Tekil doğal bitkilerin açık kanallardaki akım karakteristikleri üzerinde etkisi

Özet

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.

Abstract

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 variation of velocity profile at the downstream of vegetation were conducted for the different combinations of water depth, velocity values, and representative “type” of the vegetation. 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.