Elucidating the importance of sediment avalanches in scour at obstacles

ABSTRACT

An experimental analysis of the dynamics of avalanches for different stages of development and turbulent flow conditions is introduced. Experiments were conducted in a laboratory flume using clear-water conditions and a video camera placed within an acrylic cylinder. The results of a proper orthogonal decomposition of the pre-processed image sequences highlighted the existence of two types of sand avalanches: Type I corresponding to a narrow and vertically long-advancing slide, and Type II characterized by an elongated and short-advancing pattern. Temporally speaking, sand avalanches were triggered aperiodically, resembling the behaviour of the horseshoe vortex system. The frequency of occurrence of avalanches decreased with time, while the estimated volume of mobilized sand increased in the same period. The sand slide magnitudes and associated inter-avalanche time were Log-Pearson 3 distributed. Sand avalanches are an important scour mechanism, which depending on the flow intensity contribute to about 40% to 60% of the scoured volume.

Keywords:

• Subaqueous sediment avalanches
• scour
• turbulent flow
• flow obstacles
• POD
• image processing

Supplemental data

Supplemental data for this article can be accessed http://dx.doi.org/10.1080/00221686.2020.1866694.

Notation

b	=	flume width (m)
$d_{50}$	=	sediment size for which 50% by weight is finer (m)
$d_s$	=	representative sediment particle diameter (m)
D	=	pier diameter (m)
$D^{\ast }$	=	Dimensionless grain diameter (–)
$\overline{f}$	=	frequency of avalanches (Hz)
F	=	Froude number (–)
$F_d$	=	effective Froude number (–)
h	=	flow depth (m)
M	=	sand avalanche magnitude ($A_{d_{50}}$)
$\overline{M}$	=	mean sand avalanche magnitude ($A_{d_{50}}$)
n	=	surface area occupied by $d_{50}$ particles ($A_{d_{50}}$)
Q	=	discharge (m${}^3$ s${}^{-1}$)
$R_e$	=	Reynolds number (–)
t	=	time (s)
$t_{\mathrm{m}\mathrm{a}\mathrm{x}}$	=	duration of the experiment (s)
$S_t$	=	Strouhal number of the avalanches (–)
$u_{ef}$	=	effective flow velocity (m s${}^{-1}$)
$u_{c,s}$	=	threshold velocity for scour initiation (m s${}^{-1}$)
U	=	bulk channel velocity (m s${}^{-1}$)
$U_c$	=	critical velocity for inception of sediment motion (m s${}^{-1}$)
V	=	total volume of the scour hole (m${}^3$)
$V_A$	=	scoured volume caused by sand avalanches (m${}^3$)
$V_{\mathrm{m}\mathrm{a}\mathrm{x}}$	=	maximum volume of the scour hole (m${}^3$)
$W^{\ast }$	=	dimensionless, effective flow work (–)
z	=	scour depth (m)
$z_{\mathrm{m}\mathrm{a}\mathrm{x}}$	=	maximum scour depth (m)
$Z^{\ast }$	=	dimensionless scour depth (–)
$\mathrm{\Delta }{t}_A$	=	inter-avalanche time (s)
ν	=	kinematic viscosity of water (m${}^2$ s${}^{-1}$)
ρ	=	water density (kg m${}^{-3}$)
${\rho }_s$	=	particle density (kg m${}^{-3}$)
${\rho }^{\prime }$	=	relative density (–)
${\sigma }_g$	=	standard deviation of sediment size distribution (–)
${\sigma }_f$	=	standard deviation of the sand avalanches frequency (Hz)
${\sigma }_M$	=	standard deviation of the sand avalanches magnitude ($A_{d_{50}}$ )
ϕ	=	repose angle (deg)

Additional information

Funding

The authors are grateful for the financial support of the Comisión Nacional de Investigación, Científica y Tecnológica (CONICYT-Chile) through grants Fondecyt Nr. 1150997 and REDES 170021.