Threshold behaviour in steep alluvial streams: key variables and geohazards

众所周知，河流系统会对城市化、水电大坝建设和气候变化等不断变化的环境条件做出反应，但用于预测这种反应的性质和程度的工具是有限的。一方面，我们有通用的概念模型来描述对一些假设变化可能发生的典型反应，但这些模型很少考虑到给定河流系统的特定配置，并且经常识别不止一种潜在的反应，导致模糊的，有时是相互矛盾的预测。另一方面，我们有详细的数值模型，利用物理定律模拟河流中单个泥沙颗粒的水流和输沙的物理过程；这些模型很少考虑影响渠道响应的相关生态过程，需要详细（且昂贵）的输入数据，并且无法预测几十年或几个世纪内发生的长期影响。一种混合计算机建模方法（称为细胞自动机模型，或CAMs）已经出现，以弥合基于描述基于大规模特征（如池和条）行为的流响应的规则的差距，但是构建它们所需的假设和简化引入了相对于其准确性的重大不确定性。我的研究小组将在实验室和实地研究控制这些更大特征行为的物理，以便我们可以在渠道特征和鱼类可用的物理栖息地方面改善预测溪流如何响应环境变化的物理基础。我们的结果可能会被纳入现有的cam或其他数值模拟框架中，用于预测与环境管理者相关的空间和时间尺度上的通道响应。拟议研究的大部分具体重点是当陡峭的山地溪流经历罕见的洪水，侵蚀其堤岸并显著改变其特征时，沉积物运输动力学变化的本质。这些事件不仅对人类生命和基础设施构成重大危害，而且它们可能是这些河道调整其边界的主要手段，不像较大的、坡度较低的河流，它们受到较少变化的水流的影响，并且通常经历相对较小的河道变化。由于加拿大科迪勒拉地区的许多社区部分建立在与这些河流相关的沉积物上，因此我们必须提高对它们的行为方式的理解，它们如何可能对各种环境变化作出反应，以及如何最好地减轻与它们的自然动态相关的危害。

While it is well known that river systems respond to changing environmental conditions such as urbanization, the construction of hydropower dams, and climate change, the tools available to predict the nature and magnitude of such responses are limited. On the one hand, we have generic conceptual models that describe the typical responses that can occur in response to some hypothetical change, but these models seldom account for the particular configuration of a given river system and often identify more than one potential response, leading to vague and sometimes contradictory predictions. On the other hand, we have detailed numeric models that simulate the physics of water flow and sediment transport of the individual sediment grains in a river using the laws of physics; these models seldom consider the relevant ecological processes that influence channel response, require detailed (and expensive) input data, and are unable to predict the long-term effects occurring over several decades or centuries. A kind of hybrid computer modelling approach (called cellular automata models, or CAMs) has emerged to bridge the gap that is based on using rules describing stream response that are based on the behavior of large-scale features such as pools and bars, but the assumptions and simplifications required to construct them introduces significant uncertainty with respect to their accuracy. My research group will study the physics controlling the behavior of these larger features in the laboratory and in the field, in order that we may improve the physical basis for predicting how streams respond to environmental change, in terms of both channel characteristics and the available physical habitat for fish. Our results may be incorporated into existing CAMs or into other numerical modelling frameworks for predicting channel response at spatial and temporal scales relevant to environmental managers. The specific focus of a large part of the proposed research is the nature of the change in sediment transport dynamics that occurs when steep, mountainous streams experience rare floods, erode their banks, and change their characteristics significantly. Not only do such events represent significant hazards to human life and infrastructure, they may be the primary means by which such channels adjust their boundaries, unlike larger, lower gradient streams which are subject to less variable stream flows, and which routinely undergo relatively minor channel changes. Since many communities in the Canadian Cordillera are built in part upon sediment deposits associated with such streams, it is important that we improve our understanding of the ways in which they behave, how they are likely to respond to various environmental changes, and how the hazards associated with their natural dynamics may best be mitigated.