Although this was written a little while ago, I thought you might find this little screed by Bob interesting.
How two men in a tinnie became three men in a boat
Two prolonged dry spells in the last few years have highlighted the importance of the Gardens' two sources of irrigation water – the lagoon and the bore. The lagoon water is used in the southern section of the Gardens (Sculpture Garden and A&E Centre) whilst the bore supplies the northern section (Fern Glade, Mossy Log, Colonnade and Rock Pool).
The bore water has salinity and dissolved salts levels that are detrimental to plant growth, particularly to some of the ferns in the Fern Glade. The recent installation of the sprinkler masts in the Fern Glade has greatly benefited the condition of the plants, but even greater benefits should flow from the use of the more suitable lagoon water. The proposed installation of an electric pump under the causeway bridge would allow watering of the Fern Glade from either the lagoon or the bore, or a 'shandy' of both.
This raised the question – how much water do we have?
As far as is known the bore has not run dry, but may have come very close at the end of 2012. The lagoon capacity is something of an unknown – we watched the level drop to 750mm below the outlet in 2012, and it's now 600mm below. So, how much water is left? Has the lagoon silted up (the colour on occasions would suggest so)?
To answer these and other questions an expedition was suggested to survey the lagoon. Equipped with Tony's tinnie, complete with depth sounder, and Bob's depth sounder calibration tool (a brick with attached rope, knotted at 1metre intervals), two men in a tinnie set off. After an exhausting paddle to the southern reaches of the lagoon (at least 200metres), the positional fixing device and operator were embarked ( Rob King with his GPS) – thus three men in a boat made the return journey to the causeway, collating positions and depths.
The results were somewhat surprising. Firstly, the depth sounder was very accurate and not confused by a possible deep silt layer. Secondly, the lagoon was much deeper than anticipated. In the lower part near the causeway-over 5 metres of water was measured, and much was over 4 metres deep. Even at the far end 2 metres was quite widespread and it was only near the two feeder watercourses was the level under a metre.
Some very rough calculations put the capacity of the full lagoon at approx. 25 Mega litres (10 Olympic swimming pools) and the current level at 80% capacity. This amount of water gives us much more confidence in using it to irrigate the Fern Glade without threatening our water security; management of this water resource will be much easier knowing its capacity.
A very worthwhile expedition!
Photo: Louise Richardson