1. Canaveral II swale west
Tidal Variability of Infragravity Waves Over Cape-Associated Shoals (EP23B-0963)
Juan F. Paniagua-Arroyave A-B, Peter N. Adams A, Arnoldo Valle-Levinson C, and Sabrina M. Parra D
A – Department of Geological Sciences, University of Florida, USA; B – Departamento de Ciencias de la Tierra, Universidad EAFIT, Colombia; C – Engineering School of Sustainable Infrastructure, University of Florida, USA; D – Naval Research Laboratory, Stennis Space Center, USA
Abstract
The influence of IG waves on the spatial and temporal patterns of particulate transport is not yet understood at cape-related shoals (i.e. inner-shelves characterized by non-uniform bathymetry). To analyze the connection between IG waves and tides, cross-spectral and cross-wavelet analyses were performed on time series data of current profiles and pressure at several locations around Cape Canaveral shoals. We focused on data collected during Spring 2014 at swales (east and west) contiguous to Canaveral II shoal. Overall during this experiment, time series of IG wave heights and pressure were coherent at ~2 cycles/day with a 95% statistical confidence at swale east. High coherence squared (>0.75) between tidal motions and IG energy at this location could be explained by changes in water depth that produced IG energy losses to sea-swell frequencies during low tide. The reason why IG energy is not following a semidiurnal variability at swale west is not understood. These results may highlight the sensitivity of IG waves generation to changes in water depth within this shoal complex. Our results agree with previous studies regarding tidal variability of IG energy in nearshore and inner-shelf environments and could be applied to improve understanding of the role of bathymetry in particulate transport at cape-associated shoals.
Swale West
Swale East
Experiment (step-wise)
Upward-looking acoustic Doppler current profilers (ADCPs) mounted with piezoresistive pressure sensors were moored at locations in Fig. 1C.
Pressure data were collected in bursts of 20 min at 2 Hz (2,400 data per burst) (Figs. 2A and 2B).
The time series constructed from the mean pressure of each burst were demeaned and detrended to get water levels (Figs. 2E).
A spectrum was calculated for each burst of pressure (Hanning windows of 256 data, 124 degrees of freedom at 95% of statistical confidence) [5] (Figs. 2C and 2D).
A significant wave height for the low frequency gravity (infragravity) band, HIG, was quantified for each burst as the area under the spectrum between frequencies and Hz [6] (Figs. 2C and 3A)
A band-passed version of HIG was calculated using a Lanczos digital filter for periods between 3 hours (~7 cpd) and 30 hours (~2 cpm) [5] (for visual analysis purposes) (Figs. 2E).
A continuous wavelet transform was quantified for time series of pressure (Figs. 3D) and HIG (Figs. 3E).
Cross-spectral coherence (squared coherence, Figs. 3B and 3C) and wavelet coherence (Figs. 3F and 3G) was quantified between time series of pressure and HIG [5, 7]. A B A B C D C D E E
Transport by IG waves at cape-related shoals
Transport processes at cape-related shoals play an important role in the behavior of oceanographic phenomena (e.g. biota–hydrodynamics feedbacks, dredging consequences, transport of sediments and contaminants) [1, 2].
The function of spatial and temporal variability of IG waves in the advection of particles at cape-related shoals remains unclear [3].
Particularly, the link between IG waves and tidal and subtidal motions is not well understood [4].
Figure 2. Ten-day segments of pressure data collected at the swales of Canaveral II shoal (A) with a zoom-in to 12 hours at red box (B), spectrograph of pressure bursts and HIG (C) with two sample spectra for black and magenta bursts in B (D), and water levels and band-passed version of HIG (E).
Results
Overall, pressure followed a semi-diurnal periodicity at h (from spectral analysis, not shown) (Figs. 2A and 3C) and HIG varied according to spectral densities (Fig. 2C) at both locations.
There was a high squared coherence (~0.74) between pressure and HIG for data at swale east. It was quantified significantly lower (~0.19) at swale west (Figs. 3B).
Phase between pressure and HIG was ~–13.8° at swale east, meaning that pressure leaded HIG by ~27 min.
There was a higher variance of HIG at swale east (right) than at swale west (left) at periods between and 2 days (8 and 0.5 cpd).
Higher coherence was observed during spring tides (day 136 in Figs. 3D, 3E and 3F).
During coherent times at ~2 cpd (Figs. 3F and 3G) phase ranged from –17.7° to 59.3° during ~10 days at swale east, whereas at swale west were ~119° lasting ~1 day.
~0.75 (significant)
Swale West
Swale East
~0.19 (not significant)
1 – Canaveral II shoal
2 – Chester shoal
3 – The Bull shoal A B A B C A C C
h~14.4 m
h~13.1 m
~–13.8° (~27 min)
Not significant D D B E E
Canaveral II
swale east
Cape Canaveral
Discussion
Semi-diurnal variability of IG energy was likely connected to tidal motions [6, 8].
High coherency (>0.75) and ~0 phases between pressure and HIG and suggests the enhancement of IG waves generation during high tide, and nonlinear losses from IG waves to swell-sea during low tide [8].
However, the tidal variability of IG energy generation at Cape Canaveral shoals remains to be validated [9], and conditions when and where tides may modulate HIG are not yet determined.
Temporal and spatial variability of coherence seem to indicate a high sensitivity of nonlinear triad interactions within Canaveral shoals. May this interpretation constitute a general rule at cape-related shoals?
Canaveral II swale west
Figure 1. Study area showing the location of mooring sites at Cape Canaveral shoals complex in the context of North America (A) and Florida peninsula (B). Double red circles in A and B highlight the location of Florida and Cape Canaveral, respectively. The map in C shows the bathymetry of the inner-shelf close to Cape Canaveral with numbers are placed near the shoals monitored (with zoom-ins in the right insets) Black circles show the actual mooring locations. Magenta circles in inset 1 correspond to the data herein analyzed. F F
T~0.5 days
T~0.5 days G G
References: [1] Phlips et al. (2006) Mar. Ecol. Prog. Ser. 322; [2] McNinch and Luettich (2000) Cont. Shelf Res. 20; [3] Aagaard and Greenwood (2008) Mar. Geol. 251; [4] Kline (2013) PhD Diss. Univ. Florida; [5] Thomson and Emery (2014) Data Analysis Methods in Physical Oceanography; [6] Okihiro and Guza (1995) J. Geophys. Res. 100; [7] Grinsted et al. (2004) Nonlinear Proc. Geoph. 11; [8] Thomson et al. (2006) Geophys. Res. Lett. 33; [9] Herbers and Burton (1997) J. Geophys. Res. 102.
Figure 3. Coherence analyses of pressure and HIG. Time series of pressure and HIG (A). Squared coherence (B) and phase between pressure and HIG (C). Wavelet transforms of pressure (D) and HIG (E). Wavelet coherence (F) and phase at period ~0.5 days, or frequency ~2 cpd indicated in F by a dashed black line (G) between pressure and HIG. Horizontal line in B and black solid contours in D through F indicate level statistical significance.
Thanks to BOEM, Fulbright Commission, University of Florida Dept. Geological Sciences, EAFIT University, and the Sediment Experimentalist Network. A. Grinsted provided the wavelet analysis package.