Measuring the capacity of dams or tanks

Whilst still on the subject of water, I thought it worth recording a simple methodology for calculating the volume of earth that needs to be excavated (or alternatively working out the capacity) to create a dam of a desired size in megalitres. The calculation is taken from the trusty 'Elders Notebook' that many farmers carry. At the bottom of the page in the notebook the source of the information is shown as Southern Rural Water (telephone 03 9742 6513).
The formula in the context of the above diagram is:
a) Length at bottom, multiplied by the width at the bottom: 30 X 9 = 270
b) Length at the top multiplied by width at the top: 126 X 89 = 11,214 (Surface area)
c) Sum of the lengths, multiplied by sum of the widths: (126 + 30) X (89 + 9) = 15,288
d) Sum a) plus b) plus c): 270 + 11,214 + 15,288 = 26,772
e) Figure at d) multiplied by the depth: 26,772 X 16= 428,352
f) Figure at e) divided by 6: 428,352/6 = 71,392. (NB. I've no idea where the '6' comes from!)
As all the measurements are in metres, the calculated capacity is in cubic metres. To convert this figures into megalitres (ML) divide the figure at f) by 1000: 71,392/1,000 = 71.392 ML
Some other bits of trivia:
* The length of 1 metre is in fact 1/10,000,000 of the distance from the North Pole through Paris to the equator. Looking at this another way, the distance from the North Pole through Paris to the equator is 10,000 km. So if the earth was a perfect sphere and the distance from the North Pole through Paris to the equator was exactly one quarter of the distance around the circumference of the earth, then the distance around the earth would be 40,000 km. Quite a hike, or swim, or perhaps more accurately a bit of both!!
* 1 gram equals the mass of 1 cubic centremetre of water at 4 degrees Celsius. Or put another way, 1 kilogram is the mass of 1 litre of water at 4 degrees Celsius.
* 1 litre equals the volume of 1 cubic decimeter (100 X 100 X 100 mm) of water
My thanks go Harvey Matthews for sharing the dam capacity calculations with me.