Yakın kıyı bar oluşumu ile batık dalgakıranlar arasında işlevsel benzeşimin araştırılması
Yakın kıyı bar oluşumu ile batık dalgakıranlar arasında işlevsel benzeşimin araştırılması
Dosyalar
Tarih
1994
Yazarlar
Mutlu, Tarkan
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Özet
Son yıllarda kıyıların hem kullanım alanı hem de önemi artmıştır. Artan deniz ticaretine paralel olarak yeni ve büyük boyutlu kıyı yapılarının inşaatlarına ihtiyaç duyulurken öte yandan turizm ve benzeri olaylar için de kıyıların korunması ve doğal yapısının bozulmaması gerekmektedir. Başka bir deyişle kıyı yapılarının projelendirilmesi sırasında artık çevresel etkilerin de göz önüne alınması zorunlu olmaktadır. Mevcut koşullar altında mühendisler yeni proje kriterleri oluşturmak daha sempatik ve daha estetik yapı tipleri bulmak arayışına girmişlerdir. Son yıllardaki araştırmaların hedefi bu yapı tipinin nasıl olacağı, etkinliğinin sınırları ve risklerinin belirlenmesidir. Bu yeni yapı tipinin tasarlanabilmesi için en uygun yöntem doğal stabil bir kıyı şeridinin, her türlü deniz koşulunda mevcut etkilere karşı davranışının anlaşılması ile gerçekleştirilebilir ve "düşük maliyetli, çevreyi bozmayan kıyı yapıları" olarak tanımlanabilir. Yapılan literatür çalışmalarında ve laboratuvar deneylerinde, normal koşullarda kıyı profilinin konkav bir şekil aldığı, fırtına koşullarında ise kıyı çizgisinden taşıdığı malzemeyi kırılma çizgisinin önüne yığarak bir kum tepesi oluşturduğu (bar) görülmüştür. Daha sonra bu bar formasyonu batık bir dalgakıran gibi çalışarak kıyı erozyonunu durdurmaktadır. Bu noktadan hareketle, mevcut çalışmada söz konusu taban şeklinin, dinamik olarak stabil bir dalgakıran ile fonksiyonel benzeşimi gerçekleştirilmeye çalışılmıştır.
There can be few people who have not walked along a sandy beach and marvelled at the variety of complex forms which may be observed in the sand. From ripples with spacings of a few centimetres, arranged in highly regular or seemingly random patterns, to larger ridges and bumps of sand, there is much to interest both the casual observer and the inquisitive scientist. Because act as a buffer between the eroding influences of waves and currents, and the land. The energy associated with storm waves is dissipates on beaches, moving sediment offshore and depositing in the nearshore zone, forming a bar just after the storm events and causing shoreline recession on a coast with dominant cross-shore sediment transport (Fig. 1). s*tll built profile offjhor* bor jtora woves (Jisiiootcc wcv«s seawc"* current Fig. 1. Profile of beach under storm conditions. The bar gradually onshore under lower post-storm wave conditions, and eventually it emerges and piles-up on the beach as a berm. A typical sequence of such a beach change was illustreated in the beach stage model by Sunamura (1989), which have been explained entirely on the following pages, but will not be mentioned here. As wave and beach conditions change, therefore, a longshore bar may or may not be present in the nearshore zone, (Rector 1954, Iwagaki and Noda 1962, Hails and Carr 1975, Miller 1976, Short 1978, Short 1979, Asbury et al. 1989, Hayes 1972, Dean 1973, Komar 1976, Larson and Kraus 1992, Silvester 1974, Monohar 1978, Sunamura 1984, Kapdaşlı, Yüksel and Mutlu 1993). xm Waves approaching to the beach interact with the nearshore morphology. When the inner bar exists in the nearshore zone, it acts as an effective filter for wave height. In the case of storm wave events, therefore, the inner bar affords considerable protection of the beach from erosion by acting as a submerged breakwater (Smith and Kraus 1991, Silvester 1974, Takeda et al. 1992.) Moreover, if the inner bar has the effects of wave filtering and beach protection an outer bar should also have the same effects. Because the efficiency of wave filtering is affected by the water depth of bar crest which will change with the offshore distance and depth of the bar, it can be considered that the degree of beach protection will vary according to the offshore distance of the bar. QR 0.5 10 20 30 40 50 c Fig. 2. Erosion/accretion demarcation curves. Parameter denotes the offshore distance of the bar formation (Takeda, I., Sunamura, K. and Sunamura, T., 1992) The condition for beach erosion on given beach changes according to the situation of bars i.e., presence or absence, offshore distance shore normal width, and probably the number of bars. When the bars act as an effective wave filter, the beach is not always eroded even by the large storm waves. From the aforementioned idea, the goal of this thesis is to achieve the simulation of nearshore bar formation with a dynamically stable submerged breakwater. Because, the best way to protect a shore is to investigate the behaviour of a stable beach profile under storm and normal conditions. The function to present disaster has been given the most priority in the design of shore protection works. However, in these few years, importance of an easy access to a coastline and pro-water front is increasingly recognized. Therefore, considerations are required from the view points of the scene of the coastline and the utilization of the beach in the design of shore protection works. To cope with these requirement new types of shore protection works have been worked out and investigated in the fields and laboratories. XIV In the thesis, one example of the procedure for the determination of the shore protection works for the beach erosion is introduced, by the help of a functional simulation. There are various kinds of coastal structures such as sea dikes, seawalls, lockgates, offshore detached breakwaters, groins, artifical reefs, wave absorbing block mounds and so on have been used to construct counter works against coastal disasters and we call them conventional coastal structures. Because of their high cost and extreme ecological impact, engineers are looking forward to a new package of solutions which some call it "Building with nature" and some call it "nature-friendly". This means that the second goal of this thesis is to design a new structure which will has the minimum environmental impact. /\y\y Fig. 3. Environmental impact of a seawall. For example groynes have been found to be ineffective in protecting a shoreline suffering from erosion. Seawalls erode the seabed in front of and downcoast of them. Beach renourishment is a transitory solution to beach denudation. Offshore detached breakwater as well as the wave absorbing block mound in front of seawalls deprive coastal views from as and prevent us from effective utilization of the coastal regions. Recently, people are getting more and more concerned with the preservation of coastal environments and easy access to the shoreline. In order to cope with these requirements and to create pro-water front, low-cost sympathic shore protection works becaming more and more precious. Shore protection, however, is an interference into the natural system around the border line between land and water. The pure conservationist will resist the building of shore protection constructions and propose to leave the shoreline to the dynamical play of water wind, moving sediments and vegetation. Sometimes this is possible. But in many cases it would lead to the natural destruction of just that same system, and with that the possibilities of man using the land behind the shoreline. xv between both extremes: "leave nature alone" and "protect the land" lies a world of difference, causing a tension field between two types of conservation: nature conservation and land conservation. There is also a number of possibilities in between. The aim of this thesis is also, to provide some insight into these possibilities because, our coasts look the most natural in the landscape. Why simulation with a submerged breakwater? Submerged breakwater is a barrier with its crest at or slightly below the still water level. In situations where complete protection from waves is not required, submerged breakwaters offer a potentially economic solution. Submerged breakwaters have been effectively used to protect harbour entrances, to reduce siltation in entrance channels, against beach erosion, and for creation of artificial fishing grounts. The advantages of submerged breakwaters as compared to subaerial breakwaters include their low cost, aesthetics and effectiveness in triggering breaking of high waves without eliminating the landward flow of water, which may be important for water quality considerations. An experimental research has been carried out in order to investigate the similarities between a longshore bar and a dynamically stable submerged breakwater. The experiments were conducted in a wave flume provided with a flap type regular wave generator. The water surface time histories were recorded using a parallel-wire resistance probe in conjunction with a HBM four channel strip chart recorder. A comprehensive laboratory investigation was undertaken to evaluate the performance of the proposed structure (simulated). Experiments have been carried out, from the most known conventional structures which were statically stable up to the proposed structure which was dynamically stable. The incident wave steepness has an important influence on the wave breaking phenomenon. Waves near the critical steepness may be induced to break by the submerged breakwater, and since wave breaking process is always accompanied by energy losses, steeper waves are likely to be attemuated more than the flatter waves. This result can easily be seen from the Ht/H1 versus H./L graphics. The amount of energy transmitted across will naturally be more, if the depth of crest sumberged is more. So it is logical to conclude that the relative depth of crest submergence, Ds/D is likely to be the most important parameter influencing the performance characteristics of the submerged breakwater. Also crest width positively influences the wave transmission over the breakwater. In consequence, if we want to order the advantages of the proposed structure; - can effect substantial wave attenuation and can be successfully used in places where only partial protection against waves is required - offers a potentially economic solution, because of its lower crest height - does not eliminate the landward flow of water, which may be important for water quality considerations xvi - more aesthetic, when compared to subaerial breakwaters, so does not endanger the recreational values of the coast - as it has no multilayered cross-section or core, even catastrophic sea conditions will not vitally effect it and it can be easily nourished.
There can be few people who have not walked along a sandy beach and marvelled at the variety of complex forms which may be observed in the sand. From ripples with spacings of a few centimetres, arranged in highly regular or seemingly random patterns, to larger ridges and bumps of sand, there is much to interest both the casual observer and the inquisitive scientist. Because act as a buffer between the eroding influences of waves and currents, and the land. The energy associated with storm waves is dissipates on beaches, moving sediment offshore and depositing in the nearshore zone, forming a bar just after the storm events and causing shoreline recession on a coast with dominant cross-shore sediment transport (Fig. 1). s*tll built profile offjhor* bor jtora woves (Jisiiootcc wcv«s seawc"* current Fig. 1. Profile of beach under storm conditions. The bar gradually onshore under lower post-storm wave conditions, and eventually it emerges and piles-up on the beach as a berm. A typical sequence of such a beach change was illustreated in the beach stage model by Sunamura (1989), which have been explained entirely on the following pages, but will not be mentioned here. As wave and beach conditions change, therefore, a longshore bar may or may not be present in the nearshore zone, (Rector 1954, Iwagaki and Noda 1962, Hails and Carr 1975, Miller 1976, Short 1978, Short 1979, Asbury et al. 1989, Hayes 1972, Dean 1973, Komar 1976, Larson and Kraus 1992, Silvester 1974, Monohar 1978, Sunamura 1984, Kapdaşlı, Yüksel and Mutlu 1993). xm Waves approaching to the beach interact with the nearshore morphology. When the inner bar exists in the nearshore zone, it acts as an effective filter for wave height. In the case of storm wave events, therefore, the inner bar affords considerable protection of the beach from erosion by acting as a submerged breakwater (Smith and Kraus 1991, Silvester 1974, Takeda et al. 1992.) Moreover, if the inner bar has the effects of wave filtering and beach protection an outer bar should also have the same effects. Because the efficiency of wave filtering is affected by the water depth of bar crest which will change with the offshore distance and depth of the bar, it can be considered that the degree of beach protection will vary according to the offshore distance of the bar. QR 0.5 10 20 30 40 50 c Fig. 2. Erosion/accretion demarcation curves. Parameter denotes the offshore distance of the bar formation (Takeda, I., Sunamura, K. and Sunamura, T., 1992) The condition for beach erosion on given beach changes according to the situation of bars i.e., presence or absence, offshore distance shore normal width, and probably the number of bars. When the bars act as an effective wave filter, the beach is not always eroded even by the large storm waves. From the aforementioned idea, the goal of this thesis is to achieve the simulation of nearshore bar formation with a dynamically stable submerged breakwater. Because, the best way to protect a shore is to investigate the behaviour of a stable beach profile under storm and normal conditions. The function to present disaster has been given the most priority in the design of shore protection works. However, in these few years, importance of an easy access to a coastline and pro-water front is increasingly recognized. Therefore, considerations are required from the view points of the scene of the coastline and the utilization of the beach in the design of shore protection works. To cope with these requirement new types of shore protection works have been worked out and investigated in the fields and laboratories. XIV In the thesis, one example of the procedure for the determination of the shore protection works for the beach erosion is introduced, by the help of a functional simulation. There are various kinds of coastal structures such as sea dikes, seawalls, lockgates, offshore detached breakwaters, groins, artifical reefs, wave absorbing block mounds and so on have been used to construct counter works against coastal disasters and we call them conventional coastal structures. Because of their high cost and extreme ecological impact, engineers are looking forward to a new package of solutions which some call it "Building with nature" and some call it "nature-friendly". This means that the second goal of this thesis is to design a new structure which will has the minimum environmental impact. /\y\y Fig. 3. Environmental impact of a seawall. For example groynes have been found to be ineffective in protecting a shoreline suffering from erosion. Seawalls erode the seabed in front of and downcoast of them. Beach renourishment is a transitory solution to beach denudation. Offshore detached breakwater as well as the wave absorbing block mound in front of seawalls deprive coastal views from as and prevent us from effective utilization of the coastal regions. Recently, people are getting more and more concerned with the preservation of coastal environments and easy access to the shoreline. In order to cope with these requirements and to create pro-water front, low-cost sympathic shore protection works becaming more and more precious. Shore protection, however, is an interference into the natural system around the border line between land and water. The pure conservationist will resist the building of shore protection constructions and propose to leave the shoreline to the dynamical play of water wind, moving sediments and vegetation. Sometimes this is possible. But in many cases it would lead to the natural destruction of just that same system, and with that the possibilities of man using the land behind the shoreline. xv between both extremes: "leave nature alone" and "protect the land" lies a world of difference, causing a tension field between two types of conservation: nature conservation and land conservation. There is also a number of possibilities in between. The aim of this thesis is also, to provide some insight into these possibilities because, our coasts look the most natural in the landscape. Why simulation with a submerged breakwater? Submerged breakwater is a barrier with its crest at or slightly below the still water level. In situations where complete protection from waves is not required, submerged breakwaters offer a potentially economic solution. Submerged breakwaters have been effectively used to protect harbour entrances, to reduce siltation in entrance channels, against beach erosion, and for creation of artificial fishing grounts. The advantages of submerged breakwaters as compared to subaerial breakwaters include their low cost, aesthetics and effectiveness in triggering breaking of high waves without eliminating the landward flow of water, which may be important for water quality considerations. An experimental research has been carried out in order to investigate the similarities between a longshore bar and a dynamically stable submerged breakwater. The experiments were conducted in a wave flume provided with a flap type regular wave generator. The water surface time histories were recorded using a parallel-wire resistance probe in conjunction with a HBM four channel strip chart recorder. A comprehensive laboratory investigation was undertaken to evaluate the performance of the proposed structure (simulated). Experiments have been carried out, from the most known conventional structures which were statically stable up to the proposed structure which was dynamically stable. The incident wave steepness has an important influence on the wave breaking phenomenon. Waves near the critical steepness may be induced to break by the submerged breakwater, and since wave breaking process is always accompanied by energy losses, steeper waves are likely to be attemuated more than the flatter waves. This result can easily be seen from the Ht/H1 versus H./L graphics. The amount of energy transmitted across will naturally be more, if the depth of crest sumberged is more. So it is logical to conclude that the relative depth of crest submergence, Ds/D is likely to be the most important parameter influencing the performance characteristics of the submerged breakwater. Also crest width positively influences the wave transmission over the breakwater. In consequence, if we want to order the advantages of the proposed structure; - can effect substantial wave attenuation and can be successfully used in places where only partial protection against waves is required - offers a potentially economic solution, because of its lower crest height - does not eliminate the landward flow of water, which may be important for water quality considerations xvi - more aesthetic, when compared to subaerial breakwaters, so does not endanger the recreational values of the coast - as it has no multilayered cross-section or core, even catastrophic sea conditions will not vitally effect it and it can be easily nourished.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1994
Anahtar kelimeler
Dalgakıran,
Kıyı şeridi,
Breakwater,
Coastal line