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Effect of apron roughness on flow characteristics and scour depth under submerged wall jets


Scour downstream of smooth and rough rigid aprons under wall jets has been studied experimentally. Effect of apron roughness on scour has been investigated, and quantification of reduction in the scour depth due to rough apron has been performed. Characteristics of velocity and turbulence over smooth and rough aprons as well as within the scour hole have been analyzed to study the behavior of the jet and its interaction with the rough apron. Results show that there is a significant reduction in the equilibrium scour depth due to roughness. A maximum of 82.8% and a minimum of 31.1% reduction in the equilibrium scour depth was observed due to inducing roughness over the rigid apron. Velocity characteristics establish the cause of reduction in the equilibrium scour depth, which is due to reduction in the erosive capacity of the jet as it moves over the rough apron. The potential core of the jet gets consumed at a much lesser length due to roughness over the apron than over a smooth apron, as the boundary layer develops at a smaller distance. Further, it was observed that it takes a smaller length for the flow to get fully developed under the rough apron as compared with the smooth apron. Based on the results of the present analysis, recommendation can be made for use of roughness over the apron to restrict scour due to wall jets.

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Data availability

All the data used in this study are available from the corresponding author upon request.


a :

Sluice opening

d 50 :

Median sediment size

d d :

Height of dune crest

d s :

Depth of maximum scour at equilibrium

d st :

Depth of maximum scour at any time t

d t :

Tailwater depth

F :

Jet Froude number

g :

Gravitational acceleration

k s :

Nikuradse's equivalent sand roughness

L :

Length of apron downstream of sluice gate

s :

Specific gravity of sediment particles

t :

Time of scouring

u :

Horizontal velocity component

u o :

Local maximum of vertical distribution of u

u + :

\((\overline{{u^{\prime}u^{\prime}}} )^{0.5} /V\)

(u +)o :

Local maximum of vertical distribution of u+

(u +)m :

Maximum value of u+ for an experimental condition


Fluctuation of u

uv'+ :

\(- \overline{{u^{\prime}v^{\prime}}} /V^{2}\)

(uv +)o :

Local maximum of vertical distribution of uv+

(uv +)m :

Maximum value of uv+ for an experimental condition

V :

Issuing jet velocity

v :

Vertical velocity component

v + :

\((\overline{{v^{\prime}v^{\prime}}} )^{0.5} /V\)

(v +)o :

Local maximum of vertical distribution of v+

(v +)m :

Maximum value of v+ for an experimental condition


Fluctuation of v

x :

Streamwise distance from the end of apron



x o :

Length of the scour hole

x s :

Distance of maximum scour depth from apron

y :

Vertical distance from the top surface of apron

ŷ :


y 1 :

Vertical distance where u: uo/2 and ∂u/∂y < 0

y 2 :

Sequent depth of free jump

δ :

Boundary layer thickness; vertical distance where u: uo

σ g :

Geometric standard deviation of sediment


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Corresponding author

Correspondence to Mohammad Aamir.

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On behalf of all authors, the corresponding author states that there is no conflict of interest.

Additional information

Communicated by Dr. Michael Nones.

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Aamir, M., Ahmad, Z. Effect of apron roughness on flow characteristics and scour depth under submerged wall jets. Acta Geophys. (2021).

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  • Jets
  • Scour
  • Hydraulic jump
  • Velocity field
  • Turbulence