Barwon Water, as a major water supplier in
Victoria, needs to know the full storage capacity
of its reservoirs. It also needs to be able to quickly and accurately determine the amount of water available in the reservoirs at any time.
A simple method to help calculate the available water is to create a depth-storage curve for each reservoir. The depth-storage curve is simply a table of the storage in the reservoir for a given depth of water. Once a depth-storage curve exists, measuring the current depth of water in the reservoir instantly gives the volume of water in the reservoir. To facilitate the use of this method of determining storage, Barwon Water requires accurate depth-storage curves for all the reservoirs.
The best method of creating a depth-storage curve is to construct an accurate three dimensional model of the reservoir. The volume of water below any height can then be calculated, and as the accuracy of the reservoir model is improved, the depth-storage curves can be updated.
The mapping of the physical shape of the reservoir above the water line can be obtained by traditional land based survey techniques and photogrammetry, but for land below the water line, when draining of the reservoir is not an option, other methods such as hydrographics(depth sounding) are needed.
Using the West Barwon Reservoir as a test case, Barwon Water has used a variety of technologies including 12d Model software, CEEMAN hydrographic software, Ortho-12d rectification software, GPS and a boat to collect sufficient data within the water’s edge to complete the reservoir model. Fergus Butcher, a Survey student working for Barwon Water decided to undertake the task and use it as a final year project under the supervision of the Barwon Water Spatial Information Coordinator, Ian McLachlan.
The existing data set provided by the team at Barwon Water was quite extensive and was put together using various surveying techniques, from feature surveying to stereo modelling using aerial photography. When the hydrographic survey was to be conducted it was vital that the water level of the storage at the time of the survey be greater than that represented in the existing data set so that redundant data could be collected and compared, thus providing a cross check.
Also of crucial importance was that all the data was in the same co-ordinate system. For this project the Australian Map Grid (AMG) and the Australian Height Datum (AHD) were used. Barwon Water surveyors had accurately determined the AMG co-ordinates of fourteen control points for surveillance monitoring which could now be used for the project. These control points were essential for the GPS work – the GPS base station needs to be set up over a known point so that it can send the differential corrections via a radio link to the boat. This means that once the AMG coordinates are computed for the ten monitoring points, any one of them can be used as the known point for the base station.
Before venturing onto the water, information useful for the navigation of the boat was prepared in 12d Model from existing Barwon Water survey data. ‘Track lines’ for the boat were also constructed in 12d Model. When loaded into the laptop on the boat, the track lines act as guidelines for the boat to help achieve an even distribution of data points over the bottom of the storage.
CEEMAN, the hydrographic software used for the project, can be configured in a number of ways. For this project the software was running on a laptop on the boat and received the echo sounding data through one USB port and GPS data from a GPS rover through another USB port.
Using the navigation data from 12d Model as background data and GPS positioning, CEEMAN could display the position of the boat within the reservoir at any given time. This allowed on screen display of the current position of the boat and the track lines which gave the helmsman the ability to steer along any selected track line and start and stop collecting data at the correct times.
CEEMAN automatically saved the collected data throughout the survey so the helmsman operating the computer never had to worry about interrupting the data collection to save work.
“It took three full days to complete the data collection” said Fergus. “On days one and three there was myself and my immediate supervisor onboard the boat, whilst a survey team of three set up the base station and then continued with separate work of their own. On day two there were three of us, again myself and my immediate supervisor and one of the surveyors.”