Added value:
Queen Mary Reservoir dates back to the Victorian period and covers nearly 9km2. Dredging operations have been underway for several years to extract aggregate form the reservoir bed. This serves the dual purpose of increasing the holding capacity whilst producing a saleable product. It is however important that any dredging does not breach the natural layer of clay beneath the shingle.

Hydrographic surveys are undertaken on line intervals of 20m to give sufficient resolution of the reservoir bed. In total, over 1 million individual soundings were recorded. An Atlas Deso 15 dual frequency echo sounder operating at 210kHz and 33kHz was used to define both the silt and hard material layers – the lower frequency soundings penetrating the softer material at the bed surface. The echo sounder was calibrated using the bar check method.

Water levels were monitored throughout the survey period using conventional levelling techniques.

Innovation:
Topographical mapping of the reservoir embankments was established using RTK GPS. All bathymetry was positioned with a Trimble Ag132 Differential GPS system incorporating medium wave broadcast corrections. Combining these techniques provides seamless data coverage from the reservoir bed to the outermost crest.

Customised Norcom Hydrographic software was utilised to allow the complete integration of all survey instruments.

Team work:
Halcrow Geomatics have a variety of trailer carried survey vessels, all with hull mounted transducers. A dedicated ‘off-road’ vehicle allows access to remote locations.

Data was presented in the form of two 3D XYZ digital terrain models – one for each sounding frequency. Drawings were also produced and supplied in both digital and paper format