WireWall: a new approach to coastal wave hazard monitoring

许多有海上边界的国家需要人造防御，以保护它们免受洪水等沿海灾害的影响。在英国，有3200公里的海岸线受到保护，特别是在海滨城镇。这是为了防止洪水，并保护人民，财产和基础设施免受大浪造成的伤害，当一场严重的风暴发生在涨潮的同时。建造坚固的海岸防御系统可能耗资巨大，通常约为每米10，000英镑，并且需要仔细规划。在规划海岸防御系统时，需要大量数据来了解未来几十年可能发生的潜在危险。为了获得特定地点的这些数据，通常意味着监测当地的潮汐、波高和海滩水平5至10年。这些数据用于数字工具（如EurOtop），以测试哪种海堤设计最合适，以及需要多高才能在未来100年内提供保护。这些工具通过估计每个设计的“漫顶危险”来实现这一点，即在暴雨条件下可能会有多少水量漫过墙。工具的准确性进行评估，检查输出对测量的越浪量在风暴期间。以前的野外实验是用放在海堤后面的大水箱来收集涌过来的水。这样的实验非常昂贵，而且很难进行，所以只进行了少数几次-通常是在结构非常不同的地点（例如堤坝），而且只有几天。他们也只提供了有限的数据，根本没有关于水的速度，这是公共安全的一个重要因素。这种测量的缺乏意味着危险的数值估计存在很大的不确定性，因此海防被过度设计为具有很大的安全裕度，因此成本可能远远超过需要。该项目旨在采用以前用于测量公海波浪的低成本仪器，并将其转换为测量沿海漫顶危险的系统（“WireWall”）。最近的技术进步使人们有可能在记录快速移动的漫顶水所需的非常高的频率下进行测量（对于速度高达100 mph的水射流，每秒几百次）。该系统将采用一个三维网格的电容线，感测与盐水的接触。该信号将用于测量英格兰西北部克罗斯比900米长海堤上易受破坏位置的漫顶量和速度。该海堤的设计寿命即将结束，对当地情况的密切监测已开始帮助设计新的海堤。该项目包括工程师，环境水力学专家和海洋学家，他们具有互补的现场，实验室和建模专业知识。我们的项目合作伙伴（塞夫顿理事会、环境署、Balfour Beatty、Marlan Maritime Technologies和Channel Coastal Observatory）参与了海岸防御系统的调试、设计和建造，其中包括政府机构和私人咨询公司。他们将提供克罗斯比现有的监测数据，并将就新海堤设计中常规使用的方法和工具提供建议。我们将使用这些信息来优化WireWall系统的配置及其在克罗斯比的部署。WireWall获得的数据将改进设计新海堤时使用的工具，通过将漫顶危险的数值估计校准到观察到的数值估计。将来，WireWall可以被纳入新海堤结构中，以便进行长期监测。观察危险状况（由于防御退化、气候变化和海平面上升）的趋势和突变的能力将支持海岸线管理计划，并提供数据，以验证实用的洪水预报系统。

Many countries with a sea border need manmade defences to protect them from coastal hazards such as flooding. In the UK 3200 kilometres of coastline are defended, particularly in seaside towns and cities. This is to prevent flooding and to protect people, property and infrastructure from the harm caused by large waves that can occur when a severe storm happens at the same time as a high tide. Building strong coastal defences can be costly, often about £10,000 per meter, and needs careful planning.When planning coastal defences a lot of data are needed to understand the potential hazards that might occur in decades to come. To obtain this data for a particular site usually means monitoring the local tides, wave heights, and beach levels for a period of 5 to 10 years. These data are used in numerical tools (e.g. EurOtop) to test which seawall design is most suitable and how high it needs to be to provide protection for the next 100 years. The tools do this by estimating the "overtopping hazard" for each design, i.e. what volume of water might come over the wall during storm conditions. Accuracy of the tools is assessed by checking outputs against measurements of overtopping volumes during storms. Field experiments have previously used large tanks placed behind the seawall to catch the water that comes over. Such experiments are very costly and can be difficult to do, so only a few have been made - usually at sites with very different structures (e.g. dikes) and for only a few days. They also only provide a limited amount of data and none at all on the speed of the water that overtops: an important factor for public safety. This lack of measurements means there is large uncertainty in the numerical estimates of the hazards, so sea defences are overdesigned to have large safety margins and may therefore cost much more than they need to.This project aims to take a low-cost instrument that has previously been used to measure waves in the open ocean, and convert it into a system ("WireWall") that will measure coastal overtopping hazard. Recent improvements in technology now make it possible to measure at the very high frequencies required to record the fast moving overtopping water (a few hundred times a second for a jet of water travelling up to 100 mph). The system will employ a 3-dimensional grid of capacitance wires that sense contact with saltwater. This signal will be used to measure the volume and speed of overtopping at vulnerable locations on the 900-meter-long seawall at Crosby in the North West of England. This seawall is reaching the end of its design life and intense monitoring of the local conditions has begun to aid the design of a new wall.This project includes engineers, environmental hydraulics experts and oceanographers who have complementary field, laboratory and modelling expertise. Our project partners (Sefton Council, Environment Agency, Balfour Beatty, Marlan Maritime Technologies and Channel Coastal Observatory) are involved in commissioning, designing and constructing coastal defences, and include government authorities and private consultancies. They will provide existing monitoring data at Crosby, and will advise on the methods and tools routinely used in the design of a new seawall. We will use this information to optimise the configuration of the WireWall system and its deployment at Crosby. Data obtained by WireWall will improve the tools used when designing the new seawall by calibrating the numerical estimates of overtopping hazards to those observed.In the future WireWall could be incorporated into new seawall structures to enable long-term monitoring. The ability to observe trends and abrupt changes in hazardous conditions (due to defence degradation, climate change and sea level rise) would support shoreline management plans and provide data to validate operational flood forecasting systems.Keywords: Shoreline monitoring; Coastal defence; Wave overtopping hazard