Vertical structure of turbulence within a depression surrounded by coral reef colonies

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Abstract

Turbulence is important in coral reefs but is difficult to measure in the field. In this paper, I present in situ measurements of waves, currents, and turbulence to study the vertical structure of turbulence within a depression (curved channel) that is surrounded by coral reef colonies on a fringing reef in Hobihu, Nan-Wan Bay, southern Taiwan. Turbulence was measured using a dual-velocimetry technique, and wave bias contamination in the turbulence is controlled using ogive curve testing of the turbulent shear stress (TSS). The observed turbulent dissipation rate (ε) is approximately five times greater than simultaneous observations over the nearby sandy bottom site, which indicates stronger mixing within the coral reef than on sandy bottoms. The low ratio of the TSS to the turbulent kinetic energy (TKE) and sweeping events indicate that energetic momentum is transported downward into the depression of the coral reef canopies. The observed value of ε exceeds the shear production rate, which suggests that transport terms or other source terms might be important. Direct evaluation of the transport terms suggests that vertical turbulent transport and advection are significant mechanisms that diffuse and convect the TKE downward into the depression. The observed TSS can be described well by the Prandtl–von Kármán eddy viscosity model and a two-equation turbulence model, which suggests that it is possible to develop a physically based model to quantify nutrient uptake and mass transfer in coral reefs under complex wave–current coastal flows.

Original languageEnglish
Pages (from-to)849-862
Number of pages14
JournalCoral Reefs
Volume34
Issue number3
DOIs
StatePublished - 8 Sep 2015

Keywords

  • Coral reef
  • Depression
  • Hydrodynamics
  • Mixing
  • Turbulence

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