Surface area of a concrete drainage canal intersection

[jonlorio]

I am trying to measure a concrete structure to compute the surface area.
I have included a sketch of the structure with the dimensions that I have measured. This is part of a drainage canal bank where two canals intersect. The banks along both canals are paved with concrete. I tried to just average the slant dimensions and average the lengths of the top and bottom arcs. Then, I simply multiplied them together to get an area. I feel this is inaccurate. I then tried to stretch it out into a trapezoid, but I feel this is even less accurate. Any help?

 

[jonlorio]

also, the horizontal dimension is 32.5', and vertical dimension from top to bottom is 10.5' roughly...if that helps any.