- In February of 2017, one of the largest spillways in the world, the one at Oroville Dam in northern California, was severely damaged during releases from heavy rain. You might remember this. I made a video about it, and then another one about the impressive feat of rebuilding the structure. In the forensic report following the incident, one of the contributing causes identified in the failure was the drainage system below the spillway. Rather than being installed below the concrete, each drain protruded into it, reducing the thickness of the concrete and making it more prone to cracking.