Boşluklu Dalgakıranların Liman İçi Sirkülasyona Etkisinin Deneysel Yöntemlerle Araştırılması

Abstract

Kıyı koruma yapısı olarak dökme taş yapılar, çok uzun yıllardır Dünya’nın birçok yerinde dalganın etkilerine karşı kıyıyı korumak için tasarlanır ve inşaa edilirler. Bu tip yapıların inşaasında taş döküm tekniğinin uygulamasındaki kolaylık ve özelleşmiş ekipman gerektirmemesi, maliyetinin düşük olması, projesinin doğru boyutlandırıldığı takdirde hasar oluşması durumunda bile işlevini devam ettirebilmesinden dolayı günümüzde halen önemini koruyan kıyı koruma yapılarıdır. Dökme taş kıyı koruma yapıları gelen dalga enerjisininin büyük kısmını sönümlemek için inşaa edilirler. Dökme taş dalgakıranlarda sınırlı geçirimlilikten dolayı, korunan yarı kapalı bölgede ya da liman içinde sirkülasyon problemi meydana gelmektedir. Meydana gelen sirkülasyon problemi su kalitesini olumsuz yönde etkilemektedir. Günümüzde, çevresel etkileri açısından incelendiğinde, boşluklu dalgakıranlar su kalitesini ve bölgesel canlı yaşam özelliklerini daha az etkilemelerinden dolayı önem kazanmaktadırlar. Bundan dolayı, bu yapıların davranışlarının, çevresel etkilerinin ve tasarım kriterlerinin araştırılması ve geliştirilmesi son yıllarda araştırmaların odaklandığı konulardan biri haline gelmiştir. Yarı kapalı bölgeye ya da liman içine sirkülasyonu sağlamak için taze su girişini sağlamak ve buradaki suyun hareketini sağlayacak akım meydana getirmek gerekmektedir. Yapılan bu çalışmanın amacı, boşluklu dalgakıranların gelen dalga etkisi altında stabilitesinin ve arkasında oluşan akımın liman içi sirkülasyona etkisinin deneysel yöntemlerle araştırılmasıdır. Çalışma kapsamında yapılan deneyler İTÜ Hidrolik laboratuarındaki düzenli ve düzensiz dalga kanalında, beş farklı geometrik yapıdaki boşluklu dalgakıran modeli oluşturulmuştur. Modeller 1/7 eğimli kum zemin üzerine yerleştirilmiştir. Bu modeller ile farklı tipte yedi adet düzenli ve dört adet düzensiz dalga kullanılarak deneyler yapılmıştır. Dalga dikliğinin artması dalgakırandan geçişi düzenli ve düzensiz dalgaların her ikisinde azalmaktadır. Göreceli olarak daha uzun periyotlu dalgalar, dalgakıran kret genişliğine bağlı olarak boşluklu dalgakıranlarda daha fazla geçmektedir. Oluşturulan modellerde, düzenli dalgaların %2 - %19 luk kısmı düzensiz dalgaların %2 - % 22 lik kısmı dalgakıran içerisinden geçmektedir. Dalgakıranın geometrik yapısına göre oluşturulan boşluklu bölümün sakin su seviyesine göre derinliği dalga geçişini etkileyen önemli bir parametredir. Boşluklu dalgakıranlarda yansıma katsayısı dökme dagakıranlara göre da az olmaktadır. Gelen dalga enerjisinin, dalgakırandan yansıma enerjisi azalmakta, enerjinin küçük bir kısmı dalgakıran içerisinden geçmekte böylece geçen dalga enerjisi oluşmaktadır. Boşluklu dalgakıranlarda poroziteden kaynaklanan geçirimlilik sonucunda dalgakırandan geçen dalga, dalgakıran arkasında bir akım meydana getirmektedir. Bu akım derinlik boyunca yatay hızların kıyıya, liman içine veya dalga kırana doğru oluşan yatay hızlar ile yukarıya ya da aşağıya doğru oluşan düşey akım hızları ile sirkülasyon sağlamaktadırlar. Boşluklu dalgakıranın boşluklu bölgesinin yerinin derinlik boyunca değiştirilmesinin sonucunda dalgakıran arkasında meydana gelen akım koşulları belirlenmiştir. Düzenli ve düzensiz dalgalar altında modellerin stabilitesi test edilmiştir.

For many years, rubble mound structures as the structures of coastal protection are constructed and designed to protect the coast against the effects of the waves. Rubble mound breakwaters are still important because of easy technical application, no need for specialized equipment, keep on working after damaged if designed well. Coastal protection structures made of rubble mound are construcred to absorb most of the energy of the incoming waves. Circulation problems may occur in the harbour or protected semi-closed area because of the limited permeability in the rubble mound breakwater. This lack of circulation has a negative effect on water quality. Nowadays, researching the environmental effects, porous breakwater are gaining importance because of having less effects on water quality and on ecosystem. For this reason, in recent years the behaviours, enviromental effects and design criterias of these structures are one of the tasks to be focused on. Circulation should be generated in order to provide fresh water into the semi-closed area or harbour and movement of the water here. The purpose of this study is to research the stability of porous breakwaters under the effect of incident waves and the effect of the flow on the harbour circulation with experimental methods. During the experiments held in ITU Hydraulics Laboratory regular and irregular wave flume, five different geometric structural porous breakwater models have been created. The models are located on sandy bottom that have 1/7 slope. Experiments were conducted with these models in different types using seven regular and four irregular waves. Effect of the flow conditions formed within harbour basin behind the breakwater due to waves passing from the porous breakwater on the two dimensional circulation has been researched with experiments. On the designed sandwich-type porous breakwaters, effect of porous section on vertical position changes of the breakwater according to still water level have been determined. In the physical model experiments, regular and irregular waves were measured and recorded with wave probes. For the wave channel, four probes in total, three for the front section, one for the rear section have been used on the breakdown model. Incident wave and transmitted wave values were calculated. Among the five porous breakwater models, first two were designed with only porous materials. In these two models with the same structure, crest width was narrowed and effect of crest width on wave incident was researched. Other three models were prepared as a three-layered design, which consists of armour layer, filter layer and core, which are in the same stone size. Materials, which were used in the experiment, have three different porosity value. On each of these three models, a completely porous section, height of which is 10 cm along the cross-section was formed. Incident wave passed from the porous section and created a flow behind the breakwater. Porous section formed on three models changed the position on the cross-section of the breakwater according to the still water level along the depth. Therefore, effect of incident wave flow from the porous breakwater and depth of porous section were also examined. It was observed that incident waves flow from the porous breakwaters in greater amounts compared to three-layered breakwaters, and increase of breakwater crest s width decreases the amount of incident wave. The fact that porosity value affects the incident wave in direct proportion was calculated from the wave values. In regular waves, it is observed that the height of transmitted wave decreases as the height of incident wave increases; on the other hand, in irregular waves, height of transmitted wave increases as the height of incident wave increases. In regular and irregular waves, as wave period, therefore, wave flow from the porous breakwater for long period waves, and as a result of this, wave flow coefficient increase. It was found that both periodical wave length and breakwater geometry have an effect on wave flow. The flow through the porous breakwater decrease while the steepness of the waves increase both for regular and irregular waves. Relatively waves having longer period, pass through the breakwater depending on the width of the crest of the breakwater. Breakwater crest width is a parameter, which affects the wave flow for porous breakwaters. Non-dimensional parameter B/L was obtained to examine this effect. In this non-dimensional parameter, wave length, therefore, wave period also become a part of the process in question. Since flow amount decreases as crest width and wave length decreases, transmission coefficient also decreases. When a wave passes from the porous structure by spreading, wave height decrease since wave energy decreases as a result of projection from the structure and viscosity loss inside the structure. Within these models, %2 - %19 of regular waves and %2 - %22 of irregular waves pass through the breakwater. The depth of porous area, considering the geometrical structure of the breakwater, is an important parameter effecting the wave transmission. Coefficient of reflection on porous breakwater is less than rubble mound breakwater. Energy of incident waves is lowered through the reflection energy of the breakwater, a little part of energy passes through the breakwater and by this way energy of passing wave is occoured. As for the (E_T) percentage of wave energy flowing through the breakwater due to porosity (E_İ) 0,013% - 3,643% of incident wave coming from the breakwater passes. For irregular waves, 0,042% - 4,707% of wave energy passes from the breakwater. In the designed breakwater model, it is observed that percentage of transmitted wave energy to incident wave energy is higher for the irregular waves than those of regular waves. It is planned that a low amount incident wave energy will pass through the breakwater. Since breakwater has a porous structure, three-dimensional velocity measurements were made with two ADV s, one on the front side of the breakwater, in order to measure the velocity of circulation behind the breakwater and incident wave with the purpose o determining flow characteristics formed by the wave behind the breakwater. Flow conditions forming behind the breakwater were evaluated by analyzing horizontal and vertical velocities on two dimensions (u and w). When vertical velocity component which forms behind the breakwater was examined, it was found that it is lower compared to horizontal (u) velocities. Magnitude of vertical velocities is of great significance in terms of circulation along the agitation and depth. In the third and fourth model, it was found that narrowing the cross section increases the flow velocities on the porous section and under the porous section. As a result of permeability of porous breakwater, the waves, passing through the breakwater, causes circulation behind the breakwater. This circulation provides circulation through horizontal speed to the coast, harbour or the breakwater with the surface circulation up and down. Circulation circumtances occured behind the breakwater are determined as a result of changing porous area of the porous breakwater through depth. Stability of models have been tested under the regular and irregular waves.