Author(s): Eric Gillies; Hamish Moir
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Abstract: High to normal flows of water over geomorphically complex gravel river beds provides the environment for salmonid fish to move up-river and find suitable locations to build redds, in which the salmonids spawn. Salmonids typically select specific regions of the bed-typically close to riffles, or at bar margins. Given that the hydraulic variable that characterizes longitudinal bed variations is the Froude number, then eco-hydraulic modelling of the variations in Froude number in a channel is a useful tool for predicting the availability of spawning habitat. In this paper we use a series of 2D hydraulic models of two separate Atlantic Salmon spawning streams in Scotland to show the growth of salmon spawning habitat from pre-restoration (straightened reaches with little geomorphic complexity) to post restoration (growing geomorphic and hydraulic complexity) to post-flood (high geomorphic complexity) conditions. We develop a method for picking out spawning habitat based on local Froude number, sediment size and pressure gradient in the stream and correlate our predicted habitat with observed salmon redds. Hydraulic forces in a stream are responsible not only for the distribution of water depths and velocities, but these forces also mobilize and sort sediment in the channel-fine material collects in low velocity pools, and coarser material on high Froude number riffle crests, and vortex structures in channel bends transport sediment towards point bars. We therefore couple sediment transport modelling with hydraulic modelling, to predict the evolution of spawning habitat (i. e. ‘ecohydromorphology’) and present results for the two streams, comparing the predicted and observed distribution of bed forms and spawning habitat as the reaches adjust to restoration.
Year: 2018