Cross-Shore, Along-Shore, and Vertical Correlation of Rotary Spectra in the Nearshore

Outline RIPEX Problem Analyses Summary of Conclusions

Nearshore Rip Cells 50 to 100 m spacing Strong rip currents scour channels Crescentic bars between Waves break farther offshore over bars COAST LINE

Cross-shore Instruments Location ~ 100m offshore Centered around MWL Approx. 0.5m – 3m depth Recorded from ~April 15, 2001 to ~May 15, 2001

Tower and Goal Post Electromagnetic Current Meters Conductivity Sensors Capacitance Wave Staff BCDV Camera Scanning Acoustic XY Altimeter Recorded 14-29 April, 2001

Instrument Locations ADV Tower PUV ADCP

Observations Storm Event on April 22, 2001. Bed forms in the surf zone prior are 20-50cm across, 10-20cm high. Bed forms after the storm have been eliminated. Character of forcing changes substantially

Before

After

Days 109-112 Days 112-114 Data Coverage f Semi-diurnal tide

Question Can the nearshore wave spectra (outside beakers) be correlated to surf zone forcing? If so, does this correlation change in different wave environments?

Methodology 2.4-hour periods are examined Day 110 (prior) and Day 113 (after) Times are centered on high tide to ensure complete data coverage and similar conditions (MWL, etc)

Rotary Spectral Analysis Examines rotational frequencies of vectors Inner spectra examine co-rotation Outer spectra examine cross-rotation Autospectra correlate to single variable spectra Cross-spectra and coherence examine interactions

Data Selection Storm event

Pressure Spectra Narrow-band swell peak Swell peaks Surf beat Infragravity Narrow-band swell peak Low infragravity energy outside surf beat Broad-band swell peak High infragravity energy Three distinct surf beats

D110 Vertical D114

Cross-Shore Coherence D110 D114

Results (Not shown) No coherence with inner spectra observed Exception is minor coherence at swell peak Outer coherence appears scattered through frequecies Outer cross-spectra show opposing peaks at Surf beat and Swell frequencies: High energy at swell peak when combining ADCP anti-clockwise with others’ clockwise High energy at surf beat when combining ADCP clockwise with others’ anti-clockwise More prominent in low infragravity energy case (20 Apr)

Conclusions Vertical rotary spectral analysis shows expected coupling within the water column Inner spectra are not significantly correlated Outer cross-spectra show opposing cross-correlation at surf beat and swell peaks. Likely due to opposing positions within rip cell ADCP outside rip cell outflow PUV inside rip cell outflow ADV outside adjacent rip cell ECM inside adjacent rip cell Surf beat is more energetic because of longer period