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

离岸流是一种强烈而狭窄的激流，它在破碎浪处向海方向延伸，并将被破碎浪输送到冲浪带的水向海方向回流。裂口流在有沙洲的高波海滩上发现，裂口以独特的渠道形式穿过沙洲。离岸流可以非常强大，流速为1-2米/秒，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”）可归因于离岸流。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多人因激流而淹死。离岸流不仅把人带到海里，还把其他物质，如沉积物、浮游生物、营养物质和污染物带到海里。因此，离岸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都认识到离岸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流动强度和模式。我们对涨潮幅度大的海滩上的离岸流的了解尤其贫乏。在一些海滩上，强烈的激流穿过沙洲，把游泳者冲到海里，而在另一些海滩上，激流在冲浪区形成了一个巨大的循环涡流。对冲浪区用水者构成的风险，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海岸环流的类型。我们认为，当所有的波浪破碎都发生在沙洲上，而没有波浪在裂口通道中破碎时，裂口流是最强的。我们假设在这种条件下，裂缝产生机制是最大的，这取决于波浪条件，潮汐和沙洲形态。这三个因素随着时间的推移而变化，其中任何一个的细微变化都可能对裂口环流产生重大影响。这项名为“离岸流的动态及对泳滩安全的影响”的研究计划，目的是透过测量不同波浪、潮汐及泳滩情况下的离岸流，并以电脑模拟辅助数据分析，以检验上述观点。我们将在北康沃尔海岸的两个高浪大潮海滩上进行为期六周的实地活动，在那里，由于激流的影响，每个海滩都需要同时救援150多人。在每次活动期间，我们将在冲浪区安装几台仪器，在固定地点测量海浪、潮汐和离岸流。此外，我们将使用大量的专业漂流者来测量完整的离岸流模式。漂浮物将被释放在冲浪区，并将根据近岸洋流模式移动。他们的位置将被持续监测（使用全球定位系统），漂船的数据将提供有用的信息，不仅对离岸流的强度，而且对流型的类型。这些漂流者被设计成像人类一样行动，因此它们的动作模仿了被动游泳者的动作。在实地活动期间收集的资料将用于发展一个计算机模型，该模型能够预测任何给定波浪、潮汐和海滩情况下的裂口流模式。然后，我们将使用这个模型来开发救生员可以使用的工具，以确定离岸流风险并制定应对这种风险的策略。该研究项目涉及皇家国家救生艇研究所（RNLI）作为合作伙伴，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将通过提供其专职人员和设施来帮助我们进行实地测量，而我们将通过研讨会、讲座、传单和其他类型的出版物将研究结果传递给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.