Sensitivity of post-storm surge dune recovery to geomorphological variability

2013年12月5日，英国海岸线遭遇了有史以来最大的风暴潮之一。在过去的60年里，只有两次风暴的规模与之相当。1953年水位达到5.6m以上，1974年达到4.7m，2013年风暴达到5.8m。后者对沙丘和海滩造成了严重的侵蚀。由于这是一次如此不寻常的事件，科学家们几乎没有机会研究在如此规模的风暴过后，受损的沙丘可以多快地恢复。了解情况将很有趣，因为沙丘在海岸防御中发挥着重要作用，形成了抵御内陆洪水的屏障，并消散了风暴能量，从而减少了对生命的风险和对基础设施的破坏。规划者必须决定如何最好地管理海岸线，不仅在目前的条件下，而且要提前考虑未来的环境可能是什么样子。英国未来的气候变化情景表明，海平面将以每年4毫米的速度上升，2055年后将上升到每年12毫米以上。同时预测，沿海风暴潮的发生频率和严重程度将会增加，风暴潮将对海岸地貌产生重要影响。这项研究将研究海岸沙丘在被风暴事件侵蚀后，在什么条件下以多快的速度恢复其形状、生态和防御功能。这是令人兴奋的，因为我们很少有机会研究英国各地大型风暴事件的后果，我们将能够收集数据，沿海规划者可以利用这些数据做出更好的海岸防御决策。对大风暴对沙丘影响的研究表明，风暴前沙丘的形状对风暴侵蚀的严重程度以及沙丘被风吹来的沉积物重建的速度起着关键作用。还没有研究的是，在向海推进的海岸上，沙丘恢复的速度有多快。这是我们知识上的一个重大差距，因为这种类型的海岸线占英格兰和威尔士沙丘背后海滩的30%。还不清楚输沙风吹来的方向的变异性如何影响沙丘恢复的速度。尽管沙丘通常与将沙子从海滩吹到沙丘上的陆上强风有关，但研究表明，吹向近海的风也会促进沙丘的形成；这种情况发生的程度取决于沙丘的形状，这可能会导致气流改变方向。有鉴于此，这个项目的实地考察工作将集中在三个地点，每个地点都有不同的风暴前沙丘高度(5至8米高)和坡度，每个地点都被2013年12月的风暴严重侵蚀。这些地点位于林肯郡海岸，海平面以每年约2米的速度上升，季节性风吹向岸上、离岸和近岸。通过每两周在每个地点测量一次沙丘的二维和三维形状，我们可以看到它们在风暴过后是如何变化的。我们亦会利用有关风速和风向的数据，计算每组地形量度之间的风沙输送模式。这些测量将在12个月内定期进行。我们在2013年12月风暴发生后立即访问了现场，并在两周后再次访问了现场。在此期间，离岸大风持续了很长一段时间，然而，即使在这很短的时间内，一个地点的沙丘体积也增加了3.21立方米。我们为这个项目收集的数据将提供风暴后沙丘恢复的细节。理想的情况是，我们还会持续测量现场的风速、风向和泥沙输送情况，但由于资源有限，这是不可能的。相反，一旦我们收集了地形数据，我们将寻求额外的支持，以使用这些数据来对沙丘上的气流和泥沙输送进行三维模拟。

On 5th December 2013 the UK coastline was exposed to one of the biggest storm surges on record. In the past 60 years only two storms have been of comparable size. In 1953, water levels reached 5.6 m above Ordnance Datum (ODN), in 1974 they reached 4.7 m ODN and the 2013 storm reached 5.8 m ODN. The latter caused substantial erosion of dunes and beaches. As this is such an unusual event, there have been few opportunities for scientists to study how rapidly damaged sand dunes can recover following a storm of this magnitude. It would be interesting to know because dunes play an important role in coastal defence, forming a barrier to inland flooding and dissipating storm energy so reducing risk to lives and damage to infrastructure. Planners have to decide how best to manage the coastline not only under present conditions, but also thinking ahead to what the environment might be like in future. Future climate change scenarios for the UK suggest sea level will rise at 4 mm per year rising to over 12 mm per year after 2055. At the same time, it is predicted that the frequency and severity of coastal storm surges will increase, and that storminess will have an important impact on coastal geomorphology. This research will study how quickly, and under what conditions, coastal dunes can recover their shape, ecological and defence functions following erosion by a storm event. It is exciting because we rarely get the opportunity to study the aftermath of large storm events around the UK, and we will be able to collect data that coastal planners can use to make better decisions about coastal defences. Studies of the impacts of large storms on dunes suggest the shape of the dunes before the storm plays a key role affecting how severe the storm erosion is, and also how rapidly the dunes are rebuilt by wind-blown sediments. What has not been studied is how quickly dunes on a seaward-advancing (prograding) coast recover. This is a major gap in our knowledge given this type of coastline accounts for 30% of dune-backed beaches in England and Wales. It is also unclear how variability in the direction from which sand-transporting winds blow affects the rate of dune recovery. Although dunes are normally associated with strong onshore winds that blow sand from the beach to the dunes, research has shown that winds blowing offshore can also contribute to dune-building; the extent to which this occurs depends on the shape of the sand dune which can cause airflow to change direction. Given the above, the fieldwork for this project will focus on three sites each of which had different pre-storm dune heights (from 5 to 8 m high) and slope gradients, and each of which was severely eroded by the December 2013 storm. The sites are located on the Lincolnshire coast which is advancing seawards at about 2 m per year, and which has seasonal winds blowing onshore, offshore and alongshore. By measuring the two- and three-dimensional shape of the dunes every two weeks at each site we can see how they change following the storm. We will also use data on wind speed and direction to work out patterns of wind-blown sediment transport between each set of topographic measurements. The measurements will be taken regularly for 12 months. We visited the field sites immediately after the December 2013 storm and then again two weeks later. During this time there was a prolonged period of very strong offshore winds, however even during this short time, the dunes at one site increased in volume by 3.21 m3. The data we collect for this project will provide detail concerning post-storm dune recovery. Ideally we would also conduct continuous measurements of wind speed, wind direction and sediment transport at the site, but that is not possible given resource constraints. Instead, once we have collected the topographic data, we will seek additional support to use this data for three-dimensional modelling of airflow and sediment transport over the dunes.