Flow in a nature-like fishway and its relation to fish behaviour

Abstract

Nature-like fishways are hydraulic structures built in order to allow fish to negotiate river obstructions. Being different than common fishways designs, nature-like fishways imitate natural rivers characteristics as they are built using simple materials that are usually present in rivers (such as, rocks, boulders, gravel and wood). Even though there is some experience about the efficiency of this type of fishways, there is a lack of knowledge about the hydraulic conditions found by fish during its migration. The flow may be characterized by applying spatial and point analysis techniques. Point analysis describes the temporal behaviour of hydraulic variables measured at a specific location, and includes autocorrelation, spectrum and quadrant analysis. Spatial analysis assesses the spatial distribution of flow variables averaged in time, including main flow variables (such as water depth, mean velocity, kinetic energy and vorticity) and turbulent variables (such as Turbulent Kinetic Energy, TKE; turbulent intensity, asymmetry, kurtosis and Reynolds shear stresses). In this study, experimental results on the flow field induced by a nature-like fishway are presented, and its relation to fish behaviour is discussed. A rocky-ramp with 5% slope following DVWK (2002) design guidelines was built along a 8.9 x 0.9 x 0.6 m laboratory flume. Acoustic Doppler velocity measurements were taken at 186 locations on a regular grid, at a low, middle, and high discharge. Point analysis consistently shows that the boulder to boulder distance is small enough to disrupt turbulent coherent structures. At boulder wakes the flow showed no predominance of sweep and ejection events, evidencing a good resting place for fish migration. The nature-like rocky-ramp offers a diversity of flow conditions controlled by the geometry of the boulders. This might allow a variety of fishes to develop their own preferred paths which may have different magnitudes of hydraulic variables depending on each particular species. Results compare well with values reported for other standard fishways