無人機光達系統應用於沙岸與藻礁地區之波浪能量消散之研究;A UAV-RTK-Lidar System measurements of wave energy dissipation over a sandy beach and an algal-reef area

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Title:	無人機光達系統應用於沙岸與藻礁地區之波浪能量消散之研究;A UAV-RTK-Lidar System measurements of wave energy dissipation over a sandy beach and an algal-reef area
Authors:	葉正陽;Yeh, Cheng-Yang
Contributors:	水文與海洋科學研究所
Keywords:	波浪能量消散;藻礁;底床粗糙度;無人機;Wave Energy Dissipation;Algal Reef;Bottom Roughness;Unmanned Aerial Vehicle
Date:	2019-12-30
Issue Date:	2020-01-07 14:23:48 (UTC+8)
Publisher:	國立中央大學
Abstract:	波浪能量消散會直接影響近岸的水動力分佈、造成地形變化與影響生態系，因此計算近岸的波浪能量消散非常重要。計算波浪能量消散，需要精準測量空間上不同點位的水深時序列資料，然而傳統上無法快速大範圍佈放壓力計，精準度也有其不足之處，因此計算其能量消散有難度。光達測距儀為另一種測量水位的儀器，隨著無人機產業和科技發展不斷進步，無人機已應用諸多海洋觀測和研究領域，然而無人機受限於低酬載重量，光達測距儀鮮少被裝載在無人機上。本研究使用一台輕量化、低成本和高效率的光達無人機，量測藻礁海岸與沙岸的波浪能量消散，並在觀測同時，在底部佈放壓力計與無人機資料做比較。本研究在藻礁海岸使用光達無人機八個小時內執行了九次量測，涵蓋一個漲潮潮汐；在沙岸七個小時內執行了八次量測，涵蓋一個漲潮潮汐。我們用壓力計比較無人機量測的水深、示性波高、波浪週期和波浪能量通率，得到兩者之間的水深均方根在藻礁海岸為3公分，在沙岸為9公分；兩者之間的示性波高均方根在藻礁海岸為4公分，在沙岸為9公分，證實無人機測量誤差小。無人機測量波浪能量消散結果，在藻礁區觀測到的總能量消散低於5 Wm-1，在沙岸區域低於15 Wm-1，總能量消散隨著潮汐增高而減少。本研究也使用無人機光達系統量測底床粗糙度，並量化摩擦能量消散，藻礁區摩擦消散占總消散能量比例大於70%，然而沙岸區僅有42%。從碎波指標發現，沙岸區域發生碎波的次數較藻礁區多，在沙岸區的參數化碎波能量消散比藻礁海岸多。得到結果，摩擦消散為藻礁海岸主要的波浪能量消散來源，碎波消散為沙岸波浪能量消散來源。本系統的優點為提供一個有效且精準地量測海岸波浪能量消散方法，並能找出主要能量消散來源。;Wave energy dissipation is a significant factor affecting coastal hydrodynamics, morphology, and ecology. However, it is difficult to be measured since accurate and spatial-distributed measurements of sea surface elevations are required to compute this quantity. Unmanned Aerial Vehicles (UAVs) have been widely used for coastal applications due to its efficiency. However, the usage of Light Detection and Ranging (Lidar) on UAVs is limited by the low payload of UAVs. A lightweight, low-cost, and compact UAV-RTK-Lidar system developed by Huang et al. (2018) was used to test the possibility for measurements of wave energy dissipation. Two field experiments of measuring energy dissipation over algal-reef and sandy beach site were conducted. Nine and eight flights of UAV measurements on the algal reef and sandy beach were conducted in 8 and 7 hours for one tidal cycle. The measurements of wave properties such as the wave height, wave period and wave energy flux by UAV were compared with those of in-situ instruments using pressure sensors. The Root Mean Square (RMS) errors for measurements of water depth and significant wave height between the two techniques are 3 cm and 9 cm over the algal reef and are 4 cm and 9 cm over the sandy beach, respectively. The results measured by UAV-lidar system is consistent with these using bottom-mounted pressure sensors. The results of wave energy dissipation over the algal reef are within 5 Wm-1 and are within 15 Wm-1 over the sandy beach. The energy dissipation is higher at low tide, while it is lower at high tide. We also used the system to observe the bottom roughness at the two sites for quantifying the frictional dissipation. The results show that more than 70% of the total energy dissipation is attributed to the bottom friction on the algal reef, while that is only 42% on the sandy beach. UAV-recorded images were analysed to determine the breaking conditions on the sea surface. Wave breaking occurs more frequently on the sandy beach than that on the algal reef. The UAV-lidar system provides a great potential to quantify the wave energy dissipation remotely and efficiently in fields.
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