The flood level factor
Much of the reworked spillway design revolves around attaining a probable maximum flood (PMF) rating, which is necessary to move forward with construction. The PMF rating is essentially a flow level that could only be reached in the most extreme flooding conditions, to ensure the structure could withstand the deluge.
After CH2M Hill took over the project from the Corps. as engineer of record, the team went about reviewing the original hydrology report at the recommendation of the Corps. It was at that point the engineers determined the PMF level for Duck River is actually more than double the Corps.’s original estimate — which means a much longer emergency spillway could be required. The report concluded a 600-foot spillway could be necessary, which is twice the size of the original 300-foot design.
“We considered two other designs, but they had disadvantages, and the costs were almost equal to what we were seeing already,” Harwell said.
Designers are now eyeing a design for a 250-foot spillway with a sand layer “fuse plug” at the top that could serve as a trapdoor to allow water to flow out to a designated area in the event of an unprecedented flood.
“Obviously, we want to make sure it’s a safe dam, in the unlikely event that a flood does occur,” Newton said. “That basically means we had to have a longer spillway to accommodate for a flood.”
Considering the dam’s location — near a major highway with residential areas nearby — the dam is labeled a high hazard dam, meaning it could cause loss of life and damage if it were to break.
The safety measures are needed because engineers determined that if the dam were to fall, it could endanger lives and structures within up to 60 miles. At its peak, the dam could potentially create a massive wave more than 40-feet high if it were to breach — which is why officials say the redesign is necessary to ensure the dam could withstand potential flood conditions.
“It’s just not practical to not have this as a high hazard dam,” Harwell said.
Trent Moore can be reached by e-mail at firstname.lastname@example.org, or by telephone at 734-2131, ext. 220.