Threshold behaviour in steep alluvial streams: key variables and geohazards

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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.