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[AOS18-11] Quantification of Wave Attenuation Effect and the Uncertainty Considering Mangrove Tree Shape
Keywords:mangroves, wave attenuation, prop root, tree shape, normalization
As climate change continues to progress, the level required for adaptation has increased even more, and existing adaptation measures have been reviewed in recent years. The Nature-based Solution (NbS) is one of the options that have been comprehensively validated in this review. This method, which utilizes the advantages of the natural environment, is highly valued for its flexibility in achieving both climate change mitigation and sustainable development. Among the areas where NbS can be applied, coastal areas are vulnerable to sea level rise, and have been the subject of active discussion in recent years. Mangroves are of particular interest as NbS in the coastal areas. Mangroves are evergreen broad-leaved trees that grow in brackish water in coastal and estuarine areas in the tropics and subtropics. The main characteristic of mangroves is the intricate structure formed by the many prop roots. This unique shape is thought to effectively reduce the velocity of waves as they pass through the tree zone, and its effectiveness has been demonstrated in the case of tsunami mitigation during the Sumatra earthquake. In addition to this protective aspect, mangrove forests also play a role as fishing grounds for fish spawning and growth, and provide a variety of benefits to local industries. These multifaceted benefits are highly valued as a comprehensive adaptation measure against climate change, and their applicability is being actively discussed. In this study, we evaluate the applicability of mangroves as a protective function against storm surges and tsunamis.
Mangroves reduce wave energy due to their shape-dependent resistance. However, because of the complexity of root shape, mangrove geometry studies are time-consuming, and few detailed studies have been conducted until the results of a detailed study are reported in 2022. Therefore, accurate estimation of the resistance due to mangroves has been postponed, even though it is the basis of the assessment. In this study, we aim to solve this problem by precisely quantifying the wave energy attenuation effect based on the mangrove tree shape. First, the vertical distribution of tree shape was analyzed based on the results of systematic field surveys to accurately represent the resistance force of mangroves in terms of tree shape. A good linear relationship was found between the height and the vertical distribution of diameter. Furthermore, based on the result that DBH follows the change of each shape parameter well, tree shape was normalized by DBH. Based on the normalization results, a method for quantifying the wave attenuation effect under practical conditions was presented. The method was used to calculate attenuation rates for various combinations of input waves and mangroves areas under various conditions. It was shown that the attenuation rate is proportional to the input wave height and increases with decreasing wave number. Under the long wave approximation, the attenuation rate increases in proportion to the square of DBH. These results provide important information for estimating changes in protection function over time. In addition, the effect of the uncertainty in the wave attenuation effect was evaluated by considering the variation in the normalized tree geometry. It was found that, as with the attenuation rate, the absolute value of the uncertainty increases simultaneously with the input wave height and DBH. The range of uncertainty was about 50% of the average attenuation rate. The results obtained here provide a basis for evaluating other applicability, including biological, mechanical, and economic assessments.
Mangroves reduce wave energy due to their shape-dependent resistance. However, because of the complexity of root shape, mangrove geometry studies are time-consuming, and few detailed studies have been conducted until the results of a detailed study are reported in 2022. Therefore, accurate estimation of the resistance due to mangroves has been postponed, even though it is the basis of the assessment. In this study, we aim to solve this problem by precisely quantifying the wave energy attenuation effect based on the mangrove tree shape. First, the vertical distribution of tree shape was analyzed based on the results of systematic field surveys to accurately represent the resistance force of mangroves in terms of tree shape. A good linear relationship was found between the height and the vertical distribution of diameter. Furthermore, based on the result that DBH follows the change of each shape parameter well, tree shape was normalized by DBH. Based on the normalization results, a method for quantifying the wave attenuation effect under practical conditions was presented. The method was used to calculate attenuation rates for various combinations of input waves and mangroves areas under various conditions. It was shown that the attenuation rate is proportional to the input wave height and increases with decreasing wave number. Under the long wave approximation, the attenuation rate increases in proportion to the square of DBH. These results provide important information for estimating changes in protection function over time. In addition, the effect of the uncertainty in the wave attenuation effect was evaluated by considering the variation in the normalized tree geometry. It was found that, as with the attenuation rate, the absolute value of the uncertainty increases simultaneously with the input wave height and DBH. The range of uncertainty was about 50% of the average attenuation rate. The results obtained here provide a basis for evaluating other applicability, including biological, mechanical, and economic assessments.