A hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redd selection at the riffle scale

Abstract

High‐resolution velocity profile measurements were taken over a series of riffles on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the fluid structure of riffles and nests (redds) where brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) spawned. Velocity profiles were obtained on a highly discretized planometric scale ranging between 20 and 40 cm grid spacings, with vertical observations occurring every 1.6 cm. From the velocity profiles, between 15 000 and 38 000 velocity measurements were obtained over each riffle on any given day of measurement. Velocity profiles were converted to streamwise velocity magnitude, flow depth, Reynolds number, Froude number, shear stress, vertical velocity components and turbulent kinetic energy per unit area to evaluate the spatial structure of the riffles and the spatial structure of redds (pits and tailspills) relative to the surrounding riffle structure. Semi‐variograms were employed to evaluate the persistence of the fluid structure based upon the metrics evaluated. Results showed that discrete velocity observations poorly described the spatial structure of the flow system and poorly correlated with redd locations. Reynolds number analysis identified a relatively consistent fluid property for distances typically 2–3 times the longitudinal length of redds. Turbulent kinetic energy per unit area consistently identified common regions on all riffles studied that corresponded with the location selections for redds where flow was identified as essentially uni‐directional. Froude number was found to be insensitive in predicting the fluid spatial structure in wadeable flow depths and relating it to the fluid structure of redds. Results indicated that a series of metrics at varying spatial scales of turbulence may be necessary to understand the spatial complexity of redd selection. Copyright © 2011 John Wiley & Sons, Ltd.

Physical laboratory analyses of intergravel flow through brown trout redds (Salmo trutta fario) in response to coarse sand infiltration

“…Similar to other studies dealing with intergravel flow at spawning sites (Carling et al, ; Tonina and Buffington, ; Marchildon et al, ), the measurements and modelling results obtained from this investigation confirm the presence of high pressure zones in pit regions and falling limbs of tails where the flow velocity is reduced to near‐zero values, and low pressure zones forming above the tail crests where the flow is being accelerated over a certain downstream distance even in a longitudinal succession of two redds. The narrow transition of low and high pressure zones occurring over bed irregularities and permeable sediment structures such as salmonid redds leads to enhanced convective flows into the porous medium, also referred to as ‘downwelling’ (Cooper, ; Thibodeaux and Boyle, ; White, ; Elliott, ; Harvey and Bencala, ; Elliott and Brooks, ; Tonina, ; Tonina and Buffington, ).…”

“…Marchildon et al () stated that the intergravel flow dynamics must be related to the specific hydrodynamics (e.g. near bottom flow velocities) of the free‐flowing water column.…”

“…In comparison with measured hydraulic characteristics at spawning redds, the quasi‐3D numeric modeling revealed good agreement, confirming the short alteration of low flow velocity zones and high flow velocity zones at the corresponding channel segments. Applications of detailed three‐dimensional models developed to analyze the hydraulics surrounding salmonid redds are rare in the literature (but see Marchildon et al, ). However, in comparison with the flow velocity measurements conducted in this study, the quasi‐3D numeric modelling approach proved to be a reliable and time‐saving way to visualize realistically the hydrodynamics near the surface structure of salmonid redds.…”