[Ray76]

The entire chute should be rebuilt. The upper portion is aged and has numerous cracks and patches. Secondly, and most importantly, is a design issue. The slope should be flat rather than a convex curve.

[meyer]

I believe that the shape followed, at least for a good portion of its slope, the hard rock beneath. That’s why the top portion stayed put, and the bottom section is intact. That part in the middle - well, it wasn’t built on sound bedrock apparently.

but you’re probably right - it needs to be rebuilt anyway. Technology has advanced a lot since the 1960s, even if work ethic has not.

[Ray76]

The spillway is not flat but has two inflection points. Beginning at the gate the spillway has a flat angle of decline for about 900 feet. It then reaches the first inflection point. The spillway begins to curve downward toward a steeper angle of decline. At the second inflection point, a little more than 1,700 feet from the gate, the downward curving stops and the spillway has a flat angle of decline at this new steeper angle until the bottom.

In my opinion it is the varying angles of decline that are a factor, if not the factor, causing the spillway failure.

The failure occurs approximately 1,735 feet from the gate.

In the following diagram the blue line illustrates the profile of the spillway and the dashed red lines illustrate the flat portions of that profile. "X" marks the location of the spillway failure. I think it is not coincidental that the failure point is at or very close to the second inflection point.

Here's a close up.

Everyone knows what happens when you drive too fast over a small rise, everyone knows what happens at the top of a hill on a roller coaster - you momentarily come out of your seat. This is what is happening to the water in the spillway. The water is, to whatever degree, flying away from the spillway at the first inflection point and colliding with the spillway at the second inflection point.

The force is tremendous.

According to a Los Angles Times article "the spillway began breaking apart when its gates were opened Feb. 7, allowing 55,000 cubic feet of water per second to roar down the slope." At 70 degrees one cubic foot of water weights a little over 62 pounds. Colder water weighs more, but let's go with 62 pounds. 62 x 55,000 = 3,410,000 pounds of water per second.

According to this November 1974 California State Water Project Bulletin the width of the spillway is 178' 8". I'll leave it to engineers to determine how many pounds per square inch of spillway 3.4 million pounds of water per second can pound. To me it seems common sense, the convex curve of the spillway is the problem.