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The influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy

The influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy
The influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy
Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied.
turbulence, bed roughness, small-scale topography, roughness length, Law of the Wall
935-956
Chirol, Clementine
4c819785-4361-4d1b-9905-27ae4e72659e
Amos, C.L.
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Lefebvre, Alice
3c33a28d-8873-4fc6-a75f-1af841d7aa8d
Umgiesser, G.
715016aa-c166-4598-89fc-cf2af5a85c78
Cucco, Andrea
843993da-3729-4579-b449-1b7b98459ef9
Chirol, Clementine
4c819785-4361-4d1b-9905-27ae4e72659e
Amos, C.L.
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Lefebvre, Alice
3c33a28d-8873-4fc6-a75f-1af841d7aa8d
Umgiesser, G.
715016aa-c166-4598-89fc-cf2af5a85c78
Cucco, Andrea
843993da-3729-4579-b449-1b7b98459ef9

Chirol, Clementine, Amos, C.L., Kassem, Hachem, Lefebvre, Alice, Umgiesser, G. and Cucco, Andrea (2015) The influence of bed roughness on turbulence: Cabras Lagoon, Sardinia, Italy. Journal of Marine Science and Engineering, 3 (3), 935-956. (doi:10.3390/jmse3030935). (PMID:jmse-89895)

Record type: Article

Abstract

Estimates of bed roughness used for predictions of sediment transport are usually derived either from simple scalars of the physical roughness (i.e., ripple height or grain size) or from the hydrodynamic roughness length (Zo) based upon velocity gradient estimates in the benthic boundary layer. Neither parameter accounts for irregular bed features. This study re-evaluates the relation between hydrodynamic roughness and physical bed roughness using high-resolution seabed scanning in the inlet of a shallow lagoon. The statistically-robust relationship, based on a 1D statistical analysis of the seabed elevation at different locations of the Cabras lagoon. Sardinia, has been obtained between Zo and the topographical bed roughness Ks by defining Ks = 2*STD + skin friction, with STD the standard deviation of the seabed elevation variations. This correlation between Ks and Zo demonstrates that the roughness length is directly influenced by irregular bed features, and that the Reynolds number accounts for the total drag of the bed: the data points collapse on the Law of the Wall curves with a fitting factor x = 0.5. Further testing must be done in other locations and in the fully-rough domain in order to test how widely those new parameters can be applied.

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More information

Submitted date: 9 June 2015
Accepted/In Press date: 11 August 2015
e-pub ahead of print date: 19 August 2015
Published date: 19 August 2015
Keywords: turbulence, bed roughness, small-scale topography, roughness length, Law of the Wall
Organisations: Ocean and Earth Science, Geology & Geophysics, Coastal & Shelf Research

Identifiers

Local EPrints ID: 380746
URI: http://eprints.soton.ac.uk/id/eprint/380746
PURE UUID: b4dd03f1-fdec-405b-9ef4-929f4494136a
ORCID for Hachem Kassem: ORCID iD orcid.org/0000-0002-5936-6037

Catalogue record

Date deposited: 20 Aug 2015 09:06
Last modified: 12 Dec 2021 04:01

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Contributors

Author: Clementine Chirol
Author: C.L. Amos
Author: Hachem Kassem ORCID iD
Author: Alice Lefebvre
Author: G. Umgiesser
Author: Andrea Cucco

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