Author(s): Smith, Schuyler Ann | Advisor(s): Edwards, Christopher; Barnard, Patrick | Abstract: AbstractThe Impacts of the 2015/2016 El Nino on California’s Sandy BeachesBy Schuyler SmithThe El Nino Southern Oscillation is the most dominant mode of interannual climate variability in the Pacific. The 2015/2016 El Nino event was one of the strongest of the last 145 years, resulting in anomalously high wave energy across the U.S. West Coast, and record coastal erosion for many California beaches (Barnard et al., 2017). Currently, 26 million people live in California’s coastal counties (2010 U.S. Census), and over 600,000 people in California will likely be at risk of coastal flooding by the end of this century due to projected sea level rise and storms (Barnard et al., 2019). To better manage our coastal resources, it is critical that we understand the impacts of both short-term climate variability and long-term climate impacts across the varied coastal settings of California. This study is the first to quantify the effects of one of the strongest El Nino events in the historical record across the entire coast of California, represented by 8000, 50-m spaced shore-normal transects across sandy beaches along the length of the state’s shoreline. The response of sandy shorelines to the extreme El Nino winter of 2015/2016 is quantified in the context of net shoreline movement, using the mean high water (MHW) line as a shoreline proxy. MHW contours were extracted from Light Detection and Ranging (LiDAR) digital elevation models (DEMS) from the Oregon border to Mexico using ArcGIS, to represent the 1998/2002, 2015 and 2016 shorelines. Both net shoreline movement values (from fall of 2015 to spring of 2016) and long-term end-point rates of change (1998/2002-2016) were calculated. Satellite-derived long-term (1984-2019) rates of shoreline change acquired from Luijendijk et al. (2018) are summarized for comparison. To determine the influence of wave energy on the coastal response observed here, wave energy flux values for the El Nino winter were calculated at the 20 m depth contour every 100 m along the entire California coastline using hindcast data generated by O'Reilly et al. (2016).We find that central and northern California experienced the most sandy beach erosion during the El Nino winter, with 96% of analyzed beaches in Central California eroding (mean = 45.7 m of erosion), compared to 89% in northern California (mean = 25.5 m of erosion), and 79% in southern California (mean = 9.7 m of erosion). Although local beach response was highly variable, much of the erosion was observed at river mouths, and on the southern side of structures impeding littoral drift, with accretion observed on the northern or upcoast side of these structures. Within west-facing embayments, more extreme erosion was observed in the north than in the south. These erosional patterns contrast to those of typical El Nino events, when the direction of alongshore transport has been observed as south to north, and accretion occurs in the northern end of embayments. In the long-term (1998/2002-2016), southern California and central California beaches are moderately accreting, while northern California is eroding on average at 79 cm per year. A significant correlation was found between cumulative wave energy flux and shoreline change during the El Nino winter across the state of California (R2 = -0.45, Pl0.001). The correlation is lower (-0.25, Pl0.001) for the 2015/2016 winter cumulative wave energy flux anomaly and shoreline change in southern California. After assessing the impact of the 2015/2016 El Nino event, spatial patterns indicate that an unusual, more northerly wave direction, extreme wave energy, and coastline orientation were key factors in the observed shoreline response. This response was markedly different from the classic El Ninos of 1982-83 and 1997-98, where more southerly storm tracks and southerly wave directions were key factors controlling shoreline behavior.

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2015/2016 年厄尔尼诺现象对加州沙滩的影响

作者（S）：史密斯，斯凯勒安 | 顾问：爱德华兹、克里斯托弗；巴纳德，帕特里克 | 摘要：摘要2015/2016 年厄尔尼诺现象对加利福尼亚沙滩的影响作者：Schuyler Smith 厄尔尼诺南方涛动是太平洋年际气候变率最主要的模式。2015/2016 年厄尔尼诺事件是过去 145 年中最强烈的事件之一，导致美国西海岸异常高的波浪能量，并创下了加利福尼亚许多海滩的海岸侵蚀记录（Barnard 等，2017）。目前，有 2600 万人居住在加利福尼亚的沿海县（2010 年美国人口普查），由于预计的海平面上升和风暴，到本世纪末，加利福尼亚有超过 60 万人可能面临沿海洪水的风险（巴纳德等人， 2019）。为了更好地管理我们的沿海资源，我们必须了解加利福尼亚不同沿海地区的短期气候变率和长期气候影响的影响。这项研究首次量化了整个加利福尼亚海岸历史记录中最强厄尔尼诺事件之一的影响，该事件的代表是沿着该州海岸线长度的沙滩上的 8000 条间隔 50 米的海岸法线断面. 沙质海岸线对 2015/2016 年极端厄尔尼诺冬季的响应是在净海岸线运动的背景下量化的，使用平均高水位 (MHW) 线作为海岸线代理。MHW 轮廓是使用 ArcGIS 从俄勒冈州边界到墨西哥的光探测和测距 (LiDAR) 数字高程模型 (DEMS) 中提取的，以表示 1998/2002、2015 和 2016 年的海岸线。计算了净海岸线移动值（从 2015 年秋季到 2016 年春季）和长期终点变化率（1998/2002-2016）。从 Luijendijk 等人处获得的卫星衍生的长期 (1984-2019) 海岸线变化率。(2018) 进行了总结以供比较。为了确定波浪能对这里观察到的海岸响应的影响，厄尔尼诺冬季的波浪能通量值是使用 O'Reilly 等人生成的后报数据在整个加利福尼亚海岸线每 100 m 的 20 m 深度等值线处计算的。(2016)。我们发现，在厄尔尼诺现象的冬季，加利福尼亚中部和北部经历了最严重的沙滩侵蚀，加利福尼亚中部 96% 的分析海滩被侵蚀（平均= 45.7 m 侵蚀），而加利福尼亚北部则为 89%（平均值 = 25.5 m 侵蚀），和 79% 在南加州（平均 = 9.7 m 的侵蚀）。尽管当地的海滩响应变化很大，但在河口和阻止沿海漂移的结构的南侧观察到了大部分侵蚀，在这些结构的北侧或上岸侧观察到了增生。在朝西的海湾内，北部比南部观察到的侵蚀更为严重。这些侵蚀模式与典型的厄尔尼诺事件形成对比，当时观察到沿岸运输的方向为从南到北，并且在海湾的北端发生增生。从长期来看（1998/2002-2016），南加州和加州中部的海滩正在适度增加，而北加州则以平均每年 79 厘米的速度侵蚀。在整个加利福尼亚州的厄尔尼诺冬季期间，累积波浪能通量与海岸线变化之间存在显着相关性（R2 = -0.45，Pl0.001）。2015/2016 年冬季累积波浪能通量异常与南加州海岸线变化的相关性较低（-0.25，Pl0.001）。在评估 2015/2016 年厄尔尼诺事件的影响后，空间模式表明异常的、更偏北的波浪方向、极端波浪能量和海岸线方向是观察到的海岸线响应的关键因素。这种反应与 1982-83 年和 1997-98 年的经典厄尔尼诺现象明显不同，后者更偏南的风暴轨迹和偏南的波浪方向是控制海岸线行为的关键因素。南加州2015/2016年冬季累积波浪能通量异常与海岸线变化的相关性较低（-0.25，Pl0.001）。在评估 2015/2016 年厄尔尼诺事件的影响后，空间模式表明异常的、更偏北的波浪方向、极端波浪能量和海岸线方向是观察到的海岸线响应的关键因素。这种反应与 1982-83 年和 1997-98 年的经典厄尔尼诺现象明显不同，后者更偏南的风暴轨迹和偏南的波浪方向是控制海岸线行为的关键因素。2015/2016 年冬季累积波浪能通量异常与南加州海岸线变化的相关性较低（-0.25，Pl0.001）。在评估 2015/2016 年厄尔尼诺事件的影响后，空间模式表明一个不寻常的、更偏北的波浪方向、极端波浪能量和海岸线方向是观察到的海岸线响应的关键因素。这种反应与 1982-83 年和 1997-98 年的经典厄尔尼诺现象明显不同，后者更偏南的风暴轨迹和偏南的波浪方向是控制海岸线行为的关键因素。极端波浪能量和海岸线方向是观察到的海岸线响应的关键因素。这种反应与 1982-83 年和 1997-98 年的经典厄尔尼诺现象明显不同，后者更偏南的风暴轨迹和偏南的波浪方向是控制海岸线行为的关键因素。极端波浪能量和海岸线方向是观察到的海岸线响应的关键因素。这种反应与 1982-83 年和 1997-98 年的经典厄尔尼诺现象明显不同，后者更偏南的风暴轨迹和偏南的波浪方向是控制海岸线行为的关键因素。