Working out the volume of a detention basin

[kazzy]

Hi, I know this may seem easy but I just want to check that my calculation of a rectangular based detention basin that has slopes around the sides is correct. It is like an upside down square based triangluar prism but is cut off at the top.
I have a formula which I would like to check is correctly giving me the volume of the pond:
It is (h/3)(A +A' + sqrt(AxA'))

h = depth of the pond.
A is the surface area on the bottom of the pond
A' is the surface area on the top water level of the pond(this is larger)
Thanks

 

[gbam]

Average end area would be fine (Area top + Area Bottom)/2 X H for a prismatic configuration