Experimental investigation of the effects of beach drainage on erosion in fine-grained beaches

Plajlar ince kumlardan iri kayalara kadar değişen büyüklükte kayaların birikmesiyle oluşmuş jeolojik bölgelerdir. Plajlar kara, hava ve deniz birleşiminde yer almakta olup morfolojileri dalga mekaniği, katı madde özellikleri ve mekaniği, plaj yeraltı suyu akımı ve mekaniği ile rüzgar etkilerinin etkileşimine bağlıdır. Sahil drenajı, sahil altına yerleştirilen perfore drenaj boruları vasıtasıyla dalga ve yeraltı sularının drene edilmesi ve böylece yeraltı su seviyesinin düşürülmesi ile sızma oranının arttırılması prensibine dayanır. Araştırmada ince taneli kuma sahip sahillerde sızma oranının arttırılması ile erozyonun nasıl değişeceği deneysel olarak incelenmiştir. Çalışmanın amacı değişik dalga özelliklerinin etkisinde drenaj borusunun farklı konumlarında profil şeklinin ve drenaj debisinin tayin edilmesidir. Bu amaçla fiziksel bir model geliştirilmiş ve basit analitik çözümler önerilmiştir. Bu çalışmalar daha sonra düzenli dalga koşullarında yapılan laboratuvar çalışmaları ile karşılaştırılmıştır. Drenaj debisinin tahmini, inşaa edilecek su toplama kuyusunun hesabında ve bu kuyudan suları denize deşarj edecek pompa kapasitesinin tayininde kullanılmaktadır. Öte yandan, değişik dalga ve drenaj borusu yerleşim yerleri için erozyon alanının tahmin edilmesi, kıyı duvarları ve kıyı kaplamaları gibi çeşitli yapıların stabilitelerinin belirlenmesi açısından önemlidir. Bu çalışmada hem drenaj debisi hem de erozyon alanı parametrelerinin tahmini için denklemler önerilmiştir. Sahil drenajı ile erozyon alanında azalma yaşanmıştır. Bunun nedeni olarak yer altı su seviyesinin düşmesi ile sızma oranında artma ve buna mukabil katı madde efektif ağırlığında artış söylenebilir. Profil verileri incelendiğinde erozyon alanındaki artışın en önemli nedeni ise dalga tırmanma yüksekliğinde yaşanan azalma görülmüştür.  Anahtar Kelimeler: Sahil drenajı, erozyon, stabilizasyon.Beaches play important roles in both the dissipation of wave energy and for the recreational purposes. They are generally in dynamic equilibrium. But, due to increasing industrial activities and population, human interference into the beaches occurs and the dynamic equilibrium deteriorates. In order to control the cross-shore and alongshore sediment transport in eroding beaches, some structures like breakwaters, groins, seawalls are constructed. These structures are composed of massive concrete blocks which lead to loss in aesthetic vision. Moreover, these structures are very difficult to relocate after construction. Beach drainage is an erosion control method studied in-situ, laboratory and numerically and offers an alternative to massive coastal control structures. Main philosophy lying under this method is lowering the water table and increasing the infiltration through the sediment matrix. As the infiltration rate increases, the probability of liquefaction and effective weight reduction of sediments will be reduced. Also, water that infiltrates during the wave runup will lead to reduction in flow velocity during both in runup and run-down phases. This reduction in flow velocity will lead to reduction in shear stresses. Recently, some numerical studies claim that for fine sediments, in case of infiltration, erosion occurs due to offshore migration of sediments. On the other hand, reports of in-situ investigations reveal that, in case of infiltration either the beach accretes or the erosion is reduced. Experiments are conducted in a narrow flume with 22m in length, 1m in width and 80cm in depth. Slope of beach profile is always kept constant (1:5), the profile and drainage flow rate data are measured.  The median grain size of the sediment is d50=0.3 mm. The diameter of the drainage pipe is 50mm and is laid all width of the flume. The pipe is perforated in 8mm holes to allow the water suction. Geotextile sheet is wrapped over the pipe to prevent the holes from sediment clogging. 6 drainage pipe locations are determined; beach profiles and drainage flow rates are measured in these locations. Between the beach head and the ground water trench, geotextile filter layer is placed to prevent clogging of the holes, which allow the ground water flow through the beach. Ground water level is always kept constant in all experiments as the same as the still water level. According to the experimental analysis, the waves which have lower height than 5.5 cm do not result in erosion in the profile in no-drain case. The area of the erosion is almost linearly increasing with wave height. In case of drainage, the wave height which does not result in erosion in the profile is observed as 8cm. Center of gravity of the offshore bar migrates landwards as the drainage pipe location approaches the intersection of beach profile and still water line. There is no direct relation between the drainage flow rate and erosion area. The important factor is that whether the runup/rundown wave water or the existing ground water is drained. The ground water is drained as the pipe location migrates landwards. According to the experimental analysis results, the most obvious reduction in the erosion area is observed when the pipe is placed in location (2). When the data measured in location (2) is compared with no-drain data, erosion area reduction rate is reduced when the wave height is increased. When the wave height is increased up to 25 cm, drainage has no advantage to reduce erosion Dimensionless drainage flow rate and dimensionless erosion area parameters are derived analytically by using the data measured in pipe locations of 1, 3, 4 and 5. Derived equations are verified by using locations 2 and 6. The increase in the infiltration rate by lowering the water table leads to reduction in the effective weight of the sediments and decrease in the flow velocity of the runup/rundown swash flows. When the profile graphs are investigated, the main reason for the erosion reduction should be the reduction of the wave runup height. Increase in the infiltration rate leads to reduction of the wave runup wave flow velocity and quantity, resulting in the decrease of runup limit of water waves. In the down-rush phase, the flow velocity is also reduced and the ability to carry sediments is decreased. Keywords: Beach drainage, dewatering, erosion, stabilisation

Beach profile response analysis during storm surges

Bu çalışmada, açıkta oluşan kum tepelerinin konumunun dalga özellikleri ve kıyıyı oluşturan malzemenin özellikleri cinsinden tanımlanması için fiziksel bir model geliştirilmiş ve dalganın kırılması ile harcanan enerji miktarı ile ilişkili basit analitik çözümler yapılmıştır. Bu çözümler daha sonra düzenli ve düzensiz dalga koşullarında yapılan labaratuvar çalışmaları ile karşılaştırılmış ve düzenli dalga ikliminde ortalama kök hatası 4.06%, düzensiz dalga ikliminde ise 6.21% olmuştur. Bütün bu çalışmalar dalganın kırılma-sından sonra katı madde taşınımı için gerekli olan birim hacimdeki enerji miktarının, dalganın kırıldığı anda birim hacimde açığa çıkan enerji miktarı ile profilin denge halinde olabilmesi için gerekli olan enerji miktarı arasındaki farka eşit olduğunu göstermiştir.  Anahtar Kelimeler: Bar oluşumu, dalga yüksekliği, dinamik stabilite, katı madde, kıyı profili.During winter seasons, an increase in wave height and decrease in wave period grounds erosion of foreshore coastal regions where eroded material accumulate at some distance offshore from the coast forming an offshore bar. Knowledge of the characteristics of an offshore bar under wave attack is of great importance in a number of coastal engineering problems particularly in coastal erosion problems. The goal of this investigation is development of a physical model to predict the actual location of offshore bar in terms of wave climates and sediment properties of beach profiles. For this purpose, a simple analytical solution based on energy dissipation relation is developed. The method presented is compared with experimental results where root mean square of 4.06% and 6.21% for regular and irregular waves respectively achieved. Throughout the research, based on dynamic equilibrium profile considerations, the energy required for sediment transport due to wave breaking per unit volume, is expressed in terms of the difference between the actual, and equilibrium levels of wave energy dissipations per unit volume. During experimental studies it is observed that when the wave height increases, the bar formation migrates further offshore progressively, increasing the volume until the equilibrium bar volume is attained.  Keywords: Bar formation, beach profile, dynamic stability, sediments, wave height

Effect of singular vegetative elements on open channel flow characteristics

Son yirmi yıldır artan çevre bilinci ile birlikte akarsu yatağındaki bitkilerin akım alanı üzerine etkisini araştıran çalışmalara olan ilgi oldukça artmıştır. Farklı türde bitki topluluklarının akım alanı üzerine etkisini araştıran pek çok araştırma yapılmış olmasına rağmen geniş gövdeli tekil ağaçlar gibi bitki türlerinin akım alanı üzerine etkisi henüz yeterince anlaşılmış değildir. Bu çalışmada bir akım ortamındaki tekil doğal bitkilerin akımın hız ve türbülans karakteristikleri üzerine etkisini araştırmayı amaçlayan iki boyutlu deneyler gerçekleştirilmiştir. Deneylerin tamamı 26 m uzunluğunda, 0.98 m genişliğinde ve 0.85 m derinliğindeki akım kanalında gerçek bitki fidanları kullanılarak gerçekleştirilmiştir. Taşkın yataklarında sıkça rastlanan bu doğa olayını analiz etmek amacıyla, geniş gövdeli ağaçlar hacim yükseklik değişimleri gözönüne alınarak başlıca üç sınıfa ayrılmıştır. Hız ölçümlerinde üç adet akustik Doppler hız ölçer kullanılmıştır. Analiz aşamasında akım doğrultusundaki ve düşeydeki zamansal ortalama hız bileşenleri, akım doğrultusundaki ve düşeydeki türbülans bileşenleri ve türbülans kinetik enerjileri araştırılmıştır. Buna ilave olarak bitkinin mansab tarafında, bitkiden belirli bir mesafede, bitkinin yapısal özelliklerinin bir fonksiyonu olarak hız profilini veren bir eşitlik elde edilmiştir. Elde edilen eşitliğin geçerliliği deney verileri aracılığı ile sınanmıştır. Ayrıca, deneyler sonucunda geçirimli yapılarına rağmen akım ortamındaki bitkilerin akımı kayda değer ölçüde etkilediği ve neden oldukları türbülans ile önemli miktarda enerjiyi kırdıkları görülmüştür. Anahtar Kelimeler: Açık kanal, hidrolik, bitki, taşkın yatağı, hız profili.In the last two decades with the increasing environment awareness there is a growing interest on studies which attempt to understand the impact of vegetation on flow field in river and flood plain systems. Although in the past great attention has been devoted to explore the impact of vegetation community on flow pattern, the effect of singular vegetative element, such as trees with large trunk, on flow and turbulence pattern is not yet known. The primary aim of the study was to explore the impact of presence of natural singular vegetative elements (trees with large trunk) in flood plains on velocity and turbulence characteristics. In order to achieve the objective stated above two dimensional experimental measurements were conducted in controlled laboratory conditions. All the experiments were performed in the flume, which is capable of supplying steady flow and regular wave, located in Hydraulic Laboratory of Istanbul Technical University. The size of the flume is 26 m in length, 0.98 m in width and 0.85 m in depth.  Tree saplings were utilized to represent the vegetative effect on flow field. In order to analyze this commonly observed nature phenomenon, trees with large trunk were classified into three groups on the basis of their volume versus height relation. The primary difference between those introduced three types was the volume increment gradient along the height. For given successive height intervals, the volume of vegetation pertain to any interval is always larger than the volume of the interval belong to the one closer to bed for Type 1, approximately constant for Type 2, smaller for Type 3. In nature, since each species is unique in terms of architectural and structural properties, three representative species were selected which could characterize those three types mentioned above. Those selected species were Pinus Pinea, Thuja Orientalis, and Cupressus Macrocarpa for Type 1, Type 2 and Type 3, respectively. Throughout the velocity measurements three Acoustic Doppler Velocimeters, two with maximum 200 Hz sampling frequency, one with maximum 25 Hz sampling frequency, were employed. During the data analysis in order to further understand of the impact of presence of vegetation on flow, the time averaged velocity and turbulence kinetic energy characteristics were examined. The experiments that aim to examine the  More specifically, the experiments on velocity profile measurements along the flume were planned to be run for the combination of 2 depth values, 5 discharge values and 3 vegetation species. At the 10 locations velocity profiles were obtained by measurements along the centerline of the flume at the downstream of vegetation. In overall, 15 velocity profiles were acquired for each test condition. At the design stage of some water resources projects, estimating the flow conditions in open channels with an adequate accuracy is important. However, the gap was detected in the literature that there is still no any method or formulation which gives the velocity profile at the downstream of vegetation depending on the vegetative characteristics. In this context, it was considered that generating a formulation which gives the velocity profile at the downstream of vegetation is necessary for river engineers. Based on these facts a formulation which gives the velocity profile at a certain distance from vegetation was introduced. The validity of the proposed formulation was tested with experimental data for verification. It was seen that the velocity results obtained by introduced formulation are in well agreement with the experimental data. It should be kept in mind that the formulation is a function of vegetative characteristics such as volume, projected area, and submergence degree. Hence approximate calculation of the volume of related vegetation in the field with acceptable accuracy is crucial for the success of the application. The similarity between whole vegetation and its branch of related vegetation can be employed as a reference during the calculation of volume of vegetation. Experimental findings revealed that despite their porous structures, the presence of vegetation considerably disturbs the flow field and dissipate a remarkable amount of energy by turbulence. Furthermore experiments showed that sub-canopy flow occurs for three types of vegetation at the downstream of vegetation in the region close to bed. With increasing compactness of vegetation the magnitude of sub-canopy flow increases. In overall assessment it was concluded that presence of any type of tree with large trunk leads to 0-70 % extra sub-canopy flow at the bottom; 30-110 % retaining effect in the region close to the water surface.Keywords: Open channel, hydraulics, vegetation, flood plain, velocity profile

Experimental investigations on tsunami runup height on permeable beachs

Tsunamiler depremlerle tetiklenebilen zemin hareketleri, heyelanlar, kayma, göçme, çökme gibi olaylar ile oluşabilmektedir. Bu dalgalar kıyı batimetrisindeki değişimle birlikte iç kısımlara doğru ilerleyerek büyük hasarlara neden olurlar. Tsunamiler özellikle kıyı çizgisinde tırmanma bölgesinde daha etkili olduğu için tırmanma yüksekliğinin önceden tahmin edilebilmesi kıyı yapılarının boyutlandırılması açısından önemli olacaktır. Bu çalışmada tsunaminin geçirimli yüzeylerde tırmanma yüksekliği deneysel olarak incelenmiştir. 1:5 eğimde geçirimsiz yüzey pürüzsüz, düz saç levha kullanılarak oluşturulmuştur. Aynı eğimde geçirimli yüzey için dane çapı 0.35 mm ve birim hacim ağırlığı 2.63 gr/cm3 olan doğal plaj malzemesi kullanılmıştır. Tırmanma yüksekliğine etki eden parametreler dalga yüksekliği, şev eğimi, su derinliği, eğimi oluşturan malzeme özellikleri olarak belirlenmiş geçirimli yüzeyler için ampirik bir  ifade önerilmiştir.Anahtar Kelimeler: Tsunami, tırmanma yüksekliği, geçirimli kıyı, deney,eğimli kıyı, kum.Tsunamis may be generated by earthquake-triggered movement of the sea bottom, landslides and collapses. With the change of nearshore bathymetry these waves progress towards inland and causes large damage. Prediction of runup height will play an important role in dimensioning coastal structure as tsunamis are more effective in the runup area at the shoreline. In the literature research on runup height, tsunami wave is liken to solitary wave and produced by horizontal movement of a vertical plate, which is a proposed by Goring (1978). In this study, for the simulation of sudden motion of the ocean bottom, tsunami wave generation system is developed by piston attached to an horizontal plate. The piston moves vertically. Experiments were carried out in the glass-side wall wave flume of 22.5 m length, 1 m width, and 0.50 m depth at the Hydraulics Laboratory, Civil Engineering Faculty, Istanbul Technical University. The beach was formed by natural beach sand and had a slope of 1 vertical to 5 horizontal. The specific gravity of sand was 2.63 g/cm3 and the diameter of sand was 0.35 mm. In the experiments it is observed that the waves are not broken. Water surface profiles and velocity values of the waves calculated and generated are compared. Runup height of tsunami waves on permeable beach is analysed and the experimental results, for impermeable slopes are compared with the runup law and it is seen that they are in consistency. Empirical equation are proposed for permeable beach by defining parameters effecting runup height, wave height, slope, water depth, and the characteristics of the material at the slopes. Keywords: Tsunami, run-up height, permeable beach, experiment, slope beach, sand