Applications of EMD to analyses of high-frequency beachface responses to Storm Bebinca in Qing’an Bay, Guangdong Province, China
Abstract: On average, five to six storms occur in the Qiongzhou Strait every year, causing significant damage to coastal geomorphology and several property losses. Tropical Storm Bebinca is the most unusual and complex storm event that has occurred in this region over the last 10 years. To detect the high-frequency beachface responses to the storm, a pressure sensor was deployed in the surf zone to record the free sea surface height, and the heights of grid pile points on the beachface were measured manually to determine beach elevation changes during this storm. Empirical Mode Decomposition and related analysis techniques were used to analyze the high-frequency topography and wave data. The results showed that: (1) the beachface response process occurred in three stages. The first stage was the rapid response stage, wherein the spring tide berm began to erode significantly, and the front edge of the beach berm reacted closely. The two beach sections resisted the harmful energy of the main storm. In the second stage, the beach slope increased after a large sediment loss on the beach berm and its front edge. To adapt to the storm energy, the beach at the low tide line began to erode, and the beach slope decreased. In the third stage, after the storm turned, the wave energy was significantly attenuated, and the beach berm eroded to resist the residual wave energy. The beachface began to oscillate and recover. (2) The main wave surface was the superimposed product of a few internal mode functions. Similar results were observed in beachface changes. High-frequency driving factors determine the local characteristics of beach evolution, and low-frequency driving factors determine the beach evolution trend. (3) The response of sediment to the storm was not a single sea-transportation, but a single- or two-way conversion driven by factors such as wave energy, swash flow, and secondary wave breaking. (4) The Ω-RTR model is not completely applicable to beach states that undergo rapid changes during storms. Therefore, it is necessary to carry out further research on beach state identification during storms.
Figure 17. Relations between the total sedimentation change and wave height. a. Pile #2, b. Pile #3, c. Pile #4, d. Pile #5, e. Pile #6, f. Pile #7. The red line in the middle represents the net elevation change under the wave height, the maximum value exceeding zero is the total deposition, and the minimum value below zero is the total erosion. The height change of each pile represents the sediment change at that pile point.
Table 1. Storm track information on Bebinca (data source: Hainan Oceanic Bureau)
Time (2018) Wind scale Wind velocity/
08:00 on Aug. 9 Grade 7 14 400 1.3–1.8 09:00 on Aug. 10 Grade 7 15 112 1.8–2.3 11:00 on Aug. 10 Grade 7 15 47 (nearest) 1.8–2.3 14:00 on Aug. 10 Grade 7 15 75 1.8–2.3 15:00 on Aug. 10 Grade 7 15 80–226 1.8–2.3 10:00 on Aug. 11 Grade 7 15 226 1.8–2.3 14:00 on Aug. 12 Grade 8 18 210 1.8–2.3 08:00 on Aug. 13 Grade 8 18 276 2.0–2.5 20:00 on Aug. 13 Grade 9 23 315 1.8–2.3 17:00 on Aug. 14 Grade 10 25 355 1.8–2.3 21:00 on Aug. 15 Grade 9 25 54 1.8–2.3 02:00 on Aug. 16 Grade 8 20 72 07:00 on Aug. 17 Xiuying Tide
Table 2. Pearson correlation coefficient R was calculated based on the daily beach response variable (the variation of the penetration at each point in the profile) on the August 9
#1 #2 #3 #4 #5 #6 #7 #1 0.536 −0.183 −0.741 0.018 0.390 0.404 #2 −0.012 −0.712 0.045 0.687 0.715 #3 0.301 −0.650 −0.314 −0.227 #4 −0.172 −0.523 −0.514 #5 0.431 0.281 #6 0.987 #7
Table 3. Pearson correlation coefficient R was calculated based on the daily beach response variable (the variation of the penetration at each point of the profile) on the August 11
#2 #3 #4 #5 #6 #7 #2 0.280 0.426 0.727 −0.397 −0.125 #3 0.459 0.411 0.379 0.751 #4 0.470 0.600 0.676 #5 −0.052 0.018 #6 0.830 #7
Table 4. Parameters of beach morphodynamic state
Date Ω RTR Aug. 8 4.98 5.6 Aug. 9 6.89 5.6 Aug. 10 5.13 5.6 Aug. 11 5.12 5.6 Aug. 12 3.26 5.6 Aug. 13 3.62 5.6 Aug. 14 3.99 5.6
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