Performance modification of vertical faced coastal structures under waves

Düşey yüzlü kıyı yapılarının dalgalar altındaki performansının arttırılması

Abstract

A sloped face maintained with "rubble-mound" technique, is mostly preferred as far as the coastal structures that are designed and constructed in order to be protected against wave effects are concerned. The main reason is not only the inexpensive and easy handling of rubble-mound operation, but also the satisfactory dissipation of incident waves on this sloped face, decreasing both the dynamic load and the reflected component. The vertical faced coastal structures, on the other side, can be constructed to fulfill many different functions with their constant breadth and suitable draft that can be used both for berthing and handling purposes. However, the vertical faced structures are poor to dissipate reflection or to limit wave run-up, compared to sloped ones, at the same time being exposed to high wave loads especially if they are plain faced. There have been several studies devoted to neutralize these disadvantages and managed to increase the performance of vertical faced structures by means of newly developed front face configurations. On the axis of this very same effort, the main goal of this study is to come up with an optimized structure configuration which will not bring a significant extra cost and will be easy to construct and mount; yet, at the same time which is capable of decreasing wave reflection, wave run-up and wave loads reliably with its high dissipation performance. First, various modifications proposed during past studies for vertical faced coastal structures were gone through. Several theoretical and experimental studies conducted on different variations of chambered breakwaters with a perforated front face have yielded quite satisfactory performance against waves, on the condition that the structure is dimensioned and designed with respect to that specific wave. These breakwaters, called "Jarlan " type after the first one to propose, may give a decreasing performance if incident wave period does not coincide with the resonance properties of their chambers. The partially perforated double chambered structure which is proposed with this study, is able to damp the wave energy with its upper chamber located just above the mean water level, without any strong dependence on the incident wave period, while performing the resonant dissipation to a specific wave frequency with its very shallow lower chamber separated from the upper chamber with a slab-like panel resting at the mean sea level. A plexiglas model of this structure, called as the flow chambered caisson, were manufactured with adjustable chamber breadths and tested under 9 sets of regular and one set of irregular waves, for different frontface perforation ratios and different relative dimensions. All the related performance parameters were derived as dimensionless parameters in order to be valid in any system and both in model and prototype scales. These parameters are reflection and runup coefficients for wave damping; pressure coefficient, force and moment coefficients for dynamic wave loads. The results of physical modeling study were compared to the performance of an equivalent plain vertical structure, tested under control experiments. Theoretical values for wave loads were also calculated in order to make further comparison with experimental results. The method proposed by Goda (1985) for wave load calculation on a vertical structure was employed and all calculated values were converted to the defined dimensionless ones. Experimental data implies that, flow chambered caisson can decrease the reflection coefficients down to 0.1 and 0.5 respectively for regular and irregular waves; also the horizontal wave force and wave moment were measured to be 25% to 35% less than the ones measured/calculated for an equivalent plain vertical faced structure. As a result, it was tried to picture an optimum structural configuration as a front faced coastal structure with its easy construction and handling with no significant extra cost; that can not only be used for port-functional purposes like quays, but also can be constructed as a breakwater in order to protect the coastal region/facility. The resulting flow chambered caisson structure is believed to satisfy these criteria to a significant degree, if a prototype application of this structure is constructed after the case-specific design is optimized with further laboratory experiments. It should be noted that open minded-ness of decision makers is a prerequisite in order to achieve more developed and advanced engineering structures.

Özet

Bu çalışmada eğimli kıyı yapılarına göre daha yüksek yansıma, daha yüksek tırmanma ve daha yüksek dalga yükleri ortaya çıkaran düşey yüzlü kıyı yapılarının, sayılan bu dezavantajlarının azaltılabilmesi amacıyla geliştirilmiş bir önyüz konfıgürasyonu tanımlanmış, bu özellikler deki bir yapı için imal edilen fiziksel model üzerinde düzenli ve düzensiz dalgalarla deneyler gerçekleştirilmiştir. Önerilen bu yapı akım odacıklı keson olarak adlandırılmıştır ve tam sakin su seviyesinde bir levha ile ayrılmış iki sığ odacığı bulunan kısmi delikli önyüze sahip bir keson formundadır. Deney sonuçlarına göre dalga yansıması, tırmanması ve dinamik dalga yükleri boyutsuz parametreler cinsinden ifade edilerek yapının performansı değerlendirilmiştir. Bu sonuçlara göre, yapının yansıma katsayısı düzenli dalgalarda 0.1 düzensiz dalgalarda da 0.5 değerlerinin altına kadar inmiştir. Ayrıca boyutsuz olarak türetilen dalga itkisi ve dinamik dalga momentinin, eşdeğer bir düz düşey yüzeyli yapıya göre bu gelişmiş yapıda % 35~% 25 mertebesinde azaltılabildiği görülmüştür. Bulunan deneysel sonuçlara ve yapılan gözlemlere göre, önerilen yapının oldukça etkili biçimde gelen dalganın enerjisini sönümleyebildiği sonucuna varılmıştır. Bunun yanında, önerilen keson sadece belli bir periyottaki gelen dalgayı değil, üst odacığı sayesinde oldukça geniş bir periyot aralığındaki dalgaları sönümleyebilmektedir. Bu noktada üst odacık genişliği yeteri kadar uzun seçilirse, yalnızca düz düşey yüzlü yapılara kıyasla değil, dalgayı odacığı içindeki rezonans özellikleriyle sönümleyen klasik tipteki perfore dalgakıranlara göre de daha avantajlı bir yapı elde edilebilmektedir.