Lavakas: investigating extreme erosion in Madagascar

极端侵蚀是马达加斯加中央高地的特征。根据世界银行的数据，这个国家的侵蚀率达到了创纪录的水平，被称为拉瓦卡的大片山坡沟壑在每一本旅游指南中都被作为该国地形遭到灾难性侵蚀的例证。Lavakas，它们被描述为.猫爪印。在地貌上，突然启动并迅速增长，在几个月内流失了约8000立方米的泥沙。它们造成了严重的基础设施问题，因为它们破坏了道路和桥梁，而且从它们发出的泥石流淹没了农田。它们被广泛引用为主要或完全是人为的起源，是马达加斯加河流高泥沙负荷的主要来源。然而，这两种解释都不是基于确凿的数据，尽管从经济和环境的角度来看，这一问题都很重要，但对侵蚀的研究很少。强有力的说法比比皆是，但没有数据来限制熔岩形成的速度，或者它们对区域沉积物收支的相对贡献；对它们形成的地质控制在很大程度上是未知的。拟议的项目将通过调查拉瓦卡的形成速度和开始形成时的地质控制来解决马达加斯加的现代侵蚀问题。通过在地理信息系统数据库中结合60年前拉瓦卡分布的航空照片测绘和今天拉瓦卡分布的实地测绘，将实现对拉瓦卡分布和大小变化的分析。这将使评估十年尺度的沉积物产生量以及拉瓦卡演变速率成为可能。宇宙成因同位素分析将得出千年尺度上的沉积物生成率，可以与测绘得出的近期侵蚀速率进行比较，以评估侵蚀速率最近是否如人们经常断言但从未证明的那样实际上有所增加。将通过对拉瓦卡的地图分布及其与不同土地利用类别的关系的地理信息系统分析，量化人类对拉瓦卡形成和生长的影响。对熔岩位置的地质控制的了解将来自于对下伏腐蚀化基岩的构造地质和坡度特征的详细测绘。该项目的结果将对人类与景观相互作用的研究具有重要意义。它们将与马达加斯加的环境规划和减缓侵蚀战略直接相关。建立有利于拉瓦卡地层的一些斜坡的地质标准，可以为道路和大坝建设组织提供预防性规划战略，从而节省大量资金。最重要的是，了解熔岩在沉积物形成中的作用；人类在拉瓦卡地层中的作用；以及马达加斯加是以高侵蚀率为自然特征还是最近的和人为的根源，这将巩固对马达加斯加生态战略的思考，并有助于更好地了解整个岛屿的外生系统。这个项目将是一个很好的方式来训练学生在处理一个与社会直接相关的科学问题的过程中。将教育和国际合作联系起来也是该项目的一个关键组成部分。本科生，包括美国人和马达加斯加人，将在实地和实验室获得密集的一对一指导和培训，以及使用各种分析工具和地理信息系统/遥感技术的实践经验。该项目将为第三世界的马达加斯加学生提供前往美国接受教育和培训的机会，并将为马达加斯加的合作者创造职业培训和研究机会。该奖项由地球科学部和国际科学和教育办公室共同资助。

Extreme erosion characterizes Madagascar's central highlands. The country has a "world-record erosion rate" according to the World Bank, and the widespread hillslope gullies called lavaka are featured in every guidebook as examples of the catastrophic denudation of the country.s landscape. Lavakas, which have been described as .cat claw-marks. on the landscape, initiate suddenly and grow rapidly, shedding sediment volumes on the order of 8000 m3 in a few months. They cause substantial infrastructural problems because they undermine roads and bridges, and because the debris flows that issue from them swamp agricultural land. They are widely cited as being largely or entirely anthropogenic in origin, and the major source of high sediment loads in Madagascar's rivers. Neither of these interpretations is based on hard data, however, and in spite of the importance of this issue, both from an economic and an environmental point of view, of erosion are largely unstudied. Strong statements abound, but there are no data to constrain the rates at which lavakas form, or their relative contribution to regional sediment budgets; and the geological controls on their formation are largely unknown. The proposed project will address the issue of modern erosion in Madagascar by investigating rates of lavaka formation and geological controls on their initiation. Analysis of changes in lavaka distribution and size will be achieved by combining air photo mapping of lavaka distributions 60 years ago and field mapping of present-day lavaka distributions in a GIS database. This will permit evaluation of decadal-scale sediment generation volumes, as well as lavaka evolution rates. Cosmogenic isotope analysis will produce sediment generation rates on the millennial scale, which can be compared with the mapping-derived rates of recent erosion to evaluate whether erosion rates have in fact increased in recent time as has been frequently asserted but never demonstrated. Quantification of human effects on lavaka formation and growth will be achieved by GIS analysis of mapped distributions of lavakas and their relationship to different land-use categories. Understanding of geologic controls on the location of lavakas will come from detailed mapping of the structural geology of underlying saprolitized bedrock and slope characteristics. The results of this project will be of significance to studies of human interactions with the landscape in general. They will have direct relevance for environmental planning and erosion mitigation strategies for Madagascar. Establishment of geologic criteria predisposing some slopes to lavaka formation could save enormous amounts of money by providing preventive planning strategies for road and dam building organizations. Above all, understanding the role of lavakas in sediment generation; the role of humans in lavaka formation; and whether Madagascar is naturally characterized by high erosion rates or whether they are of recent and anthropogenic origin, will solidify thinking about ecological strategizing in Madagascar and contribute to a better understanding of the islands exogenic system as a whole. This project will be a wonderful way to train students in the process of approaching a scientific problem that has direct societal relevance. Linking education and international collaboration is also a key component of this project. Undergraduate students, both American and Malagasy, will get intense one-on-one mentoring and training in the field and in the lab, as well as hands-on experience with a variety of analytical tools and GIS/remote sensing techniques. The project will provide opportunities for third-world Malagasy students to travel to the U.S. for education and training, and will also create career training and research opportunities for Malagasy collaborators.This award is co-funded by the Division of Earth Science and the Office of International Science and Education.