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Estimating the probability of egg loss due to scour and fill under high flows Glawdel, Joanna

Abstract

Sediment transportation occurs during high flow events in gravel bed rivers resulting in a change in bed elevations. Some areas of the river experience a net degradation (scour) and others net aggradation (fill). During these events, incubating salmon eggs can be scoured from their pockets or sediment may be deposited above them, preventing intergravel flow and the emergence of fry. The purpose of this thesis is to develop a framework for estimating the probability of egg loss due to scour and fill for a range of possible high flow events in a river. The developed framework consists of four steps. Steps one and two are the application of 2-dimensional hydrodynamic and morphodynamic models. The hydrodynamic model provides outputs of velocity, depth and shear stress at specified locations within the river. In the second step, these results are input into a morphodynamic model that simulates bed elevation changes during a transient simulation of the event. In the third step for a range of events, pre and post-event bed elevations are compared and the values of scour and fill depth are described by probabilistic distributions. For a specific high flow event, given a specific egg burial depth, a relationship between the proportion of egg loss due to scour and fill may be determined based on these distributions. In the final step, uncertainty in the depth of egg burial is accounted for by developing an egg loss model using reliability analysis that determines the probability of not meeting a target egg survival rate. The developed methodology can be applied to any gravel river and is applicable to any salmon species. A case study of the Campbell River, British Columbia using the 2D hydrodynamic and morphodynamic models, River 2D and R2DM, is developed to demonstrate the methodology. For the case study, the Generalized Pareto Distribution is recommended to describe scour and fill in high flow events in spawning areas.

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