Accounting for uncertainty in volumes of seabed change measured with repeat multibeam sonar surveys

Seafloors of unconsolidated sediment are highly dynamic features; eroding or accumulating under the action of tides, waves and currents. Assessing which areas of the seafloor experienced change and measuring the corresponding volumes involved provide insights into these important active sedimentation processes. Computing the difference between Digital Elevation Models (DEMs) obtained from repeat Multibeam Echosounders (MBES) surveys has become a common technique to identify these areas, but the uncertainty in these datasets considerably affects the estimation of the volumes displaced. The two main techniques used to take into account uncertainty in volume estimations are the limitation of calculations to areas experiencing a change in depth beyond a chosen threshold, and the computation of volumetric confidence intervals. However, these techniques are still in their infancy and, as a result, are often crude, seldom used or poorly understood. In this article, we explored a number of possible methodological advances to address this issue, including: (1) using the uncertainty information provided by the MBES data processing algorithm CUBE, (2) adapting fluvial geomorphology techniques for volume calculations using spatially variable thresholds and (3) volumetric histograms. The nearshore seabed off Warrnambool harbour - located in the highly energetic southwest Victorian coast, Australia - was used as a test site. Four consecutive MBES surveys were carried out over a four-months period. The difference between consecutive DEMs revealed an area near the beach experiencing large sediment transfers - mostly erosion - and an area of reef experiencing increasing deposition from the advance of a nearby sediment sheet. The volumes of sediment displaced in these two areas were calculated using the techniques described above, both traditionally and using the suggested improvements. We compared the results and discussed the applicability of the new methodological improvements. We found that the spatially variable uncertainty derived from the CUBE algorithm provided the best results (i.e. smaller confidence intervals), but that similar results can be obtained using as a fixed uncertainty value derived from a reference area under a number of operational conditions.

The 45 year old health check - feasibility and impact on practices and patient behaviour

Background: The 45 year old health check (MBS item 717) for patients aged 45-49 years was introduced in 2006. This study evaluated its impact on preventive care and patient reported risk factors. Methods: A quantitative and qualitative study was conducted in eight general practices in Sydney, New South Wales. It involved follow up surveys of 118 patients taken both before the check and 3 months after. Practice staff were trained and supported to conduct the health checks and appropriate interventions. Results: There was ambivalence among some of the general practitioners toward the health check, but most found it feasible. The reported frequency of GP advice relating to each of the SNAP (smoking, nutrition, alcohol, and physical activity) risk factors increased; patient referrals, however, were infrequent. Patients' readiness to change their diet and exercise habits improved as a result of the check, with respondents showing an increase in both the consumption of vegetables and the frequency of physical activity. There was no change in body mass index, smoking or alcohol consumption. Discussion: The health check was associated with a short term improvement in diet and physical activity behaviours. Mechanisms to enhance referral need to be developed.