WireWall: a new approach to coastal wave hazard monitoring

许多有海洋边界的国家需要人工防御，以保护它们免受洪水等沿海灾害的影响。在英国，3200公里的海岸线受到了保护，尤其是在海滨城镇。这是为了防止洪水，并保护人员、财产和基础设施免受巨浪造成的伤害，巨浪可能会在强风暴和涨潮同时发生时发生。建造坚固的海岸防御工事成本高昂，通常每米约1万英镑，而且需要仔细规划。在规划海岸防御时，需要大量数据来了解未来几十年可能发生的潜在危害。要获得某一地点的数据，通常需要监测当地的潮汐、浪高和海滩高度，监测时间为5至10年。这些数据用于数值工具（例如EurOtop），以测试哪种海堤设计最合适，以及在未来100年内提供保护所需的高度。这些工具通过估算每个设计的“溢出危险”来做到这一点，即在暴风雨条件下可能溢出墙的水量。这些工具的准确性是通过检查暴风雨期间的溢出量的测量结果来评估的。以前的现场实验使用的是放置在海堤后面的大型水箱来收集流过的水。这样的实验非常昂贵，而且很难做，所以只做了几次——通常是在结构非常不同的地点（例如堤防），而且只有几天的时间。它们也只能提供有限数量的数据，而且根本没有关于海水溢出速度的数据，而这是公共安全的一个重要因素。缺乏测量意味着对危害的数值估计存在很大的不确定性，因此，海防系统被过度设计，以获得很大的安全边际，因此可能花费比实际需要的多得多。这个项目的目标是采用一种低成本的仪器，这种仪器以前被用来测量开阔海域的海浪，并将其转化为一个系统（“WireWall”），以测量海岸漫溢的危险。最近技术的进步使得测量快速移动的水流所需的极高频率成为可能（高达100英里/小时的水流每秒几百次）。该系统将采用电容线的三维网格来感应与盐水的接触。这个信号将被用来测量英格兰西北部克罗斯比900米长的海堤脆弱部位的超水量和速度。这个海堤的设计寿命即将结束，对当地情况的密切监测已经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，他们具有互补的领域、实验室和建模专业知识。我们的项目合作伙伴（Sefton Council, Environment Agency, Balfour Beatty， Marlan Maritime Technologies和Channel Coastal Observatory）参与了海岸防御的调试，设计和建造，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，并就新海堤设计中常用的方法和工具提出建议。我们将利用这些信息来优化WireWall系统的配置及其在Crosby的部署。WireWall获得的数据将改进设计新海堤时使用的工具，通过校准观测到的溢顶风险的数值估计。将来，WireWall可以被整合到新的海堤结构中，以实现长期监测。在危险条件下（由于防御退化、气候变化和海平面上升）观察趋势和突然变化的能力将支持海岸线管理计划，并为验证可操作的洪水预报系统提供数据。关键词：岸线监测；近海防御;浪漫过危险

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