AUV-based bed roughness mapping over a tropical reef

Jaramillo, Sergio; Pawlak, Geno

doi:10.1007/s00338-011-0731-9

Cited by 27 publications

(26 citation statements)

References 23 publications

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“…Physical roughness heights between SGN and SGS from the bathymetry survey (Figure a) range from 0.1 to 0.4 m, with a standard deviation of

σ_{r} = 0.13

m, similar to other coral reef flats [ Nunes and Pawlak , ; Péquignet et al ., ; Jaramillo and Pawlak , ; Huang et al ., ]. The ratio of wave hydrodynamic roughness to physical roughness is

z_{o} / σ_{r} \approx 0.6

.…”

Section: Discussionmentioning

confidence: 99%

Surface gravity wave transformation across a platform coral reef in the Red Sea

et al. 2016

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The transformation of surface gravity waves across a platform reef in the Red Sea is examined using 18 months of observations and a wave transformation model developed for beaches. The platform reef is 200 m across, 700 m long, and the water depth varies from 0.3 to 1.2 m. Assuming changes in wave energy flux are due to wave breaking and bottom drag dissipation, the wave transformation model with optimal parameters characterizing the wave breaking (γm = 0.25) and bottom drag (hydrodynamic roughness zo = 0.08 m) accounts for 75%–90% of the observed wave‐height variance at four sites. The observations and model indicate that wave breaking dominates the dissipation in a 20–30 m wide surf zone while bottom drag dominates the dissipation over the rest of the reef. Friction factors (drag coefficients) estimated from the observed wave energy balance range from fw = 0.5 to fw = 5 and increase as wave‐orbital displacements decrease. The observed dependence on wave‐orbital displacement is roughly consistent with extrapolation of an empirical relationship based on numerous laboratory studies of oscillatory flow. As a consequence of the dependence on wave‐orbital displacement, wave friction factors vary temporally due to changes in water depth and incident wave heights, and spatially across the reef as the waves decay.

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“…Physical roughness heights between SGN and SGS from the bathymetry survey (Figure a) range from 0.1 to 0.4 m, with a standard deviation of

σ_{r} = 0.13

m, similar to other coral reef flats [ Nunes and Pawlak , ; Péquignet et al ., ; Jaramillo and Pawlak , ; Huang et al ., ]. The ratio of wave hydrodynamic roughness to physical roughness is

z_{o} / σ_{r} \approx 0.6

.…”

Section: Discussionmentioning

confidence: 99%

Surface gravity wave transformation across a platform coral reef in the Red Sea

et al. 2016

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“…Previous studies in a variety of fields suggest the relationship between hydrodynamic and physical roughness is complex, depending, for example, on the ratio of roughness frontal area to bed area (e.g., Raupach et al 1991;Britter and Hanna 2003;Jimenez 2004; see also Monismith et al 2015). Consequently, determining a useful characterization of the physical roughness over coral reefs that is relevant to bottom stress is a major challenge (Nunes and Pawlak 2008;Zawada et al 2010;Rosman andHench 2011, Jaramillo andPawlak 2011;Hearn 2011). Accounting for the water depth dependence of the drag coefficients to get accurate estimates of hydrodynamic roughness is an important first step.…”

Section: Discussionmentioning

confidence: 99%

Coral Reef Drag Coefficients – Water Depth Dependence

Lentz

Davis

Churchill

et al. 2017

Journal of Physical Oceanography

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A major challenge in modeling the circulation over coral reefs is uncertainty in the drag coefficient because existing estimates span two orders of magnitude. Current and pressure measurements from five coral reefs are used to estimate drag coefficients based on depth-average flow, assuming a balance between the cross-reef pressure gradient and the bottom stress. At two sites wind stress is a significant term in the cross-reef momentum balance and is included in estimating the drag coefficient. For the five coral reef sites and a previous laboratory study, estimated drag coefficients increase as the water depth decreases consistent with open channel flow theory. For example, for a typical coral reef hydrodynamic roughness of 5 cm, observational estimates, and the theory indicate that the drag coefficient decreases from 0.4 in 20 cm of water to 0.005 in 10 m of water. Synthesis of results from the new field observations with estimates from previous field and laboratory studies indicate that coral reef drag coefficients range from 0.2 to 0.005 and hydrodynamic roughnesses generally range from 2 to 8 cm. While coral reef drag coefficients depend on factors such as physical roughness and surface waves, a substantial fraction of the scatter in estimates of coral reef drag coefficients is due to variations in water depth.

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“…To quantify the global scaling behavior, we computed the power spectral density, which has been previously used in the coral reef literature to quantify how topography varies as a function of spatial scale (Jaramillo & Pawlak, ; Nunes & Pawlak, ). The power spectrum was computed following Welch's () method

P ((), k) = \frac{1}{nNU} \sum_{i = 0}^{N - 1} {|{false\sum}_{m = 0}^{n - 1} w_{m} b_{m + iv} e^{- 2 π kmj}|}^{2},

where N is the number of segments for which the Fourier transform is computed, n is the number of points in a given segment, w is a window function (Hann used here),

j = \sqrt{- 1}

, and v is the number of points a successive segment is offset; for example, for a 50% window overlap v = n /2.…”

Section: Methodsmentioning

confidence: 99%

“…Methods that have been applied to bottom topography include the variation method, which quantifies the scaling behavior from variations in elevation extremes as a function of spatial scale (Zawada et al, ; Zawada & Brock, ), and the box counting method, based on the number of boxes of progressively smaller sizes needed to cover a curve or surface (Martin‐Garin et al, ; Purkis et al, ; Purkis et al, ). Other approaches include the change in spectral density (Jaramillo & Pawlak, ; Nunes & Pawlak, ) or rugosity (Knudby & LeDrew, ) across a range of spatial scales. A limitation of the aforementioned techniques is that only a single global scaling exponent, known as the Hurst exponent, can be computed for a given curve or surface.…”

Section: Introductionmentioning

confidence: 99%

Collapsing Complexity: Quantifying Multiscale Properties of Reef Topography

2019

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Seafloor topography affects a wide range of physical and biological processes; therefore, collapsing the three‐dimensional structure of the bottom to roughness metrics is a common challenge in studies of marine systems. Here we assessed the properties captured by metrics previously proposed for the seafloor, as well as metrics developed to characterize other types of rough surfaces. We considered three classes of metrics: properties of the bottom elevation distribution (e.g., standard deviation), length scale ratios (e.g., rugosity), and metrics that describe how topography varies with spatial scale (e.g., Hölder exponents). The metrics were assessed using idealized topography and natural seafloor topography data from airborne lidar measurements of a coral reef. We illustrate that common roughness metrics (e.g., rugosity) can have the same value for topographies that are geometrically very different, limiting their utility. Application of the wavelet leaders technique to the reef data set demonstrates that the topography has a power law scaling behavior, but it is multifractal so a distribution of Hölder exponents is needed to describe its scaling behavior. Using principal component analysis, we identify three dominant modes of topographic variability, or ways metrics covary, among and within reef zones. Collectively, the results presented here show that coral reef topography is both multiscale and multifractal. While individual metrics that capture specific topography properties relevant to a given process may be suitable for some studies, many applications will require a set of metrics that includes statistics that capture how topography varies with spatial scale.

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AUV-based bed roughness mapping over a tropical reef

Cited by 27 publications

References 23 publications

Surface gravity wave transformation across a platform coral reef in the Red Sea

Surface gravity wave transformation across a platform coral reef in the Red Sea

Coral Reef Drag Coefficients – Water Depth Dependence

Collapsing Complexity: Quantifying Multiscale Properties of Reef Topography

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