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Effect of the Bed-Sediment Layer on the Scour Caused by a Jet

  • Research Article - Civil Engineering
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Abstract

Scour caused by a water jet impinging the bed-sediment layer is a significant concern for hydraulic engineers. Although several studies investigated the maximum scour depth on the non-cohesive bed-sediment layer, the effect of the bed-sediment layer’s thickness on the scour was not studied. This study investigated the effect of the thickness of the non-cohesive sediment layer at the canal bed on the depth of the scour caused by a water jet. The dimensionless parameters affecting the depth of the scour were obtained via dimensional analysis. Experiments were conducted on two different, non-cohesive bed-sediment layers at the bottom of a rectangular canal for different jet Froude numbers. Experimental results indicated that the depth of the scour increases with the thickness of the bed-sediment layer; this is because as the thickness of the sediment layer increases, the penetration depth of the air bubbles (dragged and enforced by the impinging water jet) through the sediment layer just under the scour hole increases. Due to the buoyancy force, as the air bubbles rise upward, they apply uplift forces and dynamic effects onto sediment particles, dislodging, suspending, and carrying the sediment particles away from the bed. If the thickness of the sediment layer is increased beyond a limiting value for a given flow condition, the maximum depth of the scour does not change (it remains almost constant). The effect of the thickness of the bed-sediment layer on the depth of the scour is present but not excessively large.

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Abbreviations

b 1 :

Distance between the axis of the water jet and right side wall of the canal

b 2 :

Distance between the axis of the water jet and left side wall of the canal

D 0J :

Inside diameter of the pipe from which the water jet issues into the atmosphere

D 50 :

Average diameter of the sediment particles of the bed-sediment layer

D 84.1 and D 15.9 :

Sieve opening sizes through which 84.1 and 15.9 % of the sediment particles pass, respectively

\({\mathsf{F}_{0}}\) :

Densimetric jet Froude number based on average diameter of sediment particles

\({\mathsf{F}_{0J}}\) :

Densimetric jet Froude number based on jet diameter

g :

Gravitational acceleration

H :

Distance from the pipe outlet to the bed along the jet centerline

h 0J :

Vertical distance between water surface and the issuing point of the jet (outlet of the pipe)

h 0k :

Vertical distance between the water surface and the top surface of the bed-sediment layer

h k :

Thickness of the bed-sediment layer

h m :

Maximum depth of scour

k :

Permeability coefficient

\({\mathsf{R}_{0J}}\) :

Jet Reynolds number

t :

Time

U 0k :

Average velocity of the canal flow

V 0J :

Average velocity of the jet at its issuing point into the atmosphere

w :

Settling velocity

\({\alpha}\) :

The angle between the jet axis and the horizontal axis (slope angle of the jet at its issuing point)

\({\rho}\) :

Density of the water

\({\rho_{k}}\) :

Density of the sediment particle

\({\mu}\) :

Dynamic viscosity of water

\({\nu}\) :

Kinematic viscosity of water

\({\sigma_{g}}\) :

Non-uniformity coefficient

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Authors and Affiliations

  1. Department of Civil Engineering, Faculty of Engineering, Gazi University, 06570, Maltepe, Ankara, Turkey

    Kerem Taştan & Nevzat Yildirim

  2. Ministry of Labour and Social Policies, Bursa, Turkey

    Peli̇n Pinar Koçak

Authors
  1. Kerem Taştan
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  2. Peli̇n Pinar Koçak
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  3. Nevzat Yildirim
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Correspondence to Kerem Taştan.

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Taştan, K., Koçak, P.P. & Yildirim, N. Effect of the Bed-Sediment Layer on the Scour Caused by a Jet. Arab J Sci Eng 41, 4029–4037 (2016). https://doi.org/10.1007/s13369-016-2093-7

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