Düzenli dalgaların yüzen dalgakıranlardan aktarılması

Abstract

Bu çalışmada, günümüzde özellikle küçük yat limanlarının veya küçük ticari gemilerin korunması için düşünülen limanlarda uygulanan eğimli yüzen dalgakıranların dalga etkisine karşı davranışı incelenmiştir. Bu amaçla Î.T.Ü. İnşaat Fakültesi Hidrolik Laboratuarında 23.2 m boyunda, 1.00 x 0.55 m enkesitli bir dalga kanalında iki boyutlu model deneyleri gerçekleştirilmiştir. Çalışmanın birinci bölümünde yüzen dalgakıranlar ile ilgili genel bilgi verilerek bu tür yapıların diğer masif tip yapılara göre avantaj ve dezavantajları ele alınmıştır. Ayrıca bu bölümde çalışmanın amacı ve bugüne kadar yapılmış olan çalışmalar hakkında detaylı bilgi verilmiştir. İkinci bölümde ise dinamik benzerlik koşulları kullanılarak boyut analizi yardımıyla eğimli yüzen dalgakı ranlardan dalgaların aktarılması olayında etkili olan parametreler belirlenmiştir. Üçüncü bölümde deneylerde kullanılacak dalgakıran modelinin geometrik özellikleri ve deneylerde kullanılan ölçü aletleri hakkında genel bilgi verilmiştir. Dördüncü bölümde ise deneyin yapıldığı dalga kanalının özellikleri ve dalga yutucuların teşkili ile ilgili bilgiler verilerek dalga üreticisi ve teorisi hakkında detaylı bilgi sunulmuştur. Beşinci bölümde deneylerden elde edilen verilerin değerlendirilmesi ve sonuçlar belirtilmiştir.

The basic purpose of installing a breakwater is to protect a part of shoreline or moored vessels from significant incident wave energy and to reduce the height of incident waves to a compatible level in the shelter area. The first application of floating structure for the reduction of surface gravity wave was considered by Joly in 1905. During the Normandy invasion of World War II, two different types of wave barriers were developed by Great Britain for off-loading of men and materials. One of these developments was a portable barge type unit which was floated into position and sunk at a specific location by filling sea water. The second one was a true floating breakwater which had a circular cross section. The increase in number of private pleasure craft and small commercial vessels has generated a demand for additional sheltered areas. And many naturally locations along coastlines have reached full capacity to accomodate of vessels. Additionally, at many of these locations, site parameters such as deep water or poor bottom conditions necessiate a floating structure. Research engineers and scientists knew the potential for floating breakwaters in certain areas and research interest has been directed towards this subject in last decades. The rouble-mound breakwaters has been widely applied to reduce surface water waves for many years. They are constructed of graded rock as a fixed pervious gravity structure. They are very expensive in water depth greater than about 6.0 m. This structure reduces incident wave reflection and wave breaking over it and viscous losses as water particles interact with the breakwater. Common type cross section is trapezoidal the construction coast increases with the depth of water at a site. The maintanance coast of breakwaters is more than floating breakwaters especially when the structure affected large wave. And armour unit fragility is common type of damages. In addition it acts as a pervious vertical barrier to shoreline process. This may interrupt longshore transport inducing local silt and some scour problems or may seperate circulation and cause water polution within a marina. Floating structure has multiple use potential for these reasons that alternative breakwater designs are of interest for small boat marina application. Floating breakwaters attenuate surface water through the mechanisms of reflection, destruction of water particle orbital motions and viscous damping. As a wave attacks the structure, some energy will be reflected, some dissipated, some will induce breakwater motions and some will pass benath the structure. The induced body motions will subsequently generate waves and the restraint of body motion will be provided. by the mooring system. In thepry the structure should provide greater wave attenuation in deep water waves as a greater percentage of the wave's avarage kinetic energy is located in the upper region of the water column with which the breakwater interacts. An attractive benefit of floating breakwaters is that their cost is relatively insensitive to water depth at a site. As the structures are buoyant, the breakwater is mobile which may facilitate realigment or removal if desired-' Of importance for marina application is the dual use potential of a floating breakwater. This may permit the structure to act as a both pier and breakwater. As the breakwater does not extend the full depth of water, interruption of littoral processes and local circulation would not be anticipated. The aim of this research is to evaluate the performance of sloping float (or inclined pontoon) breakwaters with respect to expected wave climate in marinas. The results of this two dimensional model study are for regular waves of intermediate water depths. Wave height in a specific site is a multi-variable phenomenon and there is not a universal equation to describe it. That's why the degree of the wave protection will be asumed by the engineer within the acceptable bounds. In chapter It the main problem is analysed and some information is given about the state-of the-art of floating breakwaters. They can be categorized into groups of; a- Box type b- Pontoon type c- Mat type d- Tethered type The present problem can be grouped into pontoon type floating breakwater. The inclined pontoon breakwater is a wave barrier that consist of a row of moored, flat slabs or panels whose mass distribution is such that in XI still water each panel fcrests with one end on the bottom and the other end protruding above the water surface [llj. Chapter 1 includes also earlier studies given in detail which are relevant to this problem. In Chapter 2; Some information is given about dynamic similarity following figures illustrates the two dimensional case found in a laboratory wave. flume. In addition to two-dimensionality, other assumptions are as follows: 1- The thickness of the float is unimportant in the sense that the mass of float, the mass of the internal ballast and hydrodynamic forces are considered to be distributed uniformly over a plane that coincides with the lower surface. 2- The restraint such as a mooring that maintains the general location of the float in the presence of waves has no effect on wave transmission. 3- The incident waves are regular; that is they form a single frequency, constant-amplitude wave train. Seaward Shoreward / // / /j / / /; Sea Bed /// / / *////**/// For the conditions assumed, it is expected that the height of the waves in the Tee of the structure (H.) depends primarily upon the values of the following independet variables? H. = height of incident waves L ss length of incident waves h a depth of water xii Ü = length of structure w = weight of a unit length of float Ü, = ballasted length w, = weight of internal ballast in a unit length of 0 float w, = Displacement of a unit length of float. Using dimensional analysis an expression for wave attenuation is derived in terms of non-dimensional quantities as following; As seen above equation, the value of C" depends upon the wave-length to the length of the float, the depth of water relative to the length of the float, the height- length ratio or steepness of waves and distribution of mass of the ballasted float. ^y.'^ and w, has been chosen dimensional constant of the breakwater tested. Therefore the above equation is re-written as follows. ) In chapter 3} Hydraulic model and the model equipment are described. For the purpose of the present study the two sloping breakwaters selected for testing each 1.46 and 1.00 m in length, both 0.96 m in width and 0.06 m in depth which were constructed of P.V.C. In chapter 4; a description of wave flume, detailed information on the wave generator, wave absorbers and the measurement of wave profiles on the shoreward and seaward direction is presented. The wave flume has a total length of 23.2 ra 1.00x0.55 m cross section. It has a rigid steel bed and the sides are lined with glass panels for the entire length of the flume for observation of the processes inside the flume. The wave generator is of flap type. The movement of the flap about hinge creates regular waves. The eccentricity of the rotating arm of flap can be adjuted to produce waves of desired height, and by adjusting motor stroke, waves of desired period can be produced. Waves generated by this facility will have wave period ranging from 0.5 to 1.0 sec. and wave height up to 12 cm..Xlll Two wave probes, one at the seaward side and the other at the shoreward side of the floating breakwater were used to measure incident and transmitted waves. The wave probes were calibrated in still water by raising and lowering them by known heights and noting the corresponding deflection of the needle in the strip chart recorder. The wave length, L, of the incident wave was computed from Airy theory relationship,,. 2 L = gT tan h (kh) 2TT where, g = gravitation constant h = water depth below SWL T = wave period k = wave number (= - ?- - ) The transmitted wave height, H., was measured directly from thewwave record obtained shoreward of the breakwater. In chapter 5; the results of the experimental studies are presented. These are as follows? 1- The controling factor of the wave transmission is the inclination of the breakwater in its rest position. 2- Transmitted wave heights are consistently lower for the 1.00 m sloping float breakwater and transmission response of both structures is strongly depend on relative depth. 3- Coefficients of transmission are relatively insensitive to wave height, 4- Sloping float breakwater's wave attenuation performance consistently increases as the wave period decreases. 5- During model tests three different type of mooring line length have been carried out for wave attenuation performance. It is seentthat coefficients of transmission increases more (about t-0. 10 % 0. 15) as the mooring line length decreases. 6- Based on two dimensional model tests for periodic waves and certain assumptions, the sloping float breakwater appears to have promise for shallow and intermediate water depths.