The Influence of Bed Roughness on Turbulence: Cabras Lagoon, Sardinia, Italy

Clémentine Chirol,Carl Amos,Alice Lefebvre,Hachem Kassem,Andrea Cucco,Georg Umgiesser

doi:10.3390/jmse3030935

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.

Highlights

The benthic boundary layer is the portion of the water column that is directly affected by bed frictional drag [1,2]
This study explores the applicability of the Law of the Wall to field data with various small-scale bed features, and evaluates the effect of compound bed roughness on the flow turbulent state in the lower part of the benthic boundary layer
We suggest that scales up to 0.4 m2 fall into the category of bed roughness and, is likely to impact flow turbulence directly

Summary

Introduction

The benthic boundary layer is the portion of the water column that is directly affected by bed frictional drag [1,2]. This layer plays an important role in the dynamics and biochemistry of ecosystems [3], and has a number of applications in fields such as marine engineering, meteorology, and oceanography [4]. Our understanding of turbulence in the benthic boundary layer is complicated by the lack of a reliable equation for the drag caused by the pressure differences at the sediment-water interface [5,6]. The stress applied to the bed in this layer depends on a number of factors: the flow characteristics, the suspended sediment concentration [6,7,8,9], and the irregularities of the bed [10]. In the case of a concentration lower than 200 mg/L (such as Cabras lagoon), only the physical roughness is considered to influence drag in the benthic boundary layer

Methods

Results

Conclusion