Ha! You get the best question award for the day!
I scratched my head for a second then I remembered an item in my collection of images (file storage on this is getting large).
The Construction Blueprints of the Main Spillway Head Sluice Gate show 8 gates noted each as 33 feet wide and over 17 feet high.
Then there was a photograph I remembered of a low water state of the reservoir that shows roads & equipment driving on the exposed apron area. (see pic below).
I hadn't gave much thought to seeing the single road going up the inlet apron to the Main Spillway Sluice Gate. But it seemed odd to me then... Until your question...
"They drive onto the main spillway chute through an open sluice gate".
That was a terrific question.
