Hydrodynamics of groyne fields in a straight river reach: insight from field experiments

This study explores the three-dimensional structure of turbulent flow around a sequence of groynes in a river reach under semi-controlled conditions. Detailed measurements were carried out in groyne fields with aspect ratios (length of groynes/distance between successive groynes) of 0.75 and 0.35 for emerged and submerged conditions and provided detailed spatial patterns of mean and turbulent fluxes of momentum. The results indicate that in mixing interfaces the vertical fluxes of turbulent momentum are dominant near the riverbed whereas horizontal fluxes dominate near the surface. Strong three-dimensional effects are reported for submerged groynes when a wake is formed at the leeward side. Groyne sequences increase flow resistance mainly by flow constriction that in turn generates an increase in velocity and bed shear stress in the main channel, rather than by production of excessive turbulence near the groynes.