1. Posidonia oceanica çayırlarının dalga özellikleri ve dalga enerjisi ile etkileşimi.
- Author
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ElgıNöz, Nilay, Tanik, Ayşegül, and Sedat Kabdaşli, M.
- Subjects
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POSIDONIA oceanica , *SEAGRASSES , *WAVE energy , *COASTAL zone management , *VEGETATION monitoring - Abstract
Submerged and emergent vegetation tends to attenuate waves, increases flow resistance and controls flow regime, especially in shallow waters where waves are affected by sea bottom. In the past three decades, magnitude of wave attenuation caused by vegetation was studied by insitu researches, numerical and physical modelling studies. Submerged and emergent vegetation includes macroalgeas like kelp species, flowering plants like seagrasses and all other vegetation which grow in water. Seagrasses which include almost 60 species, appear in estuaries and seas all over the world except Antarctica coasts. They are fully submerged flowering plants and they act as habitats of important coastal ecosystem components. Magnitude of wave attenuation due to vegetation is strongly related with the ratio of vegetation in the water column. Wave attenuation is a function of vegetation characteristics like morphology, stem density, buoyancy, elasticity and also a function of hydrodynamic conditions including water depth, wave height, wave period and wave direction. Posidonia oceanica is endemic seagrass specie in Mediterranean Sea under protection by international conventions. Since Posidonia oceanica is widely spread around the Turkish coasts, it is selected to be modelled physically. The aim of this study is to investigate the ratio of wave attenuation and energy dissipation caused by Posidonia oceanica meadows. In sandy shores, wave energy reaching the shore causes sediment movement on the coast and re-shapes the coastal profile. In summer time sediment deposition occurs while in winter time erosion and offshore bar evolution occurs. If there is vegetation on the coast there is an additional energy dissipation component in the energy balance. In this case, an additional energy is needed to move the sediment. Therefore the magnitude of the bar evolution, due to storm waves during winter, will be changed if there is vegetation. In this study wave energy in a vegetated area and unvegetated area under same wave conditions were determined experimentally. Experiments were conducted in ITU Hydraulics Laboratory using 1:10 scaled Posidonia oceanica models manufactured from nickel-chrome string and wall plugs. Approximately 3000 models were manufactured and these models were placed on a sandy coast with slope 1:5 in the wave flume. The stem density of the models was 8732 stem/m2 in a 0.3 m2 area which satisfies 87 stem/m2 stem density in prototype. Model seagrass meadow was submerged at all times during the experiments. The wave flume was divided into two sections with a glass wall perpendicular to the coast to measure wave heights both in the vegetated and non-vegetated areas simultaneously. Surface elevation measurements were taken with resistance type wave gauges, during 90 seconds for regular waves and 300 seconds for irregular waves. Analogue surface elevation data was converted into digital data with an analogue-digital converter card. Each single wave height and period was determined using zero up-crossing method and wave statistics were calculated with dos based programs. Experiments were conducted for 6 regular and 4 irregular waves with different wave steepness. After these 10 experiments, vegetated area was moved onshore and, the experiments were repeated for the same waves. The results of the study showed that Posidonia oceanica meadows attenuate waves and dissipate wave energy significantly and this finding is compatible with the previous researches. It is found that the affection ratio of the wave height is around 5-22% for regular and 6-24% for irregular waves. The ratio of wave height in the vegetated side to non vegetated side is between 0.78-0.90 and this ratio is correlated with neither offshore wave height nor wave period. Wave attenuation increases when the vegetated area moves towards onshore. Wave energy dissipated by the vegetated area is around 25% of the total energy. Since wave attenuation is strongly related with the morphology of the vegetation, finding a wave attenuation ratio covering every species is not applicable so experiments for species with different morphology should be conducted to estimate wave attenuation due to vegetation. [ABSTRACT FROM AUTHOR]
- Published
- 2011