Apron and macro roughness as scour countermeasures downstream of block ramps

ABSTRACT

A block ramp is a grade-control structure commonly used to stabilize the river bed with a high efficiency of energy dissipation. This study investigates the effects of using macro roughness and a downstream apron as scour countermeasures on the maximum scour depth downstream of block ramps in straight rivers. All the experiments have been carried out in a horizontal channel and in clear water conditions (no sediment transport). A wide range of hydraulic conditions including densimetric Froude numbers, water drop heights, and tailwater depths for three different rough ramp slopes (1 V:3 H, 1 V:5 H, and 1 V:7 H) was tested. The results have been compared with the results from a smooth ramp (no roughness), which has been considered as a reference. Results show that both macro roughness on the ramp and downstream aprons work well as scour countermeasures while as a comparison, using macro roughness (big-size stones) on the ramp is more effective than downstream aprons to reduce the scour depth. The other finding of this study shows that increasing the downstream apron decreases the length of the scour hole, consequently decreasing the volume of eroded material.

KEYWORDS:

• Aprons
• block ramp
• gradecontrol structure
• macro roughness
• scour countermeasures

Disclosure statement

No potential conflict of interest was reported by the authors.

Notation

The following symbols are used in this paper:	=
B	=	channel width (m);
d50	=	stilling basin mean particle diameter (m);
ds	=	maximum scour depth (m);
Dh	=	hydraulic depth (m);
f, f’	=	functional symbol;
F	=	V/(gDh)0.5 Froude number (−);
Fd	=	particle densimetric Froude number (−);
Gs	=	ρs / ρ (−);
g	=	gravity acceleration (ms−2);
ksM	=	macro roughness size (m);
L	=	length of ramp (m);
LA	=	length of apron (m);
P	=	height of structure (m);
Q	=	flow discharge (m3s−1);
R	=	VDh/υ Reynolds number (−);
S0	=	slope of block ramp (−);
y	=	flow depth over the ramp (m);
yt	=	tailwater depth (m);
Δy	=	difference between water surface elevations up and downstream of structure (m);
ρ, ρs	=	water and sediment density (kg m−3);
η	=	Fd2 Δy/P scour self-similar character (−)
υ	=	kinematic viscosity