Rapid response survey of the 20th May 2006 Montserrat lahar deposits: a snapshot of lahar system dynamics and associated hazard

2006年5月20日，蒙特塞拉特的苏弗里尔山火山经历了其11年喷发历史上第二大火山穹顶坍塌。全天的大雨导致了火山碎片充斥的洪水(称为拉哈斯)，极大地改变了火山四面的山谷。拉哈尔火山对世界各地的许多火山来说是一个相当大的危险，在火山喷发后的几年里继续发生，造成了大量的死亡。它们特别危险，因为它们难以预测，移动迅速，可以携带大量沉积物和碎片(包括巨石)。它们可以极大地改变河谷和随后的河流行为，并有可能削弱和摧毁先前存在的建筑物，并淹没和掩埋它们。这样的洪水本质上很难研究；它们的发生很难预测，它们的产品很快就会被随后的事件和人类活动改变。5月20日的拉哈尔洪水是蒙特塞拉特一系列洪水中最具破坏性的极端洪水。上一次发生类似事件是在2000年3月。这是一个研究这一极端事件的沉积和形态影响的独特机会，并将新的信息与我们的降雨量记录和对较小事件的沉积的研究相结合，以了解蒙特塞拉特的各种拉哈尔火山。该项目将有助于分析拉哈尔系统在连续雨季中的较长期反应，以及个别主要拉哈尔对河流行为的影响的持久性。新的数据，再加上我们的流动监测设备的重新安装，应有助于更好地了解该系统和减少蒙特塞拉特的灾害，并将对全球了解这些危险流动作出重大贡献。为了实现这一点，在随后的流动进行实质性的人为修改或自然返工之前，必须立即调查破坏和沉积情况。必须趁记忆犹新时收集目击者的报告。我们将详细绘制和记录5月20日在贝勒姆山谷发生的事件造成的地貌变化，并对其他地点进行较少细节的记录。我们将记录岩浆沉积的范围和特征，并将这些数据与降雨量和目击者数据相结合，以解释水流行为在空间和时间上的变化。我们将把这些观察结果与之前和后来的流动情况联系起来，以便更准确地描述蒙特塞拉特不断演变的拉哈尔危害。为了实现这一目标，在接下来的5个月中将有三个调查期。在这些实地考察期间，我们还将修理和重新安装5月20日损坏的UEA雨量计网络和流量监测设备，以便能够远程监测2006年雨季及随后的活动。然后，声学流量监测器的实时数据将在MVO获得，以帮助降低风险。

On 20th May 2006 the Soufriere Hills Volcano on Montserrat experienced the second largest volcanic-dome collapse in its 11 year eruptive history. Heavy rain throughout the day resulted in volcanic-debris-charged floods (called lahars) that dramatically changed the valleys on all sides of the volcano. Lahars are a considerable hazard on many volcanoes worldwide, continuing to occur for some years after eruption and accounting for significant numbers of fatalities. They are especially dangerous as they are difficult to predict, move rapidly and can transport lots of sediment and debris (including boulders). They can drastically alter river valleys and subsequent river behaviour and they have the potential to undercut and destroy pre-existing structures as well as inundate and bury them. Such floods are inherently difficult to study; their occurrence is hard to predictable and their products rapidly altered by subsequent events and human activity. The 20th May lahars were at the most devastating extreme of the range of floods on Montserrat. The last occurrence of similar events was in March 2000. This is a unique opportunity to study the deposits and morphological impact of this extreme event and combine the new information with our rainfall records and studies of the deposits of smaller events, to understand the full range of lahars on Montserrat. The project will contribute towards analysis of the longer-term response of the lahar system through successive rainy seasons and the longevity of the influence of individual major lahars on stream behaviour. The new data coupled with the re-installation of our flow monitoring equipment should lead to a better understanding of the system and to hazard mitigation on Montserrat and will be a substantial contribution to the global understanding of these hazardous flows. To achieve this, immediate survey of the devastation and deposits is essential before substantive anthropogenic modification or natural reworking by subsequent flows. Eyewitness reports must be collected while memories are fresh. We will map and document the morphological change resulting from the 20th May events in the Belham Valley in detail and other sites in less detail. We will record the extent and character of the lahar deposits and combine these data with rainfall and eyewitness data to interpret the variation in flow behaviour in space and time. We will relate these observations to those of both earlier and subsequent flows to provide a more accurate picture of evolving lahar hazard on Montserrat. To achieve this there will be three survey periods spread over the next 5 months. During these field visits we will also repair and re install the UEA rain gauge network and flow monitoring equipment damaged on the 20th May to enable remote monitoring of activity during the 2006 rainy season and subsequently. The real-time data from the acoustic flow monitor will then be available at the MVO to aid risk reduction.