I'm not exactly a geometry teacher, and there's probably an easier way of explaining this, but here ya' go, I hope it's clear..... Perhaps someone will submit an easier way of calculating it.
I find it easier to break down the dam's dimensions into three separate parts, and then calculate the volume of each part by way of multiplying the length x width x height. This gives you the number of cubic feet in each part......... To figure the volume of the two triangular cross sections which represent the slope on each face of the dam, (as shown in the sketch) I find it easier to double the triangle and make it a rectangle. After finding the volume of the rectangle, divide it by 2. This gives you the volume of the triangle..... Add together the cubic feet from each of the 3 parts and divide the total by 27 to determine the number of cubic yards.
Center section calculation:
200' x 20' x 20' = 80,000 cubic feet
Right side rectangle (4 to 1 slope) calculation:
200' x 80' x 20' = 320,000 cubic feet
320,00 divided by 2 = 160,000 cubic feet
Left side rectangle (3 to 1 slope) calculation:
200' x 60' x 20' = 240,000 cubic feet
240,000 divided by 2 = 120,000 cubic feet
80,000 + 160,000 + 120,000 = 360,000 cubic feet
360,000 divided by 27 = 13,333 cubic yards
No guarantee of accuracy, so be sure and check my math. lol
