Industrial History: 2010 Lake Delhi Dam Collapse near Delhi, IA

Friday, June 18, 2021

2010 Lake Delhi Dam Collapse near Delhi, IA

(Satellite, the V-shaped weir replaces the earth embankment that failed)

I learned of this July 24, 2010 dam breach from a TV show. There was no loss of life, but the downstream towns of Hopkinton and Monticello along with some farms did suffer damage.

Photo courtesy of Iowa Wing Civil Air Patrol via RenewableEnergyWorld

The design capacity of the spillway was 25,000 cfs , but one of the three gates would not open all of the way. The 495' long embankment had a concrete core wall. The spillway of the 83-year-old dam could not pass the heavy rain that fell for three days.

Julie Koehn/The Gazette

Photo courtesy of Iowa Department of Natural Resources via RenewableEnergyWorld
The breach of Delhi Dam in progress. The core wall is still in place.

The original configuration.

Photo courtesy of Iowa Department of Natural Resources via RenewableEnergyWorld

The following information, except for the Global Earth images, comes from RenewableEnergyWorld.

The dam was built between 1922-29 for hydroelectric power with "a 61-foot long conventional reinforced concrete powerhouse containing two S. Morgan Smith turbines with two Westinghouse generators (each rated at 750 kW)." The generation of electricity was terminated in 1968. Problems with the dam were identified in 2008 and repairs were still in progress during the July 2010 rain event.

The normal reservoir is at elevation 896' and holds 3,790 acre-feet. At the dam crest of 904.8' the reservoir holds 9,920 acre-feet. The spillway crest is at 879.8', and its capacity rating of 25,000 cfs is at 900'. The spillway capacity is 32,000 cfs at the crest height of 904.8'. The three vertical lift gates above the ogee spillway are 25' x17'.

Note that it went from a 100% headcut to a 60' breach in just two minutes.

The reinforced concrete wall stopped 6' below the crest of the dam at 898.8'. That 6' gap allowed internal erosion of the embankment. The concrete wall probably slowed down the rate of the dam breach, but it failed after the disappearance of the embankment that was intended to hold the wall in a vertical position. Even if they could have opened the spillway gates all of the way, the dam would probably have breached because the water would have exceeded the top of the core wall by 2.4' for about a day. Even if the operators had lowered the reservoir by 10' at the beginning of the rain event, the dam would have probably been breached because the reservoir did not have enough storage capacity.

The breach of Delhi Dam did not cause any loss of life. This is attributed to several factors: the concrete core wall likely slowed down the rate of the dam breach; warning of dam failure was issued several hours before the breach; the flood wave was dissipated in farm fields, which reduced the level of flooding in the downstream communities of Hopkinton and Monticello; and door to door warnings were issued in Hopkinton and Monticello resulting in evacuation of residents whose homes were subsequently inundated.

"There were design weaknesses at Delhi Dam that an experienced dam engineer should have recognized, which likely would have led to additional investigations." The embankment was replaced with a fixed weir that can safely pass the extreme flow from a severe rain event.