A bathymetric survey of Winston Lagoon

Full description

During the 1986-87 Expedition to Heard Island, a 3m inflatable boat was depoted at the shores of Winston Lagoon, on the islands' south-east coast. The boat was to allow access to the important Long Beach Elephant Seal harems for periods when flooding from the lagoon prevented passage across its spit. The availability of the boat together with a 'Furuno' echo sounder, a stabilised, floating, transducer platform (constructed by a crew member from Nella Dan), and field assistance allowed a bathymetric survey of Winston Lagoon to be conducted.

Winston Lagoon depth work was done from 9/1/1987-14/1/1987 in the rare calm periods. We (the researchers) lived in the nearby Paddick Valley hut and sheltered there in rough weather. We only ran transects in calm weather. The map used was the largest Heard Island map available in 1986. 30 transects were run across the lake from known points on the map recognisable from the shore. We calibrated the echo sounder (a marine device) for fresh water by checking a range of measured depths against a weighted fibre-glass tape. Water samples were taken from a range of depths to the bottom and the lake was fresh throughout. Lake was very opaque with a secchi depth of 0.46m.

Lineage

Progress Code: completed

Statement: An area in the centre of the lagoon was deeper than the range of the echo-sounder and its depth was determined by lowering a length of weighted rope. Further information about the work completed was obtained from an email written by Roger Kirkwood: 'There was no proper GPS or trig data. I don't have any of the raw data - There were bathymetric strip charts, field note books, the original map (on pages of graph paper sticky taped together). I assumed Harry had kept all this. Below is a description of the survey. I drew the outline of the lake based on the 1980 island map. We started beside the little SE lake. Jenny Scott and I did about six crossings, Jenny steering toward a point on the opposite coast and travelling at constant speed, me running the echo-sounder and marking the strip chart, then picking out another point to head toward. After 6 transects, the wind picked up and we stopped. A few weeks later, Harry and I based ourselves at Paddick Valley and on windless opportunities did another 17 transects, trying to cover all areas. In the area that was too deep for the echo-sounder, we lowered a water sample-bottle on a metre-marked line a few times to get the maximum depth points that are on the map. The bottle hit the bottom each time and came up with fresh water. Using the length of paper on the strip chart and distances between 10 m depth intervals, and the distance across the piece of graph paper the map outline was drawn on, the bathymetry was constructed. I hand-drew the lines between the points. This was done each night so if there were gaps or discrepancies we could focus on those areas on the next opportunity. We checked the accuracy of the echo sounder against a length of weighted line in the Derwent before going to Heard. Although we only got to depths of up to 20 m, the accuracy was spot on. The accuracy of the transects seemed pretty good. We got the same depths when transect lines overlapped. The transducer was located in the middle of a specially constructed pontoon that ensured it always pointed vertically downwards. The overall map relied on the accuracy of the outline of the lagoon in the 1980 map. If this was out, the bathymetry was out accordingly. If it is possible to stretch the outline and contained bathymetry to fit the actual dimensions of the lagoon, there would be little error. Our positioning for the water-bottle drops was derived from estimation. I guess we could have had an error of up to 20 m. We tried to keep the dinghy stationary and couldn't account for currents at depth, though we didn't notice any. If any movement occurred, the depth would have been over-estimated. We felt this would have been minimal (probably 1-3 m).'