assessment of Detached Breakwater Crest Level on Sediment Siltation Efficiency behind the Breakwaters. |
| Dept | Transportation Technology Research Center |
| Year | 2024 |
| Month | 3 |
| Price | 500 |
| Summary | The impact of waves is the primary driving force behind coastal sediment transport and changes in beach morphology. This study assesses the influence of the detached breakwater crest elevation on the effectiveness of sedimentation behind the breakwater. Combining internet resources and utilizing historical images from Google Earth, along with on-site inspections and consultation with planning, design, and construction management authorities, the study collects and synthesizes data on the types and conditions of existing detached breakwater layouts along the southwestern coast of Taiwan. This involves understanding the initial design cross-sectional dimensions and elevation configurations of these breakwaters. Building on the concept of "detached breakwater effects", the study introduces the beach morphological changes behind artificially submerged headland-bay, which physically composites of 4 detached breakwaters with one of it appearing in high crest form, implemented as part of the Qijin Coastal Protection Project in Kaohsiung. These serve as recognition models to examine the factors controlling changes in the initial and current crest elevations of detached breakwaters. Through practical case comparisons, the study aims to inspect, analyze, and evaluate the effects of offshore breakwater crest elevations on sedimentation behind the breakwater and the resulting changes in the coastal landscape. The study confirms that the detached breakwater's crest elevation and impermeability are critical mechanisms controlling sedimentation behind the breakwater. Additionally, it identifies the strong swashing currents and swashing circulations induced by diffracted waves and broken waves, with flow velocity u≈ √𝑔𝐻𝑏 (where 𝐻𝑏 is the significant wave height and 𝑔 is the acceleration due to gravity) as significant factors influencing beach reshaping. The main conclusions of the study and the adaptation strategies for enhancing sedimentation efficiency presented behind detached breakwaters are outlined as following. 1.The impact and strong scouring action derived from the breaking of nearshore waves are the main drivers of changes in sedimentation behind detached breakwaters. 2.The effectiveness of the " detached breakwater effect" or "headland-bay effect" usually accompanies strong swash circulations and dominant sand transport, influenced by the diffracted wave action behind the nearshore detached breakwater or headland. The effectiveness depends on the strength of wave diffraction behind the detached breakwater or headland. 3.On sandy beaches, the crest elevation and impermeability of detached breakwaters control the strength of the " detached breakwater effect" or "diffraction-induced embayment effect", affecting sedimentation efficiency behind the breakwater. 4.For gravel or eroded beaches, even with high crest elevation and impermeability, detached breakwaters may fail to form sand spits or attached sandbars without a proper supply of sand. 5.The " detached breakwater effect" has 3-D spatial and temporal variation characteristics, especially when the crest elevation and diffracted wave height are high. The phenomenon of reshaping the beach profile and forming sand spits or attached sandbars behind the breakwater becomes more apparent. 6.In adverse marine weather conditions, detached breakwaters, especially those with insufficient crest elevation or top height, may transform into submerged breakwaters due to the combined effects of large waves, storm surges, and settlement or collapse of the breakwater structure. This transformation leads to the conversion of diffracted waves, which were originally shielded by the breakwater, into transmitted waves. This, in turn, directly erodes the naturally formed sand spits or attached sandbars behind the breakwater, potentially causing beach erosion. 7.In the same coastal section, neighboring detached breakwaters with inadequate crest elevation due to subsidence, collapse, or reconstruction will cause narrowing of the breakwater, increased permeability, and a subsequent weakening of the embayment effect of diffracted waves. This leads to enhanced scouring by transmitted waves, resulting in differences in sedimentation efficiency and coastal changes due to the altered characteristics of wave erosion behind the breakwater. 8.To enhance sedimentation efficiency behind detached breakwaters and promote the reshaping of sand spits or attached sandbars, adjustments and coping strategies should focus on increasing the width and height of the breakwater, strengthening its impermeability, and complementing with artificial sand sources or beach nourishment. This approach effectively reduces the intensity of transmitted waves behind the breakwater, enhances the embayment effect of diffracted waves, and achieves the goal of improving sedimentation and coastal protection efficiency behind the offshore breakwater. |
| Post date | 2024/03/28 |
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