Field Investigation of the Influence of Bed Texture on Aeolian Saltation

风对沉积物的输送是各种环境问题的关键组成部分，从土壤侵蚀和污染物输送到沙尘暴和沙漠化。本研究将对风沙输沙过程的基本机制提出一个新的假设。大多数比灰尘大的风吹来的颗粒在一个被称为跳跃的过程中移动-沿着地面的一系列跳跃或反弹。长期以来，跃变的理论模型一直基于这样一个基本假设：风速的增加会导致粒子反弹得更高更远，从而导致传输速率的增加。然而，基于这一假设的运输模式已被证明无法始终如一地产生准确的预测。初步研究表明，跳跃的基本概念可能是错误的-颗粒不会随着风速的增加而反弹得更高更远，相反，反弹的大小似乎受到床沉积物的惯性的限制。在强风的作用下，增加的弹跳颗粒动能被转移到河床沉积物中，导致额外的颗粒喷射到弹跳中，从而增加了运输速率。本研究将检验这一假设，并量化床层粒度对跃跃啤酒花尺寸的影响。将进行一项实地试验，其中将采用沉积物捕集器来测量在床上不同高度和不同水平距离移动的沉积物比例。这些捕集器将安装在人工床的下风处，这些人工床是由经过筛分的狭窄粒度范围的沉积物组成的。预计从大颗粒层反弹的颗粒会比从细颗粒层反弹的颗粒跳得更高更远，而且这种反弹的尺寸在很大程度上与风速无关。数值模拟将用于重建产生所测量的输运变化所需的跳跃跳的分布，并量化跳的尺寸如何作为床粒度的函数而变化。这些发现将用于构建一个新的跃变过程理论模型，该模型将直接纳入本研究中记录的床层粒度的影响。这项研究批判性地考察了当前风吹泥沙运输科学观点的一个基本组成部分。这可能会改变人们对这一过程如何运作的普遍看法，其结果可能适用于其他环境中的沉积物运输。有许多与风吹颗粒物有关的环境危害，加强对跃变的了解将有助于了解和有效管理这些问题。虽然粉尘在悬浮物中“漂浮”，并不是直接通过跃移运输的，但人们普遍认为，跃移颗粒的影响是产生和喷射细粒的原因。因此，改善对跃变的了解将有助于改进从表土、营养物和有毒金属到经常附着在细颗粒上的化学污染物和微生物等各种物质排放的模拟。从经济角度来看，美国国家研究委员会最近公布了一份与风吹沉积物相关的非现场成本估计，仅在美国西部，每年的成本就超过10亿美元。更好地了解所涉及的过程将有助于减轻这些问题并减少其经济影响。这项研究也将提供教育机会和研究经验给几位研究生，他们将参与这个项目，并有助于促进新成立的地貌过程实验室的发展，这是一个多所大学的倡议，旨在应用地貌知识来解决环境问题。

The transport of sediment by wind is a key component of environmental problems ranging from soil erosion and contaminant transport, to dust storms and desertification. This research will evaluate a new hypothesis regarding the basic mechanics of the process of wind-blown sediment transport. Most wind-blown particles larger than dust are moved in a process known as saltation - a series of hops or bounces along the ground. Theoretical models of saltation have long been based on the fundamental assumption that increasing wind speeds cause particles to bounce higher and farther, resulting in the well-documented increase in the rate of transport. However, transport models based on this assumption have proven unable to consistently produce accurate predictions. Preliminary research has indicated that the underlying conceptualization of saltation may be in error - particles do not bounce higher and farther with increasing wind speed, rather, the size of the bounce appears to be limited by the inertia of the bed sediments. The increased kinetic energy of saltating particles under stronger winds is transferred to the bed sediments, causing ejection of additional grains into saltation and thereby increasing the rate of transport. This study will test this hypothesis, and quantify the influence of bed grain-size on the dimensions of saltation hops. A field experiment will be conducted in which sediment traps will be employed to measure the proportions of sediment moving at various elevations above the bed and various horizontal distances. These traps will be installed downwind of artificial beds composed of sediments that have been sieved into narrow grain-size ranges. It is expected that grains bouncing off beds of large particles will hop higher and farther than those bouncing off beds of finer material, and that hop dimensions will be largely independent of wind speed. Numerical modeling will be used to reconstruct the distribution of saltation hops needed to generate the measured variations in transport, and to quantify how hop dimensions change as a function of the bed grain-size. These findings will be used to construct a new theoretical model of the saltation process that will directly incorporate the influence of bed grain-sizes as documented in this study.This research critically examines a fundamental component of the current scientific view of wind-blown sediment transport. It is likely to alter the prevailing view of how the process works, and the results will potentially be applicable to sediment transport in other environments. There are many environmental hazards associated with wind-blown particulates, and enhanced understanding of saltation will contribute to understanding and effective management of these problems. Although dusts 'float' in suspension and are not directly transported by saltation, it is generally accepted that the impacts of saltating grains are responsible for dislodging and ejecting fines. Hence, improved understanding of saltation will contribute to improvement in modeling emissions of substances ranging from topsoil, nutrients and toxic metals to the chemical contaminants and microorganisms that often attach to fine particles. From an economic standpoint, the National Research Council recently published an estimate of off-site costs associated with wind-blown sediment, which in the western U.S. alone exceed $1 billion per year. Improved understanding of the processes involved will aid in mitigating these problems and reducing their economic impact. This research will also provide educational opportunities and research experience for several graduate students who will participate in the project, and help to foster the development of the newly formed Geomorphic Process Laboratory, a multi-university initiative aimed at applying geomorphic knowledge to solve environmental problems.