The rubber dam that burst at Tempe Town Lake was fully intact until the moment it blew out, air pressure readings of the dam reveal.
Records released Monday show Tempe had known for two years about an air leak, a glass bottle jammed into the rubber and a bulge, but the city has determined those problems had nothing to do with the structure failing.
"Those three things were at the bottom and the rip was at the top," city spokeswoman Kris Baxter-Ging said.
The dam split along a seam July 20, causing it to completely deflate and release nearly all the water from the 220-acre lake.
Tempe monitors air pressure in the dams every minute, which showed the dam had about 14 pounds of pressure inside at 9:44 p.m. One minute later, pressure dropped to about 8 pounds.
Also, surveillance video of the dam from two angles shows a sudden failure. Tempe has posted video clips at www.youtube.com/Tempe11Video.
Police have determined there was no foul play involved.
The city inspects the rubber dams every month, except when water is flowing through the Salt River. No inspections were done from January through May when the river flowed.
The inspections found a small air leak, though city officials could never find the exact source underneath the bladder. An air pump was able to compensate for the leak. A 2-inch bulge also existed, which the city said was a distortion that's normal in the rubber dams. Also, a glass bottle was pushed into the surface, likely when the bladder was lowered during a flood. The city determined it was safer to leave the bottle in place than to pull it out.
Tempe was to begin work last Wednesday to replace the four rubber sections, but the damaged one was thought to be in better shape than two other sections set to be replaced first.
The manufacturer, Bridgestone Industrial Products, had recommended first replacing two sections that had been patched in 2002, Baxter-Ging said. The repairs were to address problems similar to conditions on the dam that burst. But when Bridgestone identified the bulge and air leak in 2008, it told the city the problem didn't warrant the same level of concern as on the sections it had previously patched.
The dams could have been replaced while a metal structure was temporarily placed behind them, allowing the lake to remain full. Now the city plans to fill the lake by Nov. 1, after the last of four new sections is in place.
For now, efforts continue to remove fish from the lake and send edible fish to wildlife rescue groups. The remaining puddles have fewer fish than expected, Baxter-Ging said.
A bulldozer will start grading the lakebed on Wednesday to remove debris and to ensure water drains instead of puddling and becoming a breeding ground for mosquitoes.