Sergio Jaramillo Uribe Geno R. Pawlak University of Hawaii AUV-Based Observations of Rough Bed Hydrodynamics

Reefs: Highly irregular/ rough bathymetry Turbulent processes affect: Waves/currents Sediment transport Nutrient dispersal Acoustics/optics The problem: Roughness scales How to relate hydrodynamic and physical roughness in reefs? Models assume homogeneous roughness. Need to measure spatial scales of roughness over a reef Roughness Spectra Nunes & Pawlak 2008: Vessel, diver surveys of bed roughness at Kilo Nalu

Objectives: Study area: Kilo Nalu Nearshore Reef Observatory, Oahu, Hawaii. REMUS AUV surveys m depth, 1 x 0.5km Sidescan: spatial context / bed classification Narrow beam altimeter: physical roughness Relate bottom type/roughness to currents Can we use DVLs to measure currents?

Instruments: Sidescan sonar (Marine Sonic Technology): 900 kHz, 6x12cm resolution Narrow beam altimeter (Imagenex): 2.5 degree beam spread, ~10cm res. DVLs (RD Instruments): 1.2MHz, 1m bins, 1Hz sampling rate REMUS (Remote Environmental Monitoring UnitS, Hydroid, Inc) Sidescan transducer CTD ADCP array Trim weights Altimeter LBL transducer Altimeter GPS antenna

Convert to geo coords Statistics (6x6m boxes) EOF Relate to Roughness AUV Sidescan Analysis

EOF variables: variance skewness entropy spectral slope anisotropy AUV Sidescan Analysis

Variance Skewness Spectral Slope Anisotropy + Entropy …

AUV Sidescan Analysis R 2 = 0.61 EOF 1: 53%

AUV Sidescan Analysis EOF modes 2 (18%), 3 (17%): anisotropy + spectral slope  Sand ripples EOF Mode 2

Ship/AUV based current measurements: (Fong & Monismith, 2004, Dereck & Fong, 2006) Known bias in vehicle's direction Need to assess data quality: Characterize bias Compare to fixed ADCPs

Velocity bias 5m 6m 7m 8m 9m 2.0m/s 2.5m/s 1.5m/s AUV speed + shoreward legs o seaward legs Should be closer (6min difference)

Bias velocity vs: Ambiguity velocity and Vehicle speed AUV speed 2.5m/s 2.0m/s 1.5m/s Should be close to zero Best result for our tests: AUV speed ~= Va = 2.5m/s

Bias mean profiles (6min difference) Along-track velocity Ringing Low SNR? Along-track bias ~1% of AUV speed => Navigation OK Currents NOT OK

DVL along-shore current profiles: NO bias observed KN2 Current vertical structure OK

Summary - Bottom classification: Sidescan EOFs separate sand, coral, rippled areas - Roughness measurements: Sand areas RMS < 3cm, Coral reef 8-14cm RMS - Velocity bias and DVL settings: Bias velocity (only along-track) ~ 1% of AUV speed Sensitive to ambiguity velocity setting Possible ringing + SNR decay with distance - Current profiles:Good agreement with fixed ADCP DVLs can resolve current vertical structure - Ongoing: Relate roughness/seabed type to currents More observations

Visual verification

Sand with ripples Coral