Let me try it this way. Draw an imaginary line at the water level through the dam. All the pipe above that line and all the way to the pond end is of negative value. All the pipe below that line is positive value. With the numbers you gave, there is no way you have any positive value on a 100 foot pipe. Really, I want to see a 3/1 ratio to the positive. That is 3 feet vertical down for every 1 foot vertical up. That means 200 feet in your case.
