Peter McComb

Comparison of a Self-Processed EM3000 Multibeam Echosounder Dataset with a QTC View Habitat Mapping and a Sidescan Sonar Imagery, Tamaki Strait, New Zealand

Abstract A methodology for automatically processing the data files from an EM3000 multibeam echosounder (Kongsberg Maritime, 300 kHz) is presented. Written in MatLab, it includes data extraction, bathymetry processing, computation of seafloor local slope, and a simple correction of the backscatter along-track banding effect. The success of the latter is dependent on operational restrictions, which are also detailed. This processing is applied to a dataset acquired in 2007 in the Tamaki Strait, New Zealand. The resulting maps are compared with a habitat classification obtained with the acoustic ground-discrimination software QTC View linked to a 200-kHz single-beam echosounder and to the imagery from a 100-kHz sidescan sonar survey, both performed in 2002. The multibeam backscatter map was found to be very similar to the sidescan imagery, quite correlated to the QTC View map on one site but mainly uncorrelated on another site. Hypotheses to explain these results are formulated and discussed. The maps and the comparison to prior surveys are used to draw conclusions on the quality of the code for further research on multibeam benthic habitat mapping.

NUMERICAL MODEL STUDIES TO SUPPORT THE SUSTAINABLE MANAGEMENT OF DREDGE SPOIL DEPOSITION IN A COMPLEX NEARSHORE ENVIRONMENT

The numerical modelling approach used to inform and guide the sustainable management of maintenance dredge spoil within a nearshore environment is presented. Port Otago is one of New Zealand’s primary commercial ports and has a recent annual maintenance dredging history of approximately 150,000 m3. Long term disposal options for these sediments need to consider the sediment budgets for adjacent beaches as well as the quality of surfing conditions at nationally-significant breaks nearby. The port is set within a natural harbour that features strong tidal flows and complex entrance morphology, with a wave climate that is frequently energetic. A multi-scale, multi-model approach, combined with an input-reduction framework, is presented. The modelling effort is supported by an extensive field dataset, including hydrodynamic measurements and historical bathymetries used to calibrate the suite of models which are subsequently used to characterize the contemporary morphodynamics. The model suite is being used for the planning of future disposal activities that sustain the valued surfing wave conditions as well as providing adequate sediment bypassing.

Potential influence of shells on multibeam backscatter imagery within the Te Matuku Marine Reserve, New Zealand

Backscatter imagery from a KONGSBERG EM3000 multibeam echosounder (300kHz) has been analyzed to investigate its potential for benthic habitat mapping. A MATLAB code has been developed to process both bathymetry and coregistered echo‐strength, which includes a simple correction to remove the typical multibeam echo strength swathe contrast introduced by the backscatter incident‐angle dependence. Although the correction algorithm does completely remove the effect, it is sufficient to generate maps of suitable quality for further analysis. The processing code was applied to a dataset from a survey performed on two sites in the Tamaki Strait, New Zealand, in August 2007, including a section of the Te Matuku marine reserve. Within the marine reserve, a 400m2 area displays a strong reflectivity that contrasts with its surroundings while a past ground‐truth data collection shows the full area is dominated by mud with only some differences in shell fragments distribution. Hypotheses to explain the origin of this r...