A framework for understanding drag parameterizations for coral reefs
In a hydrodynamic sense, a coral reef is a complex array of obstacles that exerts a net drag force on water moving over the reef. This drag is typically parameterized in ocean circulation models using drag coefficients (C D) or roughness length scales (z0); however, published CD for coral reefs span two orders of magnitude, posing a challenge to predictive modeling. Here we examine the reasons for the large range in reported CD and assess the limitations of using CD and z0 to parameterize drag on reefs. Using a formal framework based on the 3-D spatially averaged momentum equations, we show that CD and z0 are functions of canopy geometry and velocity profile shape. Using an idealized two-layer model, we illustrate that CD can vary by more than an order of magnitude for the same geometry and flow depending on the reference velocity selected and that differences in definition account for much of the range in reported CD values. Roughness length scales z 0 are typically used in 3-D circulation models to adjust C D for reference height, but this relies on spatially averaged near-bottom velocity profiles being logarithmic. Measurements from a shallow backreef indicate that z0 determined from fits to point measurements of velocity profiles can be very different from z0 required to parameterize spatially averaged drag. More sophisticated parameterizations for drag and shear stresses are required to simulate 3-D velocity fields over shallow reefs; in the meantime, we urge caution when using published C D and z0 values for coral reefs. Copyright 2011 by the American Geophysical Union.
Duke Scholars
Published In
DOI
EISSN
Publication Date
Volume
Issue
Related Subject Headings
- 3709 Physical geography and environmental geoscience
- 3708 Oceanography
- 3706 Geophysics
- 0406 Physical Geography and Environmental Geoscience
- 0405 Oceanography
- 0404 Geophysics
Citation
Published In
DOI
EISSN
Publication Date
Volume
Issue
Related Subject Headings
- 3709 Physical geography and environmental geoscience
- 3708 Oceanography
- 3706 Geophysics
- 0406 Physical Geography and Environmental Geoscience
- 0405 Oceanography
- 0404 Geophysics