Dynamics of Rip currents and Implications for Beach Safety (DRIBS)

激流是冲浪区的强而窄的水流，它向破碎波的海面延伸，并将通过破碎波输送到冲浪区的水向海返回。激流出现在高海浪的海滩上，沙洲上的裂口以不同的水道形式穿过沙洲。激流可以很强，流速为1-2m/S，是冲浪区用水者的主要危险。根据救生员的记录，英国海滩上超过68%的事故(救援)可以归因于激流。据报道，澳大利亚和美国也有类似的比例，仅在佛罗里达州，每年就有100多人因激流溺水身亡。激流不仅将人们带到海洋中，还会将其他物质，如沉积物、浮游生物、营养物和污染物。因此，激流对海滩侵蚀和冲浪区水质也很重要。海岸科学家和救生员很好地认识到激流对海滩安全的重要性，但我们并不完全了解是什么控制了它们的水流强度和模式。对于潮差较大的海滩上的激流，我们的理解尤其糟糕。在一些海滩上，强烈的撕裂穿过栅栏，将游泳者卷到海里，而在其他海滩上，激流在冲浪区内形成了一个巨大的循环涡流。冲浪区用水者面临的风险，以及海滩侵蚀和冲浪区冲刷的可能性，将在很大程度上取决于冲刷环流的类型。我们认为，当所有的波浪破碎都发生在沙洲上，而没有波浪在撕裂水道中破裂时，撕裂流最强。我们假设，在这样的条件下，撕裂的产生机制是最大的，这取决于波浪条件、潮汐和沙洲的形态。这三个因素都随着时间的推移而变化，其中任何一个因素的细微变化都可能对撕裂循环产生重大影响。该项目的总体目标是通过测量各种海浪、潮汐和海滩条件下的激流，并用计算机模型补充数据分析，来检验这一想法。我们将在康沃尔北部海岸的两个高海浪、大潮汐海滩上进行为期6周的实地行动，由于激流，每个海滩上超过150人的大规模救援活动需要同时救援。在每一次活动中，我们将在冲浪区安装几台仪器，在固定位置测量海浪、潮汐和激流。此外，我们将使用大量的专业漂移器来测量完整的裂流模式。漂流者将在冲浪区被释放，并将根据近岸洋流模式移动。他们的位置将被持续监测(使用GPS)，来自漂流者的数据将不仅提供关于激流强度的有用信息，而且还将提供关于流型的有用信息。漂流者被设计成行为像人类，因此他们的动作模仿了被动游泳者的动作。在实地活动中收集的信息将被用于开发一个计算机模型，该模型能够预测任何给定的波浪、潮汐和海滩条件下的撕裂水流模式。然后，我们将使用此模型开发可供救生员使用的工具，以确定当前的撕裂风险，并制定应对此风险的策略。该研究项目由皇家国家救生艇研究所(RNLI)作为合作伙伴参与，RNLI将参与工作的所有阶段。这种伙伴关系将是互惠互利的：RNLI将通过提供其专职工作人员和设施来帮助我们进行实地测量，我们将通过研讨会、讲座、传单和其他类型的出版物将研究成果传递给RNLI。更重要的是，研究结果将被纳入RNLI的风险评估程序和资源管理工具。

Rip currents are strong and narrow currents in the surf zone that extend seaward of the breaking waves and return water seaward that has been transported into the surf zone by breaking waves. Rip currents are found on high-wave beaches with bars with the rips cutting through the bars in the form of distinct channels. Rip currents can be very strong with flow velocities of 1-2 m/s and are the main hazard to surf zone water users. According to lifeguard records, over 68% of incidents ('rescues') on UK beaches can be attributed to rip currents. A similar percentage is reported from Australia and the USA and, in Florida alone, over 100 people drown each year due to rip currents. Rip currents not only transport people out to sea, but also other material, such as sediment, plankton, nutrients and pollutants. Rip currents are therefore also important for beach erosion and surf zone water quality. The importance of rip currents for beach safety is well recognised by coastal scientists and lifeguards, but we do not fully understand what controls their flow strength and pattern. Our understanding is particularly poor for rip currents on beaches with a large tide range. On some beaches, strong rips cut through bars and sweep swimmers out to sea, whereas on other beaches the rip current develops a large circulating eddy within the surf zone. The risks posed to surf zone water users, and the potential for beach erosion and surf zone flushing, will depend strongly on the type of rip circulation. We believe that rip currents are strongest when all wave breaking occurs on the bar and none of the waves break in the rip channel. We hypothesise that under such conditions the rip generation mechanism is maximised and this depends on wave conditions, tide and bar morphology. All three factors vary over time and even subtle changes in any of them may have significant repercussions for the rip circulation. The overall aim of this project, Dynamics of Rip currents and Implications for Beach Safety (DRIBS), is to test this idea by measuring rip currents under a variety of wave, tide and beach conditions, and complementing the data analysis with computer modelling. We will conduct a 6-week field campaign on two high-wave, large-tidal beaches along the north Cornish coast where mass rescue events of upwards of 150 people per beach have required simultaneous rescue due to rip currents. During each of these campaigns, we will install several instruments in the surf zone that will measure waves, tides and rip currents at fixed locations. In addition, we will use a large number of specialist drifters that measure the complete rip current pattern. The drifters will be released in the surf zone and will move according to the nearshore current pattern. Their location will be continuously monitored (using GPS) and the data from the drifters will provide useful information not only on the strength of the rip current, but also on the type of flow pattern. The drifters are designed to behave like human beings and their movement therefore mimics that of passive bathers. The information collected during the field campaigns will be used to develop a computer model that is able to predict the rip flow pattern for any given wave, tide and beach condition. We will then use this model to develop tools that can be used by lifeguards to determine the rip current risk and develop strategies to deal with this risk. This research project involves the Royal National Lifeboat Institution (RNLI) as a Partner and the RNLI will be involved during all stages of the work. The partnership will be mutually beneficial: the RNLI will help us with the field measurements by making available their dedicated staff and facilities, and we will pass on the research findings to the RNLI via workshops, lectures, leaflets and other types of publications. More importantly, the research findings will be incorporated into the RNLI's risk assessment procedures and resource management tools.