Bathymetric Surveying Using Acoustic Methods
Client: Auckland Council
PDP was engaged by Auckland Council to develop and test a methodology to assess the accumulation of sediments within stormwater treatment ponds in the Auckland region. As the Auckland Council owns and manages around 350 wet ponds within the region, the chosen methodology needed to be reliable and cost effective.
- Methodology development and testing
- Sedimentation surveying
- Asset assessment
- Staff training
Client: Auckland Council
During the project, we assessed two methods:
- The first method is a commonly used technique which involved collecting point depth measurements using a probe. Depth measurements were taken to identify the surface level of the sediment, and the depth of the underlying liner material (considered to be the depth at which the user feels resistance as the probe is pushed into the sediment). GPS co-ordinates were collected along with the two depth measurements. For a typical pond, measurements were commonly collected at a sampling resolution of 1 per 10m².
- The second method involved the use of an Acoustic Doppler Current Profiler (ADCP). The ADCP was mounted within a remote control boat (Q Boat). Using this method, a depth measurement (using different soundwave frequencies) can be recorded each second. A typical resulting pond survey using the ADCP method would therefore have thousands of point measurements. The ADCP can measure pond depths up to 40 m deep. Data obtained from the ADCP was quality assured within SonTek’s Hydrosurveyor software, then exported to mapping software, where a digital terrain model can be produced. The bathymetric data was then compared to as-built drawings of the ponds. As-built drawings were either digitised (or directly imported if already available in GIS format) in the mapping software to define the bathymetry of the pond prior to the pond becoming operative (i.e. before receiving sediment). The difference between the ADCP data and the as-built bathymetry data defines the quantity of sediment that has accumulated within the pond over time.
Comparative analysis between the two methods was conducted. Key findings were:
- Manual methods were found to be penetrating the underlying pond liner. Consequently, the manual method was overestimating sediment volumes by 10 to 20%.
- The ADCP method typically took a longer time to obtain data in the field (due to setting up the boat), but the subsequent desktop work was much faster. We do however consider that if the survey area were increased, the ADCP methodology would become more cost effective.
- Due to the greater number of data points collected, the error associated with the ADCP method was much less than the manual method.
- Using the radio control boat for the ADCP method significantly reduced health and safety risks compared to the manual method, due to the reduced need for staff to access the water.
- Using the ADCP’s low frequency soundwaves, the ADCP was found to be able to penetrate through medium weed growth and obtain usable data.
- Using the ADCP method, an accurate assessment of the pond’s sedimentation zones can be established. This is beneficial if dredging costs are considered high, i.e. rather than electing to dredge the entire pond, the ADCP can identify the primary depositional zones which can therefore reduce associated dredging costs.
A Digital Elevation Model output from the ADCP methodology
Currently, PDP are working with the equipment supplier, SonTek (San Diego), to further utilise the low frequency soundwave produced by the ADCP to determine the underlying depth of the pond liner. This work is being carried out as some of the ponds in the region do not have as-built plans available to complete the analysis. To date this work has achieved promising results, but the method to determine the underlying pond liner depth is still being tested and refined.
As a result of this project, Auckland Council has now adopted the ADCP method as their preferred method for assessing the accumulation of sediments within stormwater treatment ponds in the Auckland region.