Physical Modeling of Spatial and Temporal Development of Local Scour at the Downstream of Bed Protection for Low Froude Number

Jungkyu Ahn,Jin Hwan Hwang,Sung Won Park

doi:10.3390/w11051041

Abstract

Local scour at the downstream of the river bed protection is one of the most important parameters for the design criteria and sustainable management of the hydraulic structures. Previously, various researches on its process in the equilibrium state have been suggested with experimental and numerical approaches. In this study, relatively long-term laboratory experiments of local scouring at the downstream of fixed bed in an open channel were conducted with mono-granular sediment bed and analyzed about maximum scour depth and its temporal development. In particular, we conducted experiments with relatively low Froude number (less than 0.5) and their duration of tests was exceeded over 700 hours. We modified the relationship between the dimensionless time and length scales of the maximum scour depth of the local scour hole based on the turbulent shear layer thickness. A new functional relationship between dominant factors and the maximum scour depth in the equilibrium state were suggested and compared with previously suggested formula. Also, from the results by nonlinear regression, Froude number was founded as a dominant factor on the prediction of equilibrium maximum scour depth.

Highlights

Local scour at the downstream of the river bed protection is one of the most significant phenomena which should be considered and predicted reasonably to design and manage the upstream hydraulic structures, such as sill, weir, and drop structures in alluvial streams
The longitudinal length of scour hole and the maximum scour depth are the most important scales of local scour based on the excessive loss of bed materials
In this study, we focused on the case, which the water depth is quite deeper than expected ym due to the water control structures at the far downstream with low Froude number

Summary

Introduction

Local scour at the downstream of the river bed protection is one of the most significant phenomena which should be considered and predicted reasonably to design and manage the upstream hydraulic structures, such as sill, weir, and drop structures in alluvial streams. Undesirable channel bed scour around the structures can threaten the safety of hydraulic structures. Local scour at the downstream of the bed protection is commonly and directly occurs by the impact of the hydraulic structure and/or its operation. There are several dimensions which can be representative for the scale of local scour at the downstream of the bed protection. The longitudinal length of scour hole (e.g., longitudinal length from the longitudinal transition to the first initial bed elevation at each time step) and the maximum scour depth (e.g., the first minimum value of bed elevation from the longitudinal transition at each time step) are the most important scales of local scour based on the excessive loss of bed materials.

Methods

Results

Conclusion