澳大利亚“黛比”热带气旋登陆前后附近海岸线变化的卫星观测

Abstract

2017年3月28日, 热带气旋“黛比”从澳大利亚昆士兰州登陆, 给当地民众的生命安全和公共财产带来了巨大威胁. 热带气旋在短时间内改变了局部海岸地形, 并使海岸线位置迅速发生变化. 针对海波因特附近的海岸进行研究, 收集了热带气旋登陆前后的两景陆地卫星图像, 结合水体指数和人工检查进行海陆分类制图, 并得到海岸线变化空间分布图. 计算表明, “黛比”登陆前后, 海波因特附近海岸区域陆地面积净增加54.50 km2, 98.2%的海岸线表现为淤积, 仅1.8%的海岸线表现为侵蚀. 准确及时的海岸线变化信息可以帮助港口恢复运营, 还可以用于发展风暴潮-波浪-泥沙数值模式.

On March 28, 2017, tropical cyclone "Debbie" made landfall on the coast of the state of Queensland in Australia. This was the most powerful tropical cyclone to hit Australia in the past 6 years. Debbie has resulted in at least five deaths and caused rail damage, power outages and flooding according to relevant reports. This cyclone halted railway transportation and several mines in Queensland had to be closed, including the coal exports terminals of Abbot Point, Dalrymple Bay and Hay Point. In a short period, Debbie resulted in a strong hydro-dynamical response. Subsequently, Debbie caused considerable coastline movement and significantly modified the coastal topography. The position of a coastline is controlled by tide level and coastal topography. Hence, we should take tide level variation into consideration to avoid its impact on the determination of coastline position. To locate the position of coastline and assess its temporal change around Hay Point, we collected tide level data from Rosslyn Bay station near the study area. Based on the tide level data, two Landsat-8 Operational Land Imager (OLI) images showing similar tide levels before and after Debbie were obtained from the United States Geological Survey (USGS) Earth Explorer website. Landsat scenes used in this paper from Landsat Surface Reflectance Climate Data Records (CDR) had been pre-processed to L1TP level (Precision and Terrain Correction). The widely used Normalized Difference Water Index (NDWI) was used to enhance water information. This water index is able to enlarge the discrimination between surface water and other types of land cover. The optimal threshold was manually determined using the frequency histogram of the NDWI image. We applied the optimal threshold to produce a land-water map. Then, inland water was filled to remove its effect on the land-ocean map. Finally, manual checking and editing were implemented in ArcGIS10.2 to ensure the quality of land-ocean maps before and after Debbie. We used the produced land-ocean map to represent the position of the coastline. Two land-ocean maps were overlaid and subtracted to generate the coastline change map. The net change of land area per 1 km×1 km cell along the coastline was calculated to characterize the spatial pattern of coastline change after cyclone Debbie. Our results suggest that the coastline significantly expanded to the ocean after the landfall of Debbie with a net land gain of 54.5 km2 along the coast around Hay Point, Australia. In our study area, 98.2% of the coastline exhibited expansion to the ocean and only 1.8% of the coastline retreated to the land. Note that we may slightly underestimate the net land gain because some regions in the study area were covered by cloud/shadow during observations. We also observed a significant increase of suspended sediment concentration around some coasts using visual judgement, which is likely to be associated with the coastline expansion around the coastal area near Hay Point. Timely and accurate information on coastline position and its change is essential to the management of sudden siltation of ports and waterways, and the subsequent resumption of the operation of wharves. Such datasets can also be helpful to develop and validate storm surge-wave-sediment numerical models. This paper used a simple application to demonstrate the great potential of satellite technology for natural disaster monitoring and assessment. In the future, more attention should be paid to the use of multi-source remotely sensed data to conduct continuous monitoring of coastline dynamics with higher spatial resolution and temporal frequency. © 2017, Science Press. All right reserved.