Engineers: Three working floodgates may have averted disaster

Contrary to complaints from downstream residents, an earlier lowering of Lake Delhi would not likely have prevented the catastrophic July 24 dam failure, according to a draft report by an independent panel of engineers.

Had all three of the dam's floodgates been working properly, however, the disaster might have been averted, the engineers said.

The engineers, in their draft report issued Friday by the Iowa Department of Economic Development, evaluated the effect of lowering the lake by as much as 10 feet at the beginning of the flood and concluded that doing so would not have made any difference.

The results of their tests indicated that the maximum reservoir water surface and the duration of high reservoir water surface elevations would have been essentially unchanged.

“This reflects the fact that the reservoir volume is relatively small in comparison to the flood volume and any space that was created would have been filled prior to experiencing the peak flood flows,” according to the engineers – William Fiedler of the U.S. Bureau of Reclamation, Wayne King of the Federal Energy Regulatory Commission and Neil Schwanz of the U.S. Army of Corps of Engineers.

The engineers said internal erosion in the earthen embankment that made up the southernmost two-thirds of the dam was the likely cause of its failure after two days of heavy rain.

Contributing factors, they said, included the design of the dam, with a reinforced concrete core wall as the primary impervious element; the low plasticity sandy clay that made up the embankment; the limited ability of the dam with a spillway capacity of about 25,000 cubic feet per second to pass a major flood; and the binding of one of the three spillway gates, which prevented its full opening during the flood.

The engineers noted that the three 25-foot-wide by 20-foot-high floodgates had been difficult to operate in the past and that cranes had been used to fully open them during previous floods.

During the recent flood, they said, the faulty gate was not opened more than 4.25 feet, which would have been well less than 25 percent of its capacity.

The dam performed as expected during the recent flood until the reservoir height reached 898.8 feet – the same height as the top of the dam's impervious core wall, the engineer panel said.

Within about eight hours, whirlpools were observed in the reservoir near the dam and sinkholes were noticed on the upper portion of the upstream face of the dam – evidence of internal erosion, which was later confirmed by seepage from the downstream slope.

The dam breach, which started around 1 p.m. July 24, resulted in a peak breach flow of about 69,000 cubic feet per second, according to the engineers' report. It drained the 9-mile-long recreational lake in a matter of hours and eventually sent more than 200,000 cubic yards of lake-bottom silt down the Maquoketa River.

The engineers also conducted flood routage scenarios to determine likely outcomes if all three floodgates had been fully operational and opened to a height of 18 feet.

Those routing results indicated that water would not have overtopped the dam but that the reservoir would have exceeded the top of the core wall by as much as 2.4 feet for 29 hours and that internal erosion would have occurred within the embankment.

“Based on the duration of seepage that likely would have occurred through the embankment, it is

judged that the dam would have suffered damage and possibly a total breach,” the engineers said.