Characteristics of the flow field within a developing scour hole at a submerged weir

Abstract

Local scour is an important design factor for submerged weirs. This study conducted a clear-water scour experiment at a submerged weir, using the particle tracking velocimetry technique to measure the flow field in the developing scour hole. The mean flow patterns, turbulence intensities and near-bed Reynolds shear stresses in the scour hole at different scour stages are presented and discussed. The submerged weir alters the mean flow significantly, forming a high-velocity zone above the weir and a vortex system near the scoured bed. As the scour develops, the high turbulence intensity zone and the high near-bed Reynolds shear stress zone both get closer to the upstream slope, and further away from the downstream slope of the scour hole. At the equilibrium stage, although the near-bed Reynolds shear stress near the upstream slope of the scour hole exceeds the critical shear stress of sediment entrainment, sediment moves in a recirculating form such that the scour hole does not further enlarge.

Keywords:

• Flow field
• particle tracking velocimetry (PTV)
• river training structure
• scour
• submerged weir

Notation

The following symbols are used in this paper:

dse	=	equilibrium scour depth (m)
d50	=	median sediment size (m)
h0	=	approach flow depth (m)
ht	=	tailwater depth (m)
N	=	number of samples (–)
O	=	origin of coordinate system (–)
t	=	time (s)
U0	=	approach average flow velocity (m s−1)
Uc	=	critical mean velocity (m s−1)
u*c	=	critical shear velocity (m s−1)
u	=	horizontal velocity (m s−1)
$\overline{u}$	=	mean horizontal velocity (m s−1)
$u^{\mathrm{\prime }}$	=	horizontal velocity fluctuation (m s−1)
v	=	vertical velocity (m s−1)
$\overline{v}$	=	mean vertical velocity (m s−1)
$v^{\mathrm{\prime }}$	=	vertical velocity fluctuation (m s−1)
x	=	longitudinal distance measured from the downstream weir face (m)
y	=	vertical distance measured from the initial flatbed (m)
${\tau }_b$	=	near-bed Reynolds shear stress (Pa)
${\tau }_c$	=	critical shear stress for horizontal bed (Pa)
${\tau }_c^{\mathrm{\prime }}$	=	critical shear stress for sloping bed (Pa)
Δ	=	relative submerged particle density (–)
$\theta$	=	bed angle measured from horizontal datum (rad)
$\rho$	=	density of water (kg m−3)
σg	=	geometric standard deviation of sediment particles (–)
$\phi$	=	submerged angle of response of sediment (rad)

Additional information

Funding

This work was funded by the National Key R&D Program of China (2019YFC1510701), the National Natural Science Foundation of China (51909177 and 51709082) and the Fundamental Research Funds for the Central Universities (YJ201935).